nutrition research study designs

Types of Research Studies and How To Interpret Them

The field of nutrition is dynamic, and our understanding and practices are always evolving. Nutrition scientists are continuously conducting new research and publishing their findings in peer-reviewed journals. This adds to scientific knowledge, but it’s also of great interest to the public, so nutrition research often shows up in the news and other media sources. You might be interested in nutrition research to inform your own eating habits, or if you work in a health profession, so that you can give evidence-based advice to others. Making sense of science requires that you understand the types of research studies used and their limitations.

The Hierarchy of Nutrition Evidence

Researchers use many different types of study designs depending on the question they are trying to answer, as well as factors such as time, funding, and ethical considerations. The study design affects how we interpret the results and the strength of the evidence as it relates to real-life nutrition decisions. It can be helpful to think about the types of studies within a pyramid representing a hierarchy of evidence, where the studies at the bottom of the pyramid usually give us the weakest evidence with the least relevance to real-life nutrition decisions, and the studies at the top offer the strongest evidence, with the most relevance to real-life nutrition decisions .

Figure 2.3. The hierarchy of evidence shows types of research studies relative to their strength of evidence and relevance to real-life nutrition decisions, with the strongest studies at the top and the weakest at the bottom.

The pyramid also represents a few other general ideas. There tend to be more studies published using the methods at the bottom of the pyramid, because they require less time, money, and other resources. When researchers want to test a new hypothesis , they often start with the study designs at the bottom of the pyramid , such as in vitro, animal, or observational studies. Intervention studies are more expensive and resource-intensive, so there are fewer of these types of studies conducted. But they also give us higher quality evidence, so they’re an important next step if observational and non-human studies have shown promising results. Meta-analyses and systematic reviews combine the results of many studies already conducted, so they help researchers summarize scientific knowledge on a topic.

Non-Human Studies: In Vitro & Animal Studies

The simplest form of nutrition research is an in vitro study . In vitro means “within glass,” (although plastic is used more commonly today) and these experiments are conducted within flasks, dishes, plates, and test tubes. These studies are performed on isolated cells or tissue samples, so they’re less expensive and time-intensive than animal or human studies. In vitro studies are vital for zooming in on biological mechanisms, to see how things work at the cellular or molecular level. However, these studies shouldn’t be used to draw conclusions about how things work in humans (or even animals), because we can’t assume that the results will apply to a whole, living organism.

Animal studies are one form of  in vivo research, which translates to “within the living.” Rats and mice are the most common animals used in nutrition research. Animals are often used in research that would be unethical to conduct in humans. Another advantage of animal dietary studies is that researchers can control exactly what the animals eat. In human studies, researchers can tell subjects what to eat and even provide them with the food, but they may not stick to the planned diet. People are also not very good at estimating, recording, or reporting what they eat and in what quantities. In addition, animal studies typically do not cost as much as human studies.

There are some important limitations of animal research. First, an animal’s metabolism and physiology are different from humans. Plus, animal models of disease (cancer, cardiovascular disease, etc.), although similar, are different from human diseases. Animal research is considered preliminary, and while it can be very important to the process of building scientific understanding and informing the types of studies that should be conducted in humans, animal studies shouldn’t be considered relevant to real-life decisions about how people eat.

Observational Studies

Observational studies in human nutrition collect information on people’s dietary patterns or nutrient intake and look for associations with health outcomes. Observational studies do not give participants a treatment or intervention; instead, they look at what they’re already doing and see how it relates to their health. These types of study designs can only identify correlations (relationships) between nutrition and health; they can’t show that one factor causes another. (For that, we need intervention studies, which we’ll discuss in a moment.) Observational studies that describe factors correlated with human health are also called epidemiological studies . 1

One example of a nutrition hypothesis that has been investigated using observational studies is that eating a Mediterranean diet reduces the risk of developing cardiovascular disease. (A Mediterranean diet focuses on whole grains, fruits and vegetables, beans and other legumes, nuts, olive oil, herbs, and spices. It includes small amounts of animal protein (mostly fish), dairy, and red wine. 2 ) There are three main types of observational studies, all of which could be used to test hypotheses about the Mediterranean diet:

• Cohort studies follow a group of people (a cohort) over time, measuring factors such as diet and health outcomes. A cohort study of the Mediterranean diet would ask a group of people to describe their diet, and then researchers would track them over time to see if those eating a Mediterranean diet had a lower incidence of cardiovascular disease.
• Case-control studies compare a group of cases and controls, looking for differences between the two groups that might explain their different health outcomes. For example, researchers might compare a group of people with cardiovascular disease with a group of healthy controls to see whether there were more controls or cases that followed a Mediterranean diet.
• Cross-sectional studies collect information about a population of people at one point in time. For example, a cross-sectional study might compare the dietary patterns of people from different countries to see if diet correlates with the prevalence of cardiovascular disease in the different countries.

Prospective cohort studies, which enroll a cohort and follow them into the future, are usually considered the strongest type of observational study design. Retrospective studies look at what happened in the past, and they’re considered weaker because they rely on people’s memory of what they ate or how they felt in the past. There are several well-known examples of prospective cohort studies that have described important correlations between diet and disease:

• Framingham Heart Study : Beginning in 1948, this study has followed the residents of Framingham, Massachusetts to identify risk factors for heart disease.
• Health Professionals Follow-Up Study : This study started in 1986 and enrolled 51,529 male health professionals (dentists, pharmacists, optometrists, osteopathic physicians, podiatrists, and veterinarians), who complete diet questionnaires every 2 years.
• Nurses Health Studies : Beginning in 1976, these studies have enrolled three large cohorts of nurses with a total of 280,000 participants. Participants have completed detailed questionnaires about diet, other lifestyle factors (smoking and exercise, for example), and health outcomes.

Observational studies have the advantage of allowing researchers to study large groups of people in the real world, looking at the frequency and pattern of health outcomes and identifying factors that correlate with them. But even very large observational studies may not apply to the population as a whole. For example, the Health Professionals Follow-Up Study and the Nurses Health Studies include people with above-average knowledge of health. In many ways, this makes them ideal study subjects, because they may be more motivated to be part of the study and to fill out detailed questionnaires for years. However, the findings of these studies may not apply to people with less baseline knowledge of health.

We’ve already mentioned another important limitation of observational studies—that they can only determine correlation, not causation. A prospective cohort study that finds that people eating a Mediterranean diet have a lower incidence of heart disease can only show that the Mediterranean diet is correlated with lowered risk of heart disease. It can’t show that the Mediterranean diet directly prevents heart disease. Why? There are a huge number of factors that determine health outcomes such as heart disease, and other factors might explain a correlation found in an observational study. For example, people who eat a Mediterranean diet might also be the same kind of people who exercise more, sleep more, have higher income (fish and nuts can be expensive!), or be less stressed. These are called confounding factors ; they’re factors that can affect the outcome in question (i.e., heart disease) and also vary with the factor being studied (i.e., Mediterranean diet).

Intervention Studies

Intervention studies , also sometimes called experimental studies or clinical trials, include some type of treatment or change imposed by the researcher. Examples of interventions in nutrition research include asking participants to change their diet, take a supplement, or change the time of day that they eat. Unlike observational studies, intervention studies can provide evidence of cause and effect , so they are higher in the hierarchy of evidence pyramid.

The gold standard for intervention studies is the randomized controlled trial (RCT) . In an RCT, study subjects are recruited to participate in the study. They are then randomly assigned into one of at least two groups, one of which is a control group (this is what makes the study controlled ). In an RCT to study the effects of the Mediterranean diet on cardiovascular disease development, researchers might ask the control group to follow a low-fat diet (typically recommended for heart disease prevention) and the intervention group to eat a Mediterrean diet. The study would continue for a defined period of time (usually years to study an outcome like heart disease), at which point the researchers would analyze their data to see if more people in the control or Mediterranean diet had heart attacks or strokes. Because the treatment and control groups were randomly assigned, they should be alike in every other way except for diet, so differences in heart disease could be attributed to the diet. This eliminates the problem of confounding factors found in observational research, and it’s why RCTs can provide evidence of causation, not just correlation.

Imagine for a moment what would happen if the two groups weren’t randomly assigned. What if the researchers let study participants choose which diet they’d like to adopt for the study? They might, for whatever reason, end up with more overweight people who smoke and have high blood pressure in the low-fat diet group, and more people who exercised regularly and had already been eating lots of olive oil and nuts for years in the Mediterranean diet group. If they found that the Mediterranean diet group had fewer heart attacks by the end of the study, they would have no way of knowing if this was because of the diet or because of the underlying differences in the groups. In other words, without randomization, their results would be compromised by confounding factors, with many of the same limitations as observational studies.

In an RCT of a supplement, the control group would receive a placebo —a “fake” treatment that contains no active ingredients, such as a sugar pill. The use of a placebo is necessary in medical research because of a phenomenon known as the placebo effect. The placebo effect results in a beneficial effect because of a subject’s belief in the treatment, even though there is no treatment actually being administered.

For example, imagine an athlete who consumes a sports drink and then runs 100 meters in 11.0 seconds. On a different day, under the exact same conditions, the athlete is given a Super Duper Sports Drink and again runs 100 meters, this time in 10.5 seconds. But what the athlete didn’t know was that the Super Duper Sports Drink was the same as the regular sports drink—it just had a bit of food coloring added. There was nothing different between the drinks, but the athlete believed that the Super Duper Sports Drink was going to help him run faster, so he did. This improvement is due to the placebo effect. Ironically, a study similar to this example was published in 2015, demonstrating the power of the placebo effect on athletic performance. 3

Figure 2.4. An example of the placebo effect

Blinding is a technique to prevent bias in intervention studies. In a study without blinding, the subject and the researchers both know what treatment the subject is receiving. This can lead to bias if the subject or researcher have expectations about the treatment working, so these types of trials are used less frequently. It’s best if a study is double-blind , meaning that neither the researcher nor the subject know what treatment the subject is receiving. It’s relatively simple to double-blind a study where subjects are receiving a placebo or treatment pill, because they could be formulated to look and taste the same. In a single-blind study , either the researcher or the subject knows what treatment they’re receiving, but not both. Studies of diets—such as the Mediterranean diet example—often can’t be double-blinded because the study subjects know whether or not they’re eating a lot of olive oil and nuts. However, the researchers who are checking participants’ blood pressure or evaluating their medical records could be blinded to their treatment group, reducing the chance of bias.

Like all studies, RCTs and other intervention studies do have some limitations. They can be difficult to carry on for long periods of time and require that participants remain compliant with the intervention. They’re also costly and often have smaller sample sizes. Furthermore, it is unethical to study certain interventions. (An example of an unethical intervention would be to advise one group of pregnant mothers to drink alcohol to determine its effects on pregnancy outcomes, because we know that alcohol consumption during pregnancy damages the developing fetus.)

VIDEO: “ Not all scientific studies are created equal ” by David H. Schwartz, YouTube (April 28, 2014), 4:26.

Meta-Analyses and Systematic Reviews

At the top of the hierarchy of evidence pyramid are systematic reviews and meta-analyses . You can think of these as “studies of studies.” They attempt to combine all of the relevant studies that have been conducted on a research question and summarize their overall conclusions. Researchers conducting a systematic review formulate a research question and then systematically and independently identify, select, evaluate, and synthesize all high-quality evidence that relates to the research question. Since systematic reviews combine the results of many studies, they help researchers produce more reliable findings. A meta-analysis is a type of systematic review that goes one step further, combining the data from multiple studies and using statistics to summarize it, as if creating a mega-study from many smaller studies . 4

However, even systematic reviews and meta-analyses aren’t the final word on scientific questions. For one thing, they’re only as good as the studies that they include. The Cochrane Collaboration is an international consortium of researchers who conduct systematic reviews in order to inform evidence-based healthcare, including nutrition, and their reviews are among the most well-regarded and rigorous in science. For the most recent Cochrane review of the Mediterranean diet and cardiovascular disease, two authors independently reviewed studies published on this question. Based on their inclusion criteria, 30 RCTs with a total of 12,461 participants were included in the final analysis. However, after evaluating and combining the data, the authors concluded that “despite the large number of included trials, there is still uncertainty regarding the effects of a Mediterranean‐style diet on cardiovascular disease occurrence and risk factors in people both with and without cardiovascular disease already.” Part of the reason for this uncertainty is that different trials found different results, and the quality of the studies was low to moderate. Some had problems with their randomization procedures, for example, and others were judged to have unreliable data. That doesn’t make them useless, but it adds to the uncertainty about this question, and uncertainty pushes the field forward towards more and better studies. The Cochrane review authors noted that they found seven ongoing trials of the Mediterranean diet, so we can hope that they’ll add more clarity to this question in the future. 5

Science is an ongoing process. It’s often a slow process, and it contains a lot of uncertainty, but it’s our best method of building knowledge of how the world and human life works. Many different types of studies can contribute to scientific knowledge. None are perfect—all have limitations—and a single study is never the final word on a scientific question. Part of what advances science is that researchers are constantly checking each other’s work, asking how it can be improved and what new questions it raises.

Self-Check:

Attributions:

• “Chapter 1: The Basics” from Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR , CC BY-NC-SA 4.0
• “ The Broad Role of Nutritional Science ,” section 1.3 from the book An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0

References:

• 1 Thiese, M. S. (2014). Observational and interventional study design types; an overview. Biochemia Medica , 24 (2), 199–210. https://doi.org/10.11613/BM.2014.022
• 2 Harvard T.H. Chan School of Public Health. (2018, January 16). Diet Review: Mediterranean Diet . The Nutrition Source. https://www.hsph.harvard.edu/nutritionsource/healthy-weight/diet-reviews/mediterranean-diet/
• 3 Ross, R., Gray, C. M., & Gill, J. M. R. (2015). Effects of an Injected Placebo on Endurance Running Performance. Medicine and Science in Sports and Exercise , 47 (8), 1672–1681. https://doi.org/10.1249/MSS.0000000000000584
• 4 Hooper, A. (n.d.). LibGuides: Systematic Review Resources: Systematic Reviews vs Other Types of Reviews . Retrieved February 7, 2020, from //libguides.sph.uth.tmc.edu/c.php?g=543382&p=5370369
• 5 Rees, K., Takeda, A., Martin, N., Ellis, L., Wijesekara, D., Vepa, A., Das, A., Hartley, L., & Stranges, S. (2019). Mediterranean‐style diet for the primary and secondary prevention of cardiovascular disease. Cochrane Database of Systematic Reviews , 3 . https://doi.org/10.1002/14651858.CD009825.pub3
• Figure 2.3. The hierarchy of evidence by Alice Callahan, is licensed under CC BY 4.0
• Research lab photo by National Cancer Institute on Unsplas h ; mouse photo by vaun0815 on Unsplash
• Figure 2.4. “Placebo effect example” by Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR

Experiments that are conducted outside of living organisms, within flasks, dishes, plates, or test tubes.

Research that is conducted in living organisms, such as rats and mice.

In nutrition, research that is conducted by collecting information on people’s dietary patterns or nutrient intake to look for associations with health outcomes. Observational studies do not give participants a treatment or intervention; instead, they look at what they’re already doing and see how it relates to their health.

Relationships between two factors (e.g., nutrition and health).

Research that follows a group of people (a cohort) over time, measuring factors such as diet and health outcomes.

Research that compares a group of cases and controls, looking for differences between the two groups that might explain their different health outcomes.

Research that collects information about a population of people at one point in time.

Looking into the future.

Looking at what happened in the past.

Factors that can affect the outcome in question.

Research that includes some type of treatment or change imposed by the researchers; sometimes called experimental studies or clinical trials.

The gold standard for intervention studies, because the research involves a control group and participants are randomized.

A “fake” treatment that contains no active ingredients, such as a sugar pill.

The beneficial effect that results from a subject's belief in a treatment, not from the treatment itself.

technique to prevent bias in intervention studies, where either the research team, the subject, or both don’t know what treatment the subject is receiving.

Neither the research team nor the subject know what treatment the subject is receiving.

Either the research team or the subject know what treatment is being given, but not both.

Researchers formulate a research question and then systematically and independently identify, select, evaluate, and synthesize all high-quality evidence from previous research that relates to the research question.

A type of systematic review that combines data from multiple studies and uses statistical methods to summarize it, as if creating a mega-study from many smaller studies.

Nutrition: Science and Everyday Application, v. 1.0 Copyright © 2020 by Alice Callahan, PhD; Heather Leonard, MEd, RDN; and Tamberly Powell, MS, RDN is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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What Makes Nutrition Research So Difficult to Conduct and Interpret?

Mara z. vitolins.

1 Department of Epidemiology and Prevention, Wake Forest School of Medicine, Winston-Salem, NC

Talsi L. Case

2 Nutrition and Foods Bachelor of Science Dietetics Program [student], Appalachian State University, Boone, NC

Conducting and interpreting nutrition research involves consideration of the research question, study design, wide variability of nutrients in foods and dietary patterns, food environment, approaches used to collect and analyze dietary data, and manner in which results are reported. This article reviews all of these considerations with regard to diabetes-related nutrition research.

Nutrition researchers are trained to examine the complex interplay between foods eaten and health and disease in individuals or populations. Given the huge potential impact of diet on a person’s health and the fact that everyone eats, it is no surprise that studies in human nutrition are crucially important. However, nutrition research has its challenges. To fully appreciate the many challenges surrounding nutrition research, it is important to understand some of the key elements involved, including research designs, the complexity of the food environment, and approaches to collecting and analyzing dietary data.

Nutrition research conducted to examine the role of diet in managing diabetes is particularly challenging because it involves assessing and intervening in patients’ lifestyle and habits, which are influenced by human nature; a food environment that is ever-changing; and data that are largely self-reported. Another challenge is how nutrition research results are described; unfortunately, nutrition research results are often simplified and misinterpreted. Entire books have described in great detail the fundamental approaches to and challenges of conducting nutrition research ( 1 , 2 ). Such a detailed discussion is beyond the scope of this article, which rather aims to create awareness of a handful of these issues.

Research Study Designs Are Important

When designing a nutrition research study, it is crucial to determine the best study design to answer the research question ( 1 , 2 ). Several research designs are commonly used to assess the impact of a dietary intervention on health and disease. Some of these include tightly controlled feeding studies, randomized clinical trials, and observational studies. Each design has advantages and disadvantages.

A feeding trial involves providing study participants with all meals, snacks, and beverages to control what they consume. Such trials provide the most control over participants’ dietary intake. Provided meals are carefully prepared, ingredients are weighed and measured, and participants are instructed to eat all the food they are provided (or to bring back what they did not eat). As an example, Bell et al. ( 3 , 4 ) used single-meal feeding trials to examine insulin dosing for protein and fat intake in participants with type 1 diabetes.

Feeding trials are expensive, and special facilities are needed to prepare and provide food for the participants. Feeding trials are usually short-term, not only because of the costs involved, but also because participants have to greatly modify their lives to adhere to eating only the foods and beverages provided by the trial. These trials usually enroll people who are quite different from the general public. For example, if a nutrition researcher wanted to study the impact of different amounts of dietary fiber on A1C levels, and wanted to include women with prediabetes who are 30–40 years of age, recruitment could prove to be difficult, and retaining participants could be even more challenging. The participants would need to be in the trial for at least 3 months to get meaningful changes in A1C; however, women in this age-group are likely to be married, have children, and be employed. To participate in such a trial, they would have to be willing to avoid eating any of the food prepared for the rest of their family, they would not be able share their trial-provided food with anyone else, and they would need to visit the research unit nearly every day to pick up meals.

The burden on participants enrolled in feeding trials is great, and the risk for nonadherence increased as the trial goes on. If participants do not strictly adhere to eating the foods they are provided, the findings of the trial can be seriously affected. Yet, it is difficult to precisely determine participants’ degree of adherence because they might eat foods other than those they were provided, share their foods with others, or not follow the protocol requiring them to bring back any foods they do not eat.

Feeding trials are not appropriate for answering questions about longer-term dietary exposures. Many diseases develop over time, so fully examining whether a particular dietary pattern affects health requires a long follow-up period and a large sample size. Randomized controlled intervention trials have been conducted to examine the effects of long-term dietary modification ( 5 – 7 ). Although these trials are not feeding trials, they still require a large budget and come with their own complexities. Research teams who have been able to conduct long-term dietary modification trials have relied on many different research methods and tools. Having close, ongoing contact with participants, carefully monitoring their adherence, and asking them to self-monitor what they eat are just a few of the key components of such trials.

These trials are labor-intensive for both researchers and participants, and in most cases, adherence has been shown to decrease after several months. Participant adherence to the intervention (and the control condition) is essential, but not always realized. Although nonadherence to protocol-specified behaviors is not a problem specific to nutrition research, the act of selecting, preparing, and eating foods is complex and repeated multiple times each day, providing many opportunities for nonadherence to the diet modification specified by the trial.

The Diabetes Prevention Program (DPP) trial is a good example of a large, randomized controlled intervention trial ( 8 ). Registered dietitians instructed participants in the intervention group to follow a healthful eating pattern, but meals were not provided. Participants had to learn approaches to reducing their fat and calorie intake to achieve a weight loss goal of 7% of baseline body weight and then to maintain their weight loss ( 8 ). They were required to self-monitor their behaviors, and dietitians reviewed this documentation regularly and provided constructive feedback. Participants attended weekly sessions during the first 24 weeks of the study. However, during the maintenance phase, contacts with the study team decreased to about twice per month. DPP participants had to remain adherent over a long period of time, so when contact frequency decreased, the research team had to work even more diligently to assist participants in meeting their study goals. If they had not done so, it would have been impossible to test the study hypothesis that lifestyle intervention would delay or prevent the development of type 2 diabetes in people with impaired glucose tolerance who were at high risk for developing the disease.

Observational studies have been used to track dietary intake in large numbers of participants and can be used to track such data over many years. Participants do not have to change their dietary behaviors to participate in an observational study. These studies are not as expensive to conduct, and the burden on participants and investigators is much lower.

Observational studies are not carefully controlled like clinical trials, so their results may be less reliable. Observational studies compare individuals who self-select to consume either a healthful or unhealthful diet, and the two groups may differ in other characteristics that could influence health ( 9 , 10 ). Nonetheless, such studies are vitally important for identifying possible connections between diet and health that can be further tested in randomized clinical trials.

Food Is Complicated

Nutrition researchers must be aware of and account for numerous food-related issues when collecting and analyzing dietary data. A few such issues are described below.

Foods and food products are available in different varieties, brands, and flavors. There are many options of the same type of food, yet the ingredients in each option may differ in ways that matter greatly to nutrition researchers. One brand of microwavable popcorn may contain butter, whereas another may contain buttery flavoring (butter-flavored coconut oil).

How food is cooked can change its nutrient profile. If vegetables are eaten fresh, many of their nutrients are preserved; however, if vegetables are boiled, many water-soluble nutrients can be lost. Many nutrients, including vitamin C, thiamin, and folic acid, are sensitive to temperature ( 11 ).

Where a food is prepared can make a substantial difference in the nutrient content because the ingredients used in their preparation can vary widely. Home-cooked foods versus restaurant-prepared foods can greatly differ in their nutrient composition.

Seasonal variations can make a difference in the nutrient content of fruits and vegetables. Also, those purchased directly from local farms have different levels of nutrients than those picked before they have ripened and been transported many miles to grocery stores.

The food environment is ever-changing, and food ingredients will continue to change, although consumers may not notice these changes at all. For example, until relatively recently, synthetic sources of trans fats were ubiquitous in bakery products as an emulsifier to improve creaming and reduce the rancidity of frying oil ( 12 ). Now, artificial trans fats are no longer contained in bakery products, or they have been greatly reduced ( 13 ). If a researcher wanted to examine the intake of cookies over time on health outcomes, using data overlapping this important change in the trans fat content of the cookies would greatly complicate this evaluation.

People Are Also Complicated

People vary in many ways, including by sex, race/ethnicity, BMI, economic status, metabolic rate, food preferences, exercise patterns, and fitness levels, among others. All of these differences could affect what study participants eat, how they metabolize what they eat, and how much they remember about what they eat.

The ability to assess dietary intake to explore diet-disease connections is essential to nutrition research but remains problematic for a number of reasons. Dietary data are typically obtained from participants who self-report what they eat ( 1 , 14 ). Common approaches used to collect dietary data include food records, food frequency questionnaires, and 24-hour recalls ( 1 , 14 ). To complete food records, participants are asked to carefully describe everything they eat or drink over a period of a few days, typically 3–4 days. Food records are burdensome to complete because participants must weigh and measure all of their foods and beverages and write all of this information down. To complete food frequency questionnaires, participants are asked to reflect on what they have typically eaten during the past 6 or 12 months, and it is known that participant recall is fallible. The 24-hour recall method collects dietary intake over a much shorter period of time, but participants can misreport what they eat.

Such misreporting can vary by participants’ personal characteristics. For example, a number of studies have shown that women and individuals who are heavier tend to underreport foods consumed ( 15 – 18 ). Underreporting of food intake by participants may reflect social desirability (i.e., not wanting to be judged by what they eat) ( 16 , 17 ). Underreporting may also reflect an inability to accurately estimate how much they actually consume. Extensive research has been conducted to identify approaches to mathematically adjust for misreported intake. Unfortunately, these corrections are not a perfect solution to fully address the problem of misreported food intake. Biomarkers have been studied to assist in the assessment of foods consumed. However, they carry with them a number of different, but still complicating, issues, including individual differences in nutrient absorption ( 19 ).

Food and Nutrition Databases Have Limitations

Food and nutrition databases are used to convert a consumed food to the specific nutrients contained in the food ( 20 ). These databases contain information about foods’ sources, profiles, nutrients, and dietary components.

It is not hard to imagine the complexity and difficulty involved in attempting to capture all of the different types and varieties of foods available to the public in one database. Each nutrient contained in each food is quantified. Significant effort goes into annually updating the content of food and nutrient databases as new foods are introduced to the marketplace almost daily. The databases are enormous because the intention is usually to include as many foods (and their variations) as possible. For example, there are more than 40 chicken soup options from which to choose in the U.S. Department of Agriculture/Agricultural Research Service Food and Nutrient Database for Dietary Studies ( 21 ).

Food and nutrient databases cannot include every component of every food. For example, nutrients are not the only components contained in foods, and the nonnutritive components in foods are usually not included in these databases. Vegetables and fruits contain many nonnutritive compounds, including dithiolthiones, monoterpenes, indoles sterols, and sulfhydryls. Although these components are not nutritional, they are in the foods research participants eat, and they may be important to health.

It is also known that food packaging may have an impact on health. Bisphenol A (BPA) is a good example of a nonnutritive but important exposure that was linked to food packaging. BPA is considered an endocrine disruptor, and the primary source of exposure to BPA is through the diet ( 22 ). BPA has been known to leach into food from internal epoxy resin coatings on cans used for canning foods such as soups and vegetables. BPA is not included in food and nutrition databases.

Reporting of Nutrition Research Findings Can Be Confusing

Once a research study is published, its results are likely to be picked up by the lay press. Such reporting is where a good deal of nutrition confusion is created. Nutrition is a topic of great interest to people, particularly those who have been diagnosed with diabetes, who often inquire about which diet is best to help them achieve target glucose levels or to lose or maintain body weight. Where they get answers to such questions matters; unfortunately, most people learn about nutrition research findings in the popular press or rely on the Internet for nutrition information. Although a wealth of nutrition information is available, some of it may be unreliable.

The online newsletter “Obesity and Energetics Offerings,” from the Indiana School of Public Health in Bloomington and the University of Alabama–Birmingham, provides some excellent examples of headlines that have failed to accurately describe the results of nutrition studies. The newsletter’s “Headline vs. Study” section on 11 October 2019 ( 23 ), for example, provided a link to an article in Women’s Health magazine titled, “Snacking on Nuts Found to Help Prevent Weight Gain” ( 24 ), followed by a link to the actual research report on which the article was based. The study was not a randomized controlled trial, the nuts were not provided to participants, and the participants self-reported both their nut intake and their weight. Although consumers reading the magazine headline might have thought that snacking on nuts would help them control their weight, all the research really found was an association—not necessarily causal—between self-reported nut intake and self-reported weight ( 25 ).

Similarly, data from the Women’s Health Initiative study cohort were used to explore links between chocolate consumption and the risk of diabetes ( 26 ). The authors cautiously presented their finding of an inverse relationship between chocolate intake and incident diabetes at moderate levels of chocolate consumption. However, press reports about the study simply suggested that eating chocolate would stave off diabetes ( 27 ).

Because patients may not be able to interpret the results of nutrition-related studies they see reported in the news, health care providers should be prepared to discuss these issues when patients inquire. Most Americans likely do not understand the nuances of such research or how links between foods or diets and chronic disease outcomes are stronger when supported by consistent evidence from basic science research, feeding studies, observational cohort studies, and randomized controlled clinical trials. People do not always realize that findings from one study cannot stand alone and that results from multiple studies are usually needed to create the robust evidence necessary for nutrition recommendations. Health care providers can help patients better understand nutrition research findings reported in the popular media by asking them to think about how many times they have read or heard about so-called “miracle foods.”

Importantly, when patients ask questions about nutrition-related news articles, they are opening the door to discussing their own dietary habits. Health care providers might consider providing simple suggestions for improving dietary intake (e.g., drink water instead of sugary beverages or eat vegetables at every meal), referral to a dietitian, and information about Internet sites that support evidence-based dietary recommendations.

To address conflicting data and nutrition recommendations, the American Diabetes Association (ADA) routinely reviews diabetes-related nutrition research and determines whether the evidence is strong enough to issue guidelines or recommend changes to the standards of care. The recently published, “Nutrition Therapy for Adults With Diabetes or Prediabetes: A Consensus Report” ( 28 ), for example, summarized the evidence supporting a new recommendation acknowledging that several different dietary patterns are acceptable for people who have prediabetes.

Everyone eats, making diet one of the most common exposures possibly influencing health and disease outcomes. Nutrition research is complicated by numerous factors inherent in this area of inquiry. Inaccurate reporting in the lay press can further complicate things and create confusion in the public. The ADA reviews data and provides consensus reports and clinical practice guidelines to reduce the confusion. Studies such as the DPP illustrate that nutrition research has enormous potential for preventing and improving the management of diabetes and improving overall health. Nutrition, with its myriad health implications, is an exciting, challenging, and ever-evolving area of research.

Article Information

Duality of interest.

No potential conflicts of interest relevant to this article were reported.

Author Contributions

M.Z.V. researched data and wrote the manuscript. T.L.C. researched data and reviewed and edited the manuscript. M.Z.V. is the guarantor of this work and, as such, had full access to all the data and takes responsibility for the integrity of the information presented.

• understanding science

The levels of evidence in nutrition research

Nutrition and health are inextricably linked. Nutrition researchers try to unravel these connections in order to arrive at reliable nutritional advice. However, not all types of research can be used to draw equally firm conclusions. Understanding the different types of study designs is important for distinguishing between reliable and less robust findings. This article explores the various study designs commonly used in nutrition research, their purpose, how strong their evidence is and discusses the strengths and limitations of each design.

Systematic reviews and meta-analysis

A single study is not enough to make a general statement with certainty about a certain link between nutrition and health. That is where systematic reviews and meta-analysis come in: in this type of research, researchers gather all relevant studies on a particular topic and analyse these collectively. As a result, the risk between a certain exposure/factor (e.g., overweight) and outcome/disease (e.g., cancer) can be estimated reasonably well. 1

In a meta-analysis, results of multiple studies are pooled, following a rigorous protocol to find all the original research studies done on a question, and weighted with statistical methods into a single summary estimate. Large and well-conducted studies with high-quality evidence are given more ‘weight’ than small or poorly conducted studies with low quality data. A meta-analysis can only be carried out if the studies look at the same research question and use similar methods to measure relevant variables.

Systematic reviews are similar to meta-analysis, but without the use of a statistical analysis. Although systematic reviews and meta-analysis can reduce bias by pooling data from all relevant studies investigating a particular topic, they are only as good as the studies they include. It is important to check whether data from flawed studies are included or if there are data from studies that use different methods to measure variables – resulting in a comparison of ‘apples and oranges.’ To reduce the risk of bias and improve reporting and transparency, using a set of guidelines called the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) is endorsed by most of the high-quality scientific journals. 2

Randomised controlled trials (RCTs)

A randomised controlled trial (RCT) is a type of intervention study where the researcher actively intervenes to change any aspect of nutrition to see what the effect this has on a certain health outcome.

In an RCT, a group of human subjects with same condition are identified and then randomly allocated to either receive the treatment (e.g., an omega-3 fatty acid supplement) or to a control group that does not receive the treatment (e.g., a ‘placebo’ supplement that looks identical to the original supplement but does not contain the substance being studied). After a defined time period, the effects in both groups are measured and compared to each other. As only one factor is deliberately changed between the groups (and other possible factors that influence the relationship are kept the same or as similar as possible), this type of research can allow us to identify cause-and-effect relationships. 1 These types of study are also used in medical research, for example to  test the effects of new drugs or vaccines.

Preferably, RCTs are performed as a double-blind study: both the researcher and the participants are unaware who is in which group. This is important because a participant’s response or a researcher’s measurement of the outcome could be impacted by knowing who is being treated. For example, the placebo effect is a well-known phenomenon where a person observes an improvement in symptoms or effects in themselves after taking a fake or non-active ‘treatment’ (e.g., pills that don’t contain any active ingredient). On top of that, true randomisation is important. If one group were in some way more ill (or less healthy) than the other group at the start, this might make this group appear to have worse outcomes, even if the administered treatment really had no effect.

RCTs also have limitations. They may not be suitable for answering certain research questions, such the effect of whole diets (e.g., ketogenic, vegan) on the prevention of chronic diseases like cancer or cardiovascular disease. It would be impossible to control compliance (e.g., how strictly the study participants stick to the prescribed diet), a huge number of subjects would be required to show a significant difference in outcome, and it would cost a huge amount of money and time. Furthermore, interventions which deliberately expose participants to something thought to be harmful (e.g., alcohol, smoking, contaminants) or withholding participants from treatment thought to improve health (e.g., certain antibiotics or chemotherapeutic agents) in order to definitely prove a cause-and-effect relationship raise ethical concerns. Time is also an important factor. The more robust and reliable studies are those lasting longer.

It is also important not to generalise results of RCTs too quickly. RCTs often have strict criteria of whom to include and exclude. If a study was only carried out on a specific group of people (e.g., middle-aged women with diabetes), the study may not be applicable to the wider population. Lastly, RCTs may often run for short time periods since they are expensive to carry out. So, they may not be able to tell us about the long-term effects of dietary patterns and changes.

Observational research

Observational research involves simply observing the habits or behaviours in large groups of people to investigate the relationship between lifestyle factors and health outcomes. The researcher does not intervene in any way but compares the health outcomes of people who make different diet or lifestyle choices. These studies are used to identify correlations and develop hypotheses for further testing. 1

For example, researchers observe that people who drink alcohol are more likely to develop lung cancer than those who don’t. However, it may also be that people who drink alcohol also tend to smoke more often and that researchers fail to include this factor into their analysis (e.g., it was not measured or not thought to influence the relationship). Here, smoking is a so-called confounding factor: a factor associated with both the exposure (drinking alcohol) as well as the outcome (lung cancer) and therefore could distort the data. In an RCTs these confounding factors are evenly distributed across the study groups (assuming randomisation is done correctly), but this is unlikely to be the case in observational research. Because of the presence of confounding factors, observational research cannot prove cause-and-effect. The interest of observational studies lies in revealing these relationships in order to address future research.

The next sections describe three common observational study designs: prospective cohort studies, case-control studies, and cross-sectional studies.

Prospective cohort studies

A prospective cohort study is a study that follows a group of people over time. At the start of the study, researchers ask participants to complete a questionnaire (e.g., about their dietary habits, physical activity levels, etc.) and may also take measurements of weight, height, blood pressure, blood profile or other biological factors. Years later, the researchers look whether the participants have developed disease and study whether exposure from the questionnaire or biological measurements are associated with the disease. 1

One of the main advantages of this design is that researchers observe participants’ natural exposures and behaviours without intervention, providing insights into real-life scenarios. It also allows researchers to examine the long-term effects of nutrition or other lifestyle-related exposures on real diseases. Since chronic diseases, such as heart disease and osteoporosis, often take many decades to develop, a cohort may be more suitable compared to a RCT where often intermediate markers for these diseases are measured (e.g., narrowing of the arteries or bone density). These markers don’t always develop into the disease.

One of the main limitations of cohort studies is that they cannot establish causation definitively due to potential confounding factors. It is also important to consider how participants’ food intake is measured. Cohorts often use what’s called a Food Frequency Questionnaires (FFQs) which measures a person’s average dietary intake over time. While FFQs are one of the best methods available to assess dietary intake, it can be hard to accurately estimate typical intake, portion size, and preparation methods. As data is self-reported, it may include subjective interpretations (e.g., participants may underreport, overreport or simply forgot their past habits: a problem called recall bias). FFQs also don’t represent lifetime behaviour patterns: people may have changed their behaviour over the intervening years (e.g., smokers may quit smoking or meat eaters may become vegetarian), resulting in misclassification of participants and potential bias. When a validated FFQ is used, generally a part of bias is limited.

In a cohort study it is also important that participants are followed up for a long time to accumulate enough data to give robust results. This means they can generally only be used to study diseases that are relatively common. Another concern in cohort studies is selection bias: those selected to be in the study differ from those not selected in some systematic way. Recruitment of participants may be done, for example, through newspapers, phone dialling, the workplace or volunteering, impacting who takes part in the study and how generalisable the results are (i.e., newspapers are often only read by older populations, phone dialling excludes those without phones, volunteering recruits more health-conscious participants, etc.). Another concern occurs if many participants are ‘lost to follow-up’ (i.e., drop out of the study) in one exposure group than another, particularly if loss is also related to the outcome being studied.

Case-control studies

In a case-control study researchers look to the past of people with a disease (called the ‘cases’) and compare it to people without the disease (the ‘controls’). These studies are most often used to study the link between an exposure and a rare outcome. 1 They usually have a smaller sample size than cohort studies and do not require follow-up.

As in cohort studies, recall bias is a problem in case-control studies. This is to even larger extent as people already have the disease of interest when the exposure information is collected or measured and so they might recall their exposure differently from people without disease. Selection bias (cases and/or controls may not be representative for the general population), confounding, and reverse causation can be limitations. Reverse causation occurs when it is challenging to identify if the outcome or exposure came first. For example, if an association was found between the consumption of non-sugar sweeteners and obesity, is this truly because of the higher consumption of non-sugar sweeteners or is it that people with obesity more frequently consumed products containing non-sugar sweeteners to help manage their weight? The choice of an appropriate control group is also one of the main difficulties of this type of study. Control groups should be carefully selected to be similar to the cases (those with the condition of interest) in all aspects except the exposure being studied (e.g., diet). This ensures that any observed difference in outcomes can be attributed to the exposure rather than confounding factors.

Cross-sectional studies

A cross-sectional study is a survey or cross-section of a random sample of the population where information about potential exposures and outcomes is collected at the same time. For example, researchers measure blood pressure and ask questions about, for example, the amount of processed meat each person eats per day. This lets them find out whether there is a link between blood pressure and the amount of processed meat consumed per day. 1

With cross-sectional studies reverse causality is again a problem: you cannot be sure whether eating processed meat affects blood pressure or vice versa, because the information was obtained at the same time. Like cohort studies, they can also be prone to selection and recall bias. Recall bias may be a particular problem, since participant’s knowledge of their health status may influence their reporting of dietary habits (e.g., a person with type 2 diabetes may recall they ate more sweets and sodas than a person without the disease).

Instead, this simple study design can be useful to investigate the possible causes of ill-health at an early stage, examine exposure that do not change over time (e.g., sex, genetic factors) or that occurred many years previously, or estimate the prevalence of dietary habits and health outcomes in a population at a specific time point. They can provide a starting point for further investigation about associations between dietary factors and health outcomes in, for example, a cohort study or RCT.

Animal and cell studies

Animal and cell (or sometimes called in vitro) studies may provide an indication of the likely effect, however, they cannot be directly applied to humans. Research with animals is an important tool in determining how humans may react when exposed to particular substances. However, because of differences in physiology and the fact that animals are routinely exposed to far higher levels of compounds than typical dietary intakes, for example, results cannot be directly applied to humans. 3 Similarly, isolated cells in a laboratory behave differently than cells in our body. For example, if a test tube shows that substance X causes a cell to burn fat faster, that does not mean that substance X will help to lose weight in humans. The human body is much more complex than can be imitated in a test tube.

For research into toxic substances, this type of research is the norm. Testing harmful or possibly toxic compounds on humans is dangerous and unethical. Animal testing is therefore used to establish safety guidelines for chemical compounds such as pesticides and environmental contaminants. Because results cannot be extended to humans and people also differ from each other, wide safety margins are used. However, the use of laboratory animals is being substantially reduced following international protocols such as those by the Organization for Economic Cooperation and Development (OECD). 4

Animal and cell research can complement evidence from observational and experimental research: they can show if there is a mechanism that explains these results. For example, observational research shows that smoking is associated with cancer, while cell studies highlight the specific harmful substances present in tobacco that contributes to the development of cancer. The certainty that a result is accurate increases when there is such a logical explanation.

Anecdotes and case studies

Anecdotes, case reports (on 1 patient) and case series (on several patients) describe a detailed report of individual patient(s) with a specific outcome and/or exposure. 5 They are important for the early identification of health problems and can generate hypotheses about potential causes. However, since they involve a limited number of people, they cannot be generalised to broader populations. A single person’s experience or opinion does not provide an objective picture. Therefore, anecdotes and case studies are regarded as low-quality evidence.

What is considered the ‘best’ evidence?

Generally, the different types of research are organised from single person’s experiences and anecdotes having the weakest certainty of evidence to systematic reviews and meta-analysis having the strongest level of evidence. These levels of evidence can be used as a guideline to judge what can be concluded from a particular study. However, they do not substitute for critical appraisal. 6 For example, a strong cohort study may be more useful than a flawed systematic review. Besides, which type of research scientists choose to conduct depends on, among other things, the research question, the amount of time available and the amount of money. Therefore, evidence may be better sorted by its usefulness for investigating a specific research question than by type of study design.

Different types of study designs should be viewed as complementary. For example, observational research can still be meaningful and illuminating when numerous studies consistently show patterns on a large scale.

RCTs are often regarded as the ‘gold standard’ for conducting research and their findings are believed to be more accurate compared to observational research as they can establish cause-and-effect relationships. However, this assumption is not always valid because the intervention/exposure being studied in RCTs may differ from those in observational studies.6 For example, dietary intakes in observational studies are not interchangeable to some exposures used in RCTs (e.g., intake of omega-3 fatty acids by eating fish is different to omega-3 fatty acids consumed in isolated supplemental form). As a result, it is not surprising that sometimes contradictory results are found between observational and experimental research. 7 When testing findings from observational research further in RCTs, it is therefore important to carefully consider the population being studied, the way the intervention (dietary change) is applied, the comparison group, and the outcome(s) measured. Even small differences in how the study is conducted can lead to varying results.

Nutrition research is expensive and complex to develop. It is therefore difficult to reach to reliable results that support evidence. A single approach is not sufficient. There are a variety of study designs used in nutrition research which are used to study a variety of different exposures and outcomes. How all these studies can lead to a conclusion depends on the certainty of evidence. A link between an exposure and outcome is more certain if: 8

• A large number of prospective cohort studies consistently show an association between exposure to A (cause) and the risk of B (effect);
• The examinations are of good methodological quality, size and duration;
• There are few studies that find the opposite;
• If feasible, experimental studies have also been carried out;
• The link found can be explained biologically.

In contrast, there is insufficient evidence if:

• There are only a small number of studies suggesting that there is a link between exposure to A (cause) and the risk of B (effect);
• The link found is weak;
• No or insufficient experimental and observational studies have been done and therefore more research is needed.

• Webb P, Bain C & Page A (2017) Essential epidemiology: an introduction for students and health professionals. Cambridge University Press.
• PRISMA. (2023). Transparent reporting of systematic reviews and meta-analyses. Retrieved from http://www.prisma-statement.org/?AspxAutoDetectCookieSupport=1 (Accessed 05/09/2023)
• Van der Worp HB et al. (2010) Can animal models of disease reliably inform human studies? PLoS Medicine, 7(3):e1000245.
• OECD. (2023). Animal Welfare. Retrieved from https://www.oecd.org/chemicalsafety/testing/animal-welfare.htm (Accessed 05/09/2023)
• Mathes T & Pieper D (2017) Clarifying the distinction between case series and cohort studies in systematic reviews of comparative studies: potential impact on body of evidence and workload. BMC medical research methodology, 17:1-6.
• Flanagan A et al. (2023). Need for a nutrition-specific scientific paradigm for research quality improvement. BMJ Nutrition, Prevention & Health e000650
• Schwingshackl L et al. (2021) Evaluating agreement between bodies of evidence from randomised controlled trials and cohort studies in nutrition research: meta-epidemiological study. British Medical Journal 374:n1864.
• World Health Organization. (2014). WHO Handbook for Guideline Development. 2nd edition. Retrieved from https://www.who.int/publications/i/item/9789241548960 (Accessed 02/08/2023)

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Study Design in Nutrition Research

Course details, course description.

Successful interventions, in research or for programs, rely on intentional design that begins with a hypothesis that can be developed into a conceptual model and translated into an intervention. This course describes this process, from conception, through design, to execution and implementation. Students are guided through generating hypotheses and introduced to specific principles of designing feasible studies—including intervention and observational studies—that address these hypotheses. Students will learn how having a critical understanding of research-based approaches can inform programmatic intervention and evaluation. Guest lectures will present real-world examples that illustrate this process. Students will gain experience in identifying appropriate funding sources and developing proposals that meet the interests and missions of potential funders. Students will also present their proposals, and review and critique the work of their classmates.  Enrollment limited to 12 students with priority given to NICBC degree program students.

Not offered since Spring 2019.

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NUTR 0207 or equivalent, NUTR 0204 or equivalent, NUTR 0202 or equivalent, and familiarity with basic methods of dietary assessment

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NUTR202 Principles of Nutrition Science

• Study Designs

Introduction

• Keeping Track of Your Search Results
• Poster Design

This section will introduce you to the following study designs frequently used in the health sciences. Each box in this section describes a particular study design and will include a definition of the design and why it is used. For Information on how to retrieve specific study designs on PubMed, please see  Retrieving Articles by Study Design in PubMed .

• Case-Control

Clinical Trials

Randomized controlled trial (rct).

• Systematic reviews/Meta-Analysis

What is a clinical trial?

“[A] clinical trial is any research study that prospectively assigns human participants or groups of humans to one or more health-related interventions to evaluate the effects on health outcomes. Clinical trials may also be referred to as interventional trials. Interventions include but are not restricted to drugs, cells and other biological products, surgical procedures, radiologic procedures, devices, behavioural treatments, process-of-care changes, preventive care, etc. This definition includes Phase I to Phase IV trials.”[1]  

Trial Phases  [1] :

I.  Clinical trials test a new biomedical intervention in a small group of people (e.g., 20-80) for the first time to evaluate safety (e.g., to determine a safe dosage range and to identify side effects).

II. Clinical trials study the biomedical or behavioral intervention in a larger group of people (several hundred) to determine efficacy and to further evaluate its safety.

III. Studies investigate the efficacy of the biomedical or behavioral intervention in large groups of human subjects (from several hundred to several thousand) by comparing the intervention to other standard or experimental interventions as well as to monitor adverse effects, and to collect information that will allow the intervention to be used safely.

IV. Studies are conducted after the intervention has been marketed. These studies are designed to monitor effectiveness of the approved intervention in the general population and to collect information about any adverse effects associated with widespread use.

Why use this study type?

• Evaluating one or more interventions (for example, drugs, medical devices, approaches to surgery or radiation therapy) for treating a disease, syndrome, or condition. [2]
• Finding ways to prevent the initial development or recurrence of a disease or condition. These can include medicines, vaccines, or lifestyle changes, among other approaches. [2]
• Evaluating one or more interventions aimed at identifying or diagnosing a particular disease or condition.[2]
• Examining methods for identifying a condition or risk factors for that condition.[2]
• Exploring and measuring ways to improve the comfort and quality of life of people with a chronic illness through supportive care.[2]

Format and features

• University of California San Francisco (UCSF) Clinical Trial Protocol Development [3]
• Transparent Reporting of Evaluations with Nonrandomized Designs (TREND) Checklist [4]
• For randomized controlled trials, see CONSORT 2010 checklist of information to include when reporting a randomised trial [5]
• WHO International Clinical Trials Registry Platform Glossary
• ClinicalTrials.gov
• UCSF Clinical trial Protocol Development
• CDC TREND Statement Checklist
• The CONSORT Group

Cohort Study

What is a cohort study.

"A cohort study tracks two or more groups forward from exposure to outcome. This type of study can be done by going ahead in time from the present (prospective cohort study) or, alternatively, by going back in time to comprise the cohorts and following them up to the present (retrospective cohort study)" [1]

Why use this type of study?

• To identify incidence and natural history of a disease [1] 
• To examine multiple outcomes after a single exposure [1] 
• As a substitution for  an experiment when experimentation is not available [2]
• Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Statement checklist for cohort studies  [3]
• Grimes DA, Schulz KF. Cohort studies: marching towards outcomes. The Lancet. 2002;359(9303):341-345.
• Chapter 5: Risk: Looking forward. In: Fletcher RH, Fletcher SW, eds. Clinical epidemiology : the essentials. 4th ed. Baltimore: Lippincott Williams & Wilkins; 2005: p. 85.
• Strengthening the Reporting of Observational Studies in Epidemiology (STROBE). STROBE checklist for cohort studies. [2007]; http://www.strobe-statement.org/fileadmin/Strobe/uploads/checklists/STROBE_checklist_v4_cohort.pdf . Accessed March 1, 2013.

What is a randomized controlled trial (RCT)?

“A study design that randomly assigns participants into an experimental group or a control group. As the study is conducted, the only expected difference between the control and experimental groups in a randomized controlled trial (RCT) is the outcome variable being studied.”[1]

• To reduces bias   [2]
• To approximate a controlled experiment.[2]
• Statistically efficient   [2]

Format and features

•   CONSORT 2010 checklist of information to include when reporting a randomised trial   [3]
• Himmelfarb Health Sciences Library (The George Washington University). Study Design 101: Randomized Controlled Trial. 2011; http://www.gwumc.edu/library/tutorials/studydesign101/rcts.html. Accessed January 30, 2012.
• Grimes DA, Schulz KF. An overview of clinical research: the lay of the land. The Lancet. 1/5/ 2002;359(9300):57-61.
• The CONSORT Group. CONSORT 2010 checklist of information to include when reporting a randomised trial. 2010; http://www.consort-statement.org/index.aspx?o=2967 . Accessed January 31, 2013.

Systematic Review and Meta-Analysis

What is a systematic review/meta-analysis.

 “A systematic review is a review of a clearly formulated question that uses systematic and explicit methods to identify, select, and critically appraise relevant research, and to collect and analyze data from the studies that are included in the review. Statistical methods (meta-analysis) may or may not be used to analyze and summarize the results of the included studies. [1]

 “Meta-analysis refers to the use of statistical techniques in a systematic review to integrate the results of included studies.” [1]

•   “Systematic reviews establish whether scientific findings are consistent and can be generalized across populations, settings, and treatment variations, or whether findings vary significantly by particular subsets.”   [2]
•   “ Meta-analyses in particular can increase power and precision of estimates of treatment effects and exposure risks.” [2]
• “Explicit methods used in systematic reviews limit bias…[which improves] reliability and accuracy of conclusions.”[2]
• PRISMA checklist pertaining “to the content of a systematic review and meta-analysis”   [3]
• The PRISMA Group. The PRISMA Statement. http://www.prisma-statement.org/statement.htm. Accessed January 31, 2013.
• Mulrow CD. Systematic Reviews: Rationale for systematic reviews. BMJ. 1994-09-03 08:00:00 1994;309(6954):597-599.
• The PRISMA Group. PRISMA 2009 Checklist. http://www.prisma-statement.org/2.1.2%20-%20PRISMA%202009%20Checklist.pdf. Accessed January 31, 2013.

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Methods in Nutrition Research

• First Online: 01 January 2012

Cite this chapter

• David R. Jacobs Jr. PhD 4 &
• Norman J. Temple PhD 5  

Part of the book series: Nutrition and Health ((NH))

1 Citations

Excellent research designs are essential for efficiently and accurately answering nutrition-related questions, the answers to which in turn are used in policy making. Such policy would include advice from the perspective of health about what foods people should eat, what foods industry should supply, and whether some compounds should be consumed as dietary supplements. Areas of inquiry range from molecular mechanisms in cells, animals, and humans; to descriptions of what population groups eat; to effects of food and nutrient intake on clinical outcomes and intermediate variables; and to behaviors, policies, and cost structures that influence food preparation and consumption.

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Critical Evaluation of Nutrition Research

Errors in the implementation, analysis, and reporting of randomization within obesity and nutrition research: a guide to their avoidance

Willett W. Nutritional epidemiology. New York: Oxford University Press; 1998.

Book   Google Scholar  

Boushey C, Harris J, Bruemmer B, Archer S, Van Horn L. Publishing nutrition research: a review of study design, statistical analysis, and other key elements of manuscript preparation, Part 1. J Am Diet Assoc. 2006;106:89–96.

Article   PubMed   Google Scholar  

Boushey C, Harris J, Bruemmer B, Archer S. Publishing nutrition research: a review of sampling, sample size, statistical analysis, and other key elements of manuscript preparation, Part 4. J Am Diet Assoc. 2008;108:679–88.

Harris J, Boushey C, Bruemmer B, Archer S. Publishing nutrition research: a review of nonparametric methods, Part 3. J Am Diet Assoc. 2008;108:1488–96.

Harris J, Gleason P, Sheean P, Boushey C, Beto J, Bruemmer B. An introduction to qualitative research for food and nutrition professionals. J Am Diet Assoc. 2009;109:80–90.

Bruemmer B, Harris J, Gleason P, Boushey C, Sheean P, Van Horn L. Publishing nutrition research: a review of epidemiological methods. J Am Diet Assoc. 2009;109:1728–37.

Gleason PM, Harris JE, Sheean PM, Boushey CJ, Bruemmer B. Publishing nutrition research: validity, reliability, and diagnostic test assessment in nutrition-related research. J Am Diet Assoc. 2010;110:409–19.

Hill AB. The environment and disease: association or causation? Proc R Soc Med. 1965;58:295–300.

PubMed   CAS   Google Scholar  

Cutler GJ, Flood A, Hannan P, Neumark-Sztainer D. Major patterns of dietary intake in adolescents and their stability over time. J Nutr. 2009;139:323–8.

Prentice RL. Dietary assessment and the reliability of nutritional epidemiology research reports. J Natl Cancer Inst. 2010;102:583–5.

Neuhouser ML, Tinker L, Shaw PA, et al. Use of recovery biomarkers to calibrate nutrient consumption self-reports in the Women’s Health Initiative. Am J Epidemiol. 2008;167:1247–59.

Beaton GH, Milner J, McGuire V, Feather TE, Little JA. Source of variance in 24-hour dietary recall data: implications for nutrition study design and interpretation. Carbohydrate sources, vitamins, and minerals. Am J Clin Nutr. 1983;37:986–95.

Friedman GD, Cutter GR, Donahue RP, Hughes GH, Hulley SB, Jacobs Jr DR, et al. CARDIA: study design, recruitment, and some characteristics of the examined subjects. J Clin Epidemiol. 1988;41:1105–16.

Article   PubMed   CAS   Google Scholar  

Bild DE, Bluemke DA, Burke GL, et al. Multi-ethnic study of atherosclerosis: objectives and design. Am J Epidemiol. 2002;156:871–81.

Stringhini S, Sabia S, Shipley M, et al. Association of socioeconomic position with health behaviors and mortality. JAMA. 2010;303:1159–66.

Sinha R, Cross AJ, Graubard BI, Leitzmann MF, Schatzkin A. Meat intake and mortality: a prospective study of over half a million people. Arch Intern Med. 2009;169:562–71.

Willett WC. Fruits, vegetables, and cancer prevention: turmoil in the produce section. J Natl Cancer Inst. 2010;102:510–1.

Riboli E, Norat T. Epidemiologic evidence of the protective effect of fruit and vegetables on cancer risk. Am J Clin Nutr. 2003;78(3 Suppl):559S–69.

Boffetta P, Couto E, Wichmann J, et al. Fruit and vegetable intake and overall cancer risk in the European Prospective Investigation into Cancer and Nutrition (EPIC). J Natl Cancer Inst. 2010;102:529–37.

Hung HC, Joshipura KJ, Jiang R, et al. Fruit and vegetable intake and risk of major chronic disease. J Natl Cancer Inst. 2004;96:1577–84.

Siscovick DS, Weiss NS, Fletcher RH, Lasky T. The incidence of primary cardiac arrest during vigorous exercise. N Engl J Med. 1984;311:874–7.

Jacobs Jr DR, McGovern PG, Blackburn H. The US decline in stroke mortality: what does ecological analysis tell us? Am J Public Health. 1992;82:1596–9.

Armstrong B, Doll R. Environmental factors and cancer incidence and mortality in different countries, with special reference to dietary practices. Int J Cancer. 1975;15:617–31.

Giovannucci E, Liu Y, Rimm EB, et al. Prospective study of predictors of vitamin D status and cancer incidence and mortality in men. J Natl Cancer Inst. 2006;98:451–9.

Elliott P, Stamler J, Nichols R, et al., for the Intersalt Cooperative Research Group. Intersalt revisited: further analyses of 24 hour sodium excretion and blood pressure within and across populations. BMJ. 1996;312:1249–53.

Google Scholar  

De Wals P, Tairou F, Van Allen MI, et al. Reduction in neural-tube defects after folic acid fortification in Canada. N Engl J Med. 2007;357:135–42.

Taylor R, Morrell S, Mamoon H, Wain G, Ross J. Decline in cervical cancer incidence and mortality in New South Wales in relation to control activities (Australia). Cancer Causes Control. 2006;17:299–306.

Roberts S, Temple NJ. Medical research: a bettor’s guide. Am J Prev Med. 2002;23:231–2.

Temple NJ. Medical research: a complex problem. In: Temple NJ, Burkitt DB, editors. Western diseases: their dietary prevention and reversibility. Totowa, NJ: Humana Press; 1994. p. 419–36.

Temple NJ. Nutrition and disease: challenges of research design. Nutrition. 2002;18:343–7.

Kish L. Statistical design for research. Hoboken, NJ: Wiley; 2004.

Kromhout D, Geleijnse JM, Menotti A, Jacobs DR. The confusion about dietary fatty acids recommendations for CHD prevention. Br J Nutr. 2011;106:627–32.

Snitz BE, O’Meara ES, Carlson MC, et al. Ginkgo biloba for preventing cognitive decline in older adults: a randomized trial. JAMA. 2009;302:2663–70.

Seshadri N, Robinson K. Homocysteine, B vitamins, and coronary artery disease. Med Clin North Am. 2000;84:215–37.

Chao CL, Tsai HH, Lee CM, et al. The graded effect of hyperhomocysteinemia on the severity and extent of coronary atherosclerosis. Atherosclerosis. 1999;147:379–86.

Stubbs PJ, Al-Obaidi MK, Conroy RM, et al. Effect of plasma homocysteine concentration on early and late events in patients with acute coronary syndromes. Circulation. 2000;102:605–10.

Bazzano LA, Reynolds K, Holder KN, He J. Effect of folic acid supplementation on risk of cardiovascular diseases: a meta-analysis of randomized controlled trials. JAMA. 2006;296:2720–6.

Albert CM, Cook NR, Gaziano JM, et al. Effect of folic acid and B vitamins on risk of cardiovascular events and total mortality among women at high risk for cardiovascular disease: a randomized trial. JAMA. 2008;299:2027–36.

Armitage JM, Bowman L, Clarke RJ, et al. Study of the Effectiveness of Additional Reductions in Cholesterol and Homocysteine (SEARCH) Collaborative Group. Effects of homocysteine-lowering with folic acid plus vitamin B12 vs placebo on mortality and major morbidity in myocardial infarction survivors: a randomized trial. JAMA. 2010;303:2486–94.

Czeizel AE, Dudás I. Prevention of the first occurrence of neural-tube defects by periconceptional vitamin supplementation. N Engl J Med. 1992;327:1832–5.

He FJ, MacGregor GA. A comprehensive review on salt and health and current experience of worldwide salt reduction programmes. J Hum Hypertens. 2009;23:363–84.

Sacks FM, Svetkey LP, Vollmer WM, et al., for the DASH-Sodium Collaborative Research Group. Effects on blood pressure of reduced dietary sodium and the Dietary Approaches to Stop Hypertension (DASH) diet. DASH-Sodium Collaborative Research Group. N Engl J Med. 2001;344:3–10.

Knekt P, Ritz J, Pereira MA, et al. Antioxidant vitamins and coronary heart disease risk: a pooled analysis of 9 cohorts. Am J Clin Nutr. 2004;80:1508–20.

Princen HM, van Duyvenvoorde W, Buytenhek R, et al. Supplementation with low doses of vitamin E protects LDL from lipid peroxidation in men and women. Arterioscler Thromb Vasc Biol. 1995;15:325–33.

Mente A, de Koning L, Shannon HS, Anand SS. A systematic review of the evidence supporting a causal link between dietary factors and coronary heart disease. Arch Intern Med. 2009;169:659–69.

Temple NJ, Basu TK. Role of beta-carotene in the prevention of cancer—a review. Nutr Res. 1988;8:685–701.

Article   CAS   Google Scholar  

Panayiotidis M, Collins AR. Ex vivo assessment of lymphocyte antioxidant status using the comet assay. Free Radic Res. 1997;27:533–7.

Duthie SJ, Ma A, Ross MA, Collins AR. Antioxidant supplementation decreases oxidative DNA damage in human lymphocytes. Cancer Res. 1996;56:1291–5.

Druesne-Pecollo N, Latino-Martel P, Norat T, et al. Beta-carotene supplementation and cancer risk: a systematic review and metaanalysis of randomized controlled trials. Int J Cancer. 2010;127:172–84.

Hollman PC, Cassidy A, Comte B, et al. The biological relevance of direct antioxidant effects of polyphenols for cardiovascular health in humans is not established. J Nutr. 2011;141:989S–1009.

Fraser J. Conflict of interest: a major problem in medical research. In: Temple NJ, Thompson A, editors. Excessive medical spending: facing the challenge. Oxford: Radcliffe Publishing; 2007. p. 20–35.

Kassirer JP. On the take: how medicine’s complicity with big business can endanger your health. New York: Oxford University Press; 2005.

Lesser LI, Ebbeling CB, Goozner M, Wypij D, Ludwig DS. Relationship between funding source and conclusion among nutrition-related scientific articles. PLoS Med. 2007;4:e5.

Levine J, Gussow JD, Hastings D, Eccher A. Authors’ financial relationships with the food and beverage industry and their published positions on the fat substitute olestra. Am J Public Health. 2003;93:664–9.

Rowe S, Alexander N, Clydesdale FM, et al. Funding food science and nutrition research: financial conflicts and scientific integrity. J Nutr. 2009;139:1051–3.

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Jacobs, D.R., Temple, N.J. (2012). Methods in Nutrition Research. In: Temple, N., Wilson, T., Jacobs, Jr., D. (eds) Nutritional Health. Nutrition and Health. Humana Press, Totowa, NJ. https://doi.org/10.1007/978-1-61779-894-8_1

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1.3: Types of Research Studies and How To Interpret Them

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The field of nutrition is dynamic, and our understanding and practices are always evolving. Nutrition scientists are continuously conducting new research and publishing their findings in peer-reviewed journals. This adds to scientific knowledge, but it’s also of great interest to the public, so nutrition research often shows up in the news and other media sources. You might be interested in nutrition research to inform your own eating habits, or if you work in a health profession, so that you can give evidence-based advice to others. Making sense of science requires that you understand the types of research studies used and their limitations.

The Hierarchy of Nutrition Evidence

Researchers use many different types of study designs depending on the question they are trying to answer, as well as factors such as time, funding, and ethical considerations. The study design affects how we interpret the results and the strength of the evidence as it relates to real-life nutrition decisions. It can be helpful to think about the types of studies within a pyramid representing a hierarchy of evidence, where the studies at the bottom of the pyramid usually give us the weakest evidence with the least relevance to real-life nutrition decisions, and the studies at the top offer the strongest evidence, with the most relevance to real-life nutrition decisions .

Figure 2.1. Hierarchy of research design and levels of scientific evidence with the strongest studies at the top and the weakest at the bottom.

The pyramid also represents a few other general ideas. There tend to be more studies published using the methods at the bottom of the pyramid, because they require less time, money, and other resources. When researchers want to test a new hypothesis , they often start with the study designs at the bottom of the pyramid , such as in vitro, animal, or observational studies. Intervention studies are more expensive and resource-intensive, so there are fewer of these types of studies conducted. But they also give us higher quality evidence, so they’re an important next step if observational and non-human studies have shown promising results. Meta-analyses and systematic reviews combine the results of many studies already conducted, so they help researchers summarize scientific knowledge on a topic.

Non-Human Studies: In Vitro & Animal Studies

The simplest form of nutrition research is an in vitro study . In vitro means “within glass,” (although plastic is used more commonly today) and these experiments are conducted within flasks, dishes, plates, and test tubes. One common form of in vitro research is cell culture. This involves growing cells in flasks and dishes. In order for cells to grow, they need a nutrient source. For cell culture, the nutrient source is referred to as media. Media supplies nutrients to the cells in vitro similarly to how blood performs this function within the body. Most cells adhere to the bottom of the flask and are so small that a microscope is needed to see them. The cells are grown inside an incubator, which is a device that provides the optimal temperature, humidity, and carbon dioxide (CO2CO2) concentrations for cells and microorganisms. By imitating the body's temperature and CO2CO2 levels (37 degrees Celsius, 5% CO2CO2), the incubator allows cells to grow even though they are outside the body.

A limitation of in vitro research compared to in vivo research is that it typically does not take digestion or bioavailability into account. This means that the concentration used might not be physiologically possible (it might be much higher) and that digestion and metabolism of what is being provided to cells may not be taken into account. Cell-based in vitro research is not as complex of a biological system as animals or people that have tissues, organs, etc. working together as well.

Since these studies are performed on isolated cells or tissue samples, they are less expensive and time-intensive than animal or human studies. In vitro studies are vital for zooming in on biological mechanisms, to see how things work at the cellular or molecular level. However, these studies shouldn’t be used to draw conclusions about how things work in humans (or even animals), because we can’t assume that the results will apply to a whole, living organism.

Animal studies are one form of in vivo research, which translates to “within the living.” Rats and mice are the most common animals used in nutrition research. Animals are often used in research that would be unethical to conduct in humans. Another advantage of animal dietary studies is that researchers can control exactly what the animals eat. In human studies, researchers can tell subjects what to eat and even provide them with the food, but they may not stick to the planned diet. People are also not very good at estimating, recording, or reporting what they eat and in what quantities. In addition, animal studies typically do not cost as much as human studies.

There are some important limitations of animal research. First, an animal’s metabolism and physiology are different from humans. Plus, animal models of disease (cancer, cardiovascular disease, etc.), although similar, are different from human diseases. Animal research is considered preliminary, and while it can be very important to the process of building scientific understanding and informing the types of studies that should be conducted in humans, animal studies shouldn’t be considered relevant to real-life decisions about how people eat.

Observational Studies

Observational studies in human nutrition collect information on people’s dietary patterns or nutrient intake and look for associations with health outcomes. Observational studies do not give participants a treatment or intervention; instead, they look at what they’re already doing and see how it relates to their health. These types of study designs can only identify correlations (relationships) between nutrition and health; they can’t show that one factor causes another. (For that, we need intervention studies, which we’ll discuss in a moment.) Observational studies that describe factors correlated with human health are also called epidemiological studies . 1

Epidemiology is defined as the study of human populations. These studies often investigate the relationship between dietary consumption and disease development. There are three main types of epidemiological studies: cross-sectional, case-control, and prospective cohort studies.

One example of a nutrition hypothesis that has been investigated using observational studies is that eating a Mediterranean diet reduces the risk of developing cardiovascular disease. (A Mediterranean diet focuses on whole grains, fruits and vegetables, beans and other legumes, nuts, olive oil, herbs, and spices. It includes small amounts of animal protein (mostly fish), dairy, and red wine. 2 ) There are three main types of observational studies, all of which could be used to test hypotheses about the Mediterranean diet:

• Cohort studies follow a group of people (a cohort) over time, measuring factors such as diet and health outcomes. A cohort study of the Mediterranean diet would ask a group of people to describe their diet, and then researchers would track them over time to see if those eating a Mediterranean diet had a lower incidence of cardiovascular disease.
• Case-control studies compare a group of cases and controls, looking for differences between the two groups that might explain their different health outcomes. For example, researchers might compare a group of people with cardiovascular disease with a group of healthy controls to see whether there were more controls or cases that followed a Mediterranean diet.
• Cross-sectional studies collect information about a population of people at one point in time. For example, a cross-sectional study might compare the dietary patterns of people from different countries to see if diet correlates with the prevalence of cardiovascular disease in the different countries.

There are two types of cohort studies: retrospective and prospective. Retrospective studies look at what happened in the past, and they’re considered weaker because they rely on people’s memory of what they ate or how they felt in the past. Prospective cohort studies, which enroll a cohort and follow them into the future, are usually considered the strongest type of observational study design.

Most cohort studies are prospective. Initial information is collected (usually by food frequency questionnaires) on the intake of a cohort of people at baseline, or the beginning. This cohort is then followed over time (normally many years) to quantify health outcomes of the individual within it. Cohort studies are normally considered to be more robust than case-control studies, because these studies do not start with diseased people and normally do not require people to remember their dietary habits in the distant past or before they developed a disease. An example of a prospective cohort study would be if you filled out a questionnaire on your current dietary habits and are then followed into the future to see if you develop osteoporosis. As shown below, instead of separating based on disease versus disease-free, individuals are separated based on exposure. In this example, those who are exposed are more likely to be diseased than those who were not exposed.

Using trans-fat intake again as the exposure and cardiovascular disease as the disease, the figure would be expected to look like this:

There are several well-known examples of prospective cohort studies that have described important correlations between diet and disease:

• Framingham Heart Study : Beginning in 1948, this study has followed the residents of Framingham, Massachusetts to identify risk factors for heart disease.
• Health Professionals Follow-Up Study : This study started in 1986 and enrolled 51,529 male health professionals (dentists, pharmacists, optometrists, osteopathic physicians, podiatrists, and veterinarians), who complete diet questionnaires every 2 years.
• Nurses Health Studies : Beginning in 1976, these studies have enrolled three large cohorts of nurses with a total of 280,000 participants. Participants have completed detailed questionnaires about diet, other lifestyle factors (smoking and exercise, for example), and health outcomes.

Observational studies have the advantage of allowing researchers to study large groups of people in the real world, looking at the frequency and pattern of health outcomes and identifying factors that correlate with them. But even very large observational studies may not apply to the population as a whole. For example, the Health Professionals Follow-Up Study and the Nurses Health Studies include people with above-average knowledge of health. In many ways, this makes them ideal study subjects, because they may be more motivated to be part of the study and to fill out detailed questionnaires for years. However, the findings of these studies may not apply to people with less baseline knowledge of health.

We’ve already mentioned another important limitation of observational studies—that they can only determine correlation, not causation. A prospective cohort study that finds that people eating a Mediterranean diet have a lower incidence of heart disease can only show that the Mediterranean diet is correlated with lowered risk of heart disease. It can’t show that the Mediterranean diet directly prevents heart disease. Why? There are a huge number of factors that determine health outcomes such as heart disease, and other factors might explain a correlation found in an observational study. For example, people who eat a Mediterranean diet might also be the same kind of people who exercise more, sleep more, have a higher income (fish and nuts can be expensive!), or be less stressed. These are called confounding factors ; they’re factors that can affect the outcome in question (i.e., heart disease) and also vary with the factor being studied (i.e., Mediterranean diet).

Intervention Studies

Intervention studies , also sometimes called experimental studies or clinical trials, include some type of treatment or change imposed by the researcher. Examples of interventions in nutrition research include asking participants to change their diet, take a supplement, or change the time of day that they eat. Unlike observational studies, intervention studies can provide evidence of cause and effect , so they are higher in the hierarchy of evidence pyramid.

Randomization: The gold standard for intervention studies is the randomized controlled trial (RCT) . In an RCT, study subjects are recruited to participate in the study. They are then randomly assigned into one of at least two groups, one of which is a control group (this is what makes the study controlled ).

Randomization is the process of randomly assigning subjects to groups to decrease bias. Bias is a systematic error that may influence results. Bias can occur in assigning subjects to groups in a way that will influence the results. An example of bias in a study of an antidepressant drug is shown below. In this nonrandomized antidepressant drug example, researchers (who know what the subjects are receiving) put depressed subjects into the placebo group, while "less depressed" subjects are put into the antidepressant drug group. As a result, even if the drug isn't effective, the group assignment may make the drug appear effective, thus biasing the results as shown below.

This is a bit of an extreme example, but even if the researchers are trying to prevent bias, sometimes bias can still occur. However, if the subjects are randomized, the sick and the healthy people will ideally be equally distributed between the groups. Thus, the trial will be unbiased and a true test of whether or not the drug is effective.

Here is another example. In an RCT to study the effects of the Mediterranean diet on cardiovascular disease development, researchers might ask the control group to follow a low-fat diet (typically recommended for heart disease prevention) and the intervention group to eat a Mediterranean diet. The study would continue for a defined period of time (usually years to study an outcome like heart disease), at which point the researchers would analyze their data to see if more people in the control or Mediterranean diet had heart attacks or strokes. Because the treatment and control groups were randomly assigned, they should be alike in every other way except for diet, so differences in heart disease could be attributed to the diet. This eliminates the problem of confounding factors found in observational research, and it’s why RCTs can provide evidence of causation, not just correlation.

Imagine for a moment what would happen if the two groups weren’t randomly assigned. What if the researchers let study participants choose which diet they’d like to adopt for the study? They might, for whatever reason, end up with more overweight people who smoke and have high blood pressure in the low-fat diet group, and more people who exercised regularly and had already been eating lots of olive oil and nuts for years in the Mediterranean diet group. If they found that the Mediterranean diet group had fewer heart attacks by the end of the study, they would have no way of knowing if this was because of the diet or because of the underlying differences in the groups. In other words, without randomization, their results would be compromised by confounding factors, with many of the same limitations as observational studies.

Placebo: In an RCT of a supplement, the control group would receive a placebo—a “fake” treatment that contains no active ingredients, such as a sugar pill. The use of a placebo is necessary in medical research because of a phenomenon known as the placebo effect. The placebo effect results in a beneficial effect because of a subject’s belief in the treatment, even though there is no treatment actually being administered. An example would be an athlete who consumes a sports drink and runs the 100-meter dash in 11.00 seconds. The athlete then, under the exact same conditions, drinks what he is told is "Super Duper Sports Drink" and runs the 100-meter dash in 10.50 seconds. But what the athlete didn't know was that Super Duper Sports Drink was the Sports Drink + Food Coloring. There was nothing different between the drinks, but the athlete believed that the "Super Duper Sports Drink" was going to help him run faster, so he did. This improvement is due to the placebo effect.

Blinding is a technique to prevent bias in intervention studies. In a study without blinding, the subject and the researchers both know what treatment the subject is receiving. This can lead to bias if the subject or researcher has expectations about the treatment working, so these types of trials are used less frequently. It’s best if a study is double-blind , meaning that neither the researcher nor the subject knows what treatment the subject is receiving. It’s relatively simple to double-blind a study where subjects are receiving a placebo or treatment pill because they could be formulated to look and taste the same. In a single-blind study , either the researcher or the subject knows what treatment they’re receiving, but not both. Studies of diets—such as the Mediterranean diet example—often can’t be double-blinded because the study subjects know whether or not they’re eating a lot of olive oil and nuts. However, the researchers who are checking participants’ blood pressure or evaluating their medical records could be blinded to their treatment group, reducing the chance of bias.

Open-label study:

Single-blinded study:

Double-blinded study:

Like all studies, RCTs and other intervention studies do have some limitations. They can be difficult to carry on for long periods of time and require that participants remain compliant with the intervention. They’re also costly and often have smaller sample sizes. Furthermore, it is unethical to study certain interventions. (An example of an unethical intervention would be to advise one group of pregnant mothers to drink alcohol to determine its effects on pregnancy outcomes because we know that alcohol consumption during pregnancy damages the developing fetus.)

VIDEO: “ Not all scientific studies are created equal ” by David H. Schwartz, YouTube (April 28, 2014), 4:26.

Meta-Analyses and Systematic Reviews

At the top of the hierarchy of evidence pyramid are systematic reviews and meta-analyses . You can think of these as “studies of studies.” They attempt to combine all of the relevant studies that have been conducted on a research question and summarize their overall conclusions. Researchers conducting a systematic review formulate a research question and then systematically and independently identify, select, evaluate, and synthesize all high-quality evidence that relates to the research question. Since systematic reviews combine the results of many studies, they help researchers produce more reliable findings. A meta-analysis is a type of systematic review that goes one step further, combining the data from multiple studies and using statistics to summarize it, as if creating a mega-study from many smaller studies . 4

However, even systematic reviews and meta-analyses aren’t the final word on scientific questions. For one thing, they’re only as good as the studies that they include. The Cochrane Collaboration is an international consortium of researchers who conduct systematic reviews in order to inform evidence-based healthcare, including nutrition, and their reviews are among the most well-regarded and rigorous in science. For the most recent Cochrane review of the Mediterranean diet and cardiovascular disease, two authors independently reviewed studies published on this question. Based on their inclusion criteria, 30 RCTs with a total of 12,461 participants were included in the final analysis. However, after evaluating and combining the data, the authors concluded that “despite the large number of included trials, there is still uncertainty regarding the effects of a Mediterranean‐style diet on cardiovascular disease occurrence and risk factors in people both with and without cardiovascular disease already.” Part of the reason for this uncertainty is that different trials found different results, and the quality of the studies was low to moderate. Some had problems with their randomization procedures, for example, and others were judged to have unreliable data. That doesn’t make them useless, but it adds to the uncertainty about this question, and uncertainty pushes the field forward towards more and better studies. The Cochrane review authors noted that they found seven ongoing trials of the Mediterranean diet, so we can hope that they’ll add more clarity to this question in the future. 5

Science is an ongoing process. It’s often a slow process, and it contains a lot of uncertainty, but it’s our best method of building knowledge of how the world and human life works. Many different types of studies can contribute to scientific knowledge. None are perfect—all have limitations—and a single study is never the final word on a scientific question. Part of what advances science is that researchers are constantly checking each other’s work, asking how it can be improved and what new questions it raises.

Attributions:

• “Chapter 1: The Basics” from Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR , CC BY-NC-SA 4.0
• “The Broad Role of Nutritional Science,” section 1.3 from the book An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0

References:

• 1 Thiese, M. S. (2014). Observational and interventional study design types; an overview. Biochemia Medica , 24 (2), 199–210. https://doi.org/10.11613/BM.2014.022
• 2 Harvard T.H. Chan School of Public Health. (2018, January 16). Diet Review: Mediterranean Diet . The Nutrition Source. https://www.hsph.harvard.edu/nutritionsource/healthy-weight/diet-reviews/mediterranean-diet/
• 3 Ross, R., Gray, C. M., & Gill, J. M. R. (2015). Effects of an Injected Placebo on Endurance Running Performance. Medicine and Science in Sports and Exercise , 47 (8), 1672–1681. https://doi.org/10.1249/MSS.0000000000000584
• 4 Hooper, A. (n.d.). LibGuides: Systematic Review Resources: Systematic Reviews vs Other Types of Reviews . Retrieved February 7, 2020, from //libguides.sph.uth.tmc.edu/c.php?g=543382&p=5370369
• 5 Rees, K., Takeda, A., Martin, N., Ellis, L., Wijesekara, D., Vepa, A., Das, A., Hartley, L., & Stranges, S. (2019). Mediterranean‐style diet for the primary and secondary prevention of cardiovascular disease. Cochrane Database of Systematic Reviews , 3 . doi.org/10.1002/14651858.CD009825.pub3
• 6Levin K. (2006) Study design III: Cross-sectional studies. Evidence - Based Dentistry 7(1): 24.
• Figure 2.3. The hierarchy of evidence by Alice Callahan, is licensed under CC BY 4.0
• Research lab photo by National Cancer Institute on Unsplas h ; mouse photo by vaun0815 on Unsplash
• Figure 2.4. “Placebo effect example” by Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR

Research methods in nutrition and dietetics: design, data analysis, and presentation

Affiliation.

• 1 University of Washington, Seattle 98195.
• PMID: 3047199

Most problems in practice may be addressed through research. To show the applicability of research to all areas of nutrition and dietetics, seven types of research designs are discussed in this article: qualitative research; case series and surveys--both categorized as descriptive research; and experimental design, quasiexperimental design, cohort (follow-up) studies, and case-control studies--the four of which are categorized as analytical research because each design tests hypotheses of causal relationships. Sample size, subject selection, and statistical analysis and interpretation are discussed as appropriate to each research design. Numerous examples are presented, along with the basic research designs. Each section and subsection is numbered so that the article can serve easily as a reference and its component parts can be accessed readily. Research provides answers to questions and, generally, raises further questions that future research can address. Among the benefits of well-designed research are answers to clearly stated research questions, useful comparisons between options, information to guide evaluations of protocols, and data to document and support one's professional activities and one's staff.

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• Clinical Trial
• Research Support, U.S. Gov't, P.H.S.
• Clinical Trials as Topic / methods
• Data Interpretation, Statistical
• Epidemiologic Methods
• Ethics, Professional
• Nutritional Physiological Phenomena*
• Random Allocation
• Research Design*
• Statistics as Topic

Grants and funding

• CA18029/CA/NCI NIH HHS/United States
• CA38552/CA/NCI NIH HHS/United States

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• Published: 15 May 2024

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The evolution of personalized nutrition

• Josef Neu   ORCID: orcid.org/0000-0002-6973-0592 1  

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Data from a large randomized trial show that a personalized diet can improve cardiometabolic health, providing support for a ‘food as medicine’ concept that, although centuries old, still lacks robust evidence.

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Mozaffarian, D., Rosenberg, I. & Uauy, R. Br. Med. J. 361 , k2392 (2018).

Article   Google Scholar  

Bermingham, K. M. et al. Nat. Med. https://doi.org/10.1038/s41591-024-02951-6 (2024).

Article   PubMed   Google Scholar  

National Nutrition Conference For Defense. J. Am. Med. Assoc. 116 , 2598–2599 (1941).

Bruins, M. J., Van Dael, P. & Eggersdorfer, M. Nutrients 11 , 85 (2019).

Millen, B. E. et al. Adv. Nutr. 7 , 438–444 (2016).

Article   CAS   PubMed   PubMed Central   Google Scholar  

Schneeman, B. O. et al. Adv. Nutr. 12 , 1051–1057 (2021).

Article   PubMed   PubMed Central   Google Scholar  

World Health Organization. https://go.nature.com/3WgrZiU (accessed 12 April 2024)

Berry, S. E. et al. Nat. Med. 26 , 964–973 (2020).

Leme, A. C. B., Hou, S., Fisberg, R. M., Fisberg, M. & Haines, J. Nutrients 13 , 1038 (2021).

Indrio, F. et al. Front Pediatr. 5 , 178 (2017).

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Phosphorus is a mineral that naturally occurs in many foods and is also available as a supplement. It plays multiple roles in the body. It is a key element of bones, teeth, and cell membranes. It helps to activate enzymes, and keeps blood pH within a normal range. Phosphorus regulates the normal function of nerves and muscles, including the heart, and is also a building block of our genes, as it makes up DNA, RNA, and ATP, the body’s major source of energy. [1]

The kidneys, bones, and intestines tightly regulate phosphorus levels in the body. If the diet lacks phosphorus or too little phosphorus is absorbed, several things happen to preserve its stores and try to maintain normal levels: the kidneys excrete less phosphorus in urine, the digestive tract becomes more efficient at absorbing phosphorus, and the bones release its stores of phosphorus into the blood. [1] The opposite actions occur in these organs if the body has adequate phosphorus stores.

Recommended Amounts

RDA:  The Recommended Dietary Allowance (RDA) for adult men and women 19+ years is 700 mg a day. Pregnancy and lactation require the same amount of phosphorus at 700 mg daily. [2]

UL:   The Tolerable Upper Intake Level (UL) is the maximum daily intake unlikely to cause harmful effects on health. The UL for phosphorus for adult men and women ages 19-70 years old is 4,000 mg daily, and for older adults 71+ years, 3,000 mg daily. The UL for pregnant and lactating women ages 14-50 years is 3,500 and 4,000 mg, respectively.

Phosphorus and Health

Kidneys help to regulate normal levels of phosphorus in the body. If the body has adequate stores of the mineral, the kidneys will push out extra phosphorus in the urine. With chronic kidney disease (CKD), the kidneys cannot perform this action and the amount of phosphorus can rise to harmful levels in the blood. Studies show that adults with CKD have higher phosphate levels than those with normal kidney function. [3] This may quicken the progression of CKD, and increase the risk of cardiovascular disease, bone disorders, and death. [4,5] It appears that patients with higher phosphate levels and more advanced levels of CKD (such as those on dialysis) have a greater risk of disease progression and death than those with milder forms of CKD. [6-9]

Even so, it is unclear if lowering phosphate levels in those with CKD improves health outcomes later on. Dietary recommendations for phosphorus depend on how much the disease has progressed, and a person’s blood level of phosphorus. Some recommendations suggest limiting animal protein while increasing plant proteins, and reading food labels to limit foods with phosphate additives. [4] Plant-based proteins like legumes, nuts, and seeds contain phytates , which interfere with the absorption of phosphorus in the gut. [10] Medications like phosphate binders are also sometimes prescribed to be taken with food to reduce the amount of phosphorus absorbed in the gut.

Some studies have found that excess phosphorus can promote the calcification, or hardening, of heart arteries and increase inflammation. [11] Higher phosphate levels may be associated with an increased risk of cardiovascular disease (CVD). A meta-analysis of six cohort studies of more than 120,000 healthy adults followed for up to 29 years showed a 36% increased risk of deaths from CVD and all causes in those with the highest levels of serum phosphorus, compared with the lowest levels. [12] Deaths from all causes were seen mainly in men, not women.

The foods highest in phosphorus are animal proteins, which also tend to be high in other components related to CVD, like saturated fat. So it is not clear if phosphorus alone is associated with an increased risk of CVD, or if high levels are an indicator of another risk factor. The research is also unclear if restricting dietary phosphorus can prevent CVD in otherwise healthy adults. [2]

Elevated phosphorus levels may disrupt the normal hormonal balance of phosphorus, calcium , and vitamin D that regulates bone health. Animal studies have shown that high dietary phosphorus intakes are detrimental to bone health. Yet the evidence is less clear in humans, partly because it is difficult to estimate accurate phosphorus intakes. Most studies measure phosphorus in the blood, which may not reflect true dietary intakes of phosphorus, since the majority of the mineral is stored in bones and the body maintains blood levels within a specific range. However, research has shown that a higher intake of phosphate additives, from various foods like cola beverages and salad dressings that are very well-absorbed in the gut, are associated with negative effects on bone metabolism. [13] These include fractures and a lower bone mineral density. [14] More research is needed in this area.

Another downside of soda

Food sources.

A variety of foods naturally contain phosphorus, and the richest sources are dairy, red meat, poultry, seafood, legumes, and nuts. Phosphorus from these foods is called organic phosphorus. It is absorbed more efficiently from animal foods than plant foods. Plant foods like seeds, legumes, and whole grains contain a storage form of phosphorus called phytates or phytic acid that can reduce the mineral’s absorption. The body lacks an enzyme needed to break down phytic acid, so as it passes through the digestive tract it can bind not only to phosphorus but other minerals like iron and zinc . Cooking, sprouting, and soaking are some food preparation techniques that help to break down phytic acid so that phosphorus is more easily absorbed.

Inorganic phosphorus is a processed form added to foods to preserve color, moisture, and texture. It is found in fast foods, deli meats, canned and bottled beverages, and many other processed foods. Phosphate additives and preservatives are a significant contributor to phosphorus intakes, comprising up to 30% in the U.S. diet. [4] Inorganic phosphorus is very easily absorbed in the gut: about 90%, compared with 40-60% from natural animal and plant foods. [15] Phosphorus is also available in supplement form.

• Dairy : milk, yogurt, cheese
• Nuts, seeds
• Whole wheat breads and cereals
• Some vegetables : asparagus, tomatoes, cauliflower
• Processed foods (as inorganic phosphorus), especially deli meats, bacon, sausage, sodas , sports drinks , and other bottled beverages

Signs of Deficiency and Toxicity

A phosphorus deficiency is called hypophosphatemia, defined by blood levels that fall below the normal range. However, blood levels of phosphate do not necessarily show the total amount of phosphorus in the body, as most of it is stored in bones and teeth. The most common causes of deficiency are kidney problems or a condition called hyperparathyroidism, in which too much parathyroid hormone is released that causes phosphorus to exit the body through urine. Also, the overuse of aluminum-containing antacids can bind to phosphorus and increase the risk of a deficiency.

A notable but less common occurrence of hypophosphatemia occurs with refeeding syndrome, seen in people with severe malnutrition. Patients who are malnourished from conditions like cancer, advanced liver disease, alcohol abuse, or anorexia nervosa may be started on supplemental nutrition feedings through a tube or vein. However because their starved state has reduced their ability to efficiently process food, reintroducing nutrition can cause problems. A sudden infusion of nutrients and calories causes an insulin surge, which results in rapid shifts in electrolytes and fluids in the blood. Blood levels of electrolyte nutrients like potassium, phosphorus, and magnesium may quickly drop. If untreated, refeeding syndrome can lead to respiratory failure, coma, cardiac arrest, and even death. The situation can be avoided by giving these electrolytes intravenously to the patient prior to the nutritional feedings.

Symptoms appearing with a phosphorus deficiency:

• Poor appetite
• Muscle weakness
• Bone disease (osteomalacia, rickets)
• Increased susceptibility to infections

A toxicity from phosphorus, called hyperphosphatemia, is rare because the body will regulate any excess levels in healthy individuals. It might occur with supplement use, but generally the use of phosphorus supplements is not common and the amount of phosphorus in them is typically not high. (2) People with hyperphosphatemia may show no symptoms; others may develop calcium deposits and hardening of soft tissues in the body, such as in the kidney, resulting from a disruption in the normal metabolism of calcium.

Did You Know?

• Phosphorus is the second most abundant mineral in the body, second to calcium . About 85% of the body’s phosphorus is stored in bones and teeth.
• Inorganic phosphorus as phosphorus additives are commonly found in foods like processed meats and baked goods as well as beverages like soda, iced teas, bottled coffee drinks, and flavored waters. Examples to look for in the ingredients list are phosphoric acid, dicalcium phosphate, sodium phosphate, and trisodium phosphate. If one is following a low phosphorus diet, it is important to be aware of these “hidden” well-absorbed dietary sources of phosphorus by reading food labels carefully.

Vitamins and Minerals

• Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington, DC: National Academies Press; 1997.
• National Institutes of Health Office of Dietary Supplements: Phosphorus Fact Sheet for Health Professionals https://ods.od.nih.gov/factsheets/Phosphorus-HealthProfessional/ . Accessed 6/8/2020.
• Moore LW, Nolte JV, Gaber AO, Suki WN. Association of dietary phosphate and serum phosphorus concentration by levels of kidney function. The American journal of clinical nutrition . 2015 Aug 1;102(2):444-53.
• Chang AR, Anderson C. Dietary phosphorus intake and the kidney. Annual review of nutrition . 2017 Aug 21;37:321-46.
• Da J, Xie X, Wolf M, Disthabanchong S, Wang J, Zha Y, Lv J, Zhang L, Wang H. Serum phosphorus and progression of CKD and mortality: a meta-analysis of cohort studies. American Journal of Kidney Diseases . 2015 Aug 1;66(2):258-65.
• Hou Y, Li X, Sun L, Qu Z, Jiang L, Du Y. Phosphorus and mortality risk in end-stage renal disease: A meta-analysis. Clinica chimica acta . 2017 Nov 1;474:108-13.
• Selamet U, Tighiouart H, Sarnak MJ, Beck G, Levey AS, Block G, Ix JH. Relationship of dietary phosphate intake with risk of end-stage renal disease and mortality in chronic kidney disease stages 3–5: The Modification of Diet in Renal Disease Study. Kidney international . 2016 Jan 1;89(1):176-84.
• Murtaugh MA, Filipowicz R, Baird BC, Wei G, Greene T, Beddhu S. Dietary phosphorus intake and mortality in moderate chronic kidney disease: NHANES III. Nephrology Dialysis Transplantation . 2012 Mar 1;27(3):990-6.
• Mehrotra R, Peralta CA, Chen SC, Li S, Sachs M, Shah A, Norris K, Saab G, Whaley-Connell A, Kestenbaum B, McCullough PA. No independent association of serum phosphorus with risk for death or progression to end-stage renal disease in a large screen for chronic kidney disease. Kidney international . 2013 Nov 1;84(5):989-97.
• Chauveau P, Koppe L, Combe C, Lasseur C, Trolonge S, Aparicio M. Vegetarian diets and chronic kidney disease. Nephrology Dialysis Transplantation . 2019 Feb 1;34(2):199-207.
• Gutiérrez OM. The connection between dietary phosphorus, cardiovascular disease, and mortality: where we stand and what we need to know. Advances in nutrition . 2013 Nov;4(6):723-9.
• Bai W, Li J, Liu J. Serum phosphorus, cardiovascular and all-cause mortality in the general population: a meta-analysis. Clinica Chimica Acta . 2016 Oct 1;461:76-82.
• Calvo MS, Tucker KL. Is phosphorus intake that exceeds dietary requirements a risk factor in bone health?. Annals of the New York Academy of Sciences . 2013 Oct;1301(1):29-35.
• Vorland CJ, Stremke ER, Moorthi RN, Hill Gallant KM. Effects of excessive dietary phosphorus intake on bone health. Current osteoporosis reports . 2017 Oct;15(5):473-82.
• Kalantar-Zadeh K, Gutekunst L, Mehrotra R, Kovesdy CP, Bross R, Shinaberger CS, Noori N, Hirschberg R, Benner D, Nissenson AR, Kopple JD. Understanding sources of dietary phosphorus in the treatment of patients with chronic kidney disease. Clinical Journal of the American Society of Nephrology . 2010 Mar 1;5(3):519-30.

Last reviewed March 2023

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