It seems that nearly every week, a new study on nutrition comes out that contradicts the latest health trend. Eggs are nutritional miracles; eggs are cholesterol-laden artery constrictors. Fat is the number one weight-loss enemy; fat is the key to weight loss.
Why is nutrition advice so contradictory and ever-changing? In a word, research. In theory, good science and research is driven by hypotheses, ideas, and concepts that need to be proven or disproven. It should be simple. But in nutritional science, that is rarely the case.
Here are some reasons why research on nutrition can go so far awry, and how to know what information to trust.
Not All Studies Are Created Equal
The quality of a study is influenced by many factors. Here are some of the big ones:
Good-quality research can be hard to apply to nutrition questions. Unfortunately, the most reliable research methods are difficult to apply to many questions about what constitutes healthy eating.
Randomized controlled trials are considered the most accurate way to gather evidence in medicine. These involve groups of participants who are selected randomly and then divided into two groups: the test group and the placebo (control) group. The idea is that since the participants were randomly selected, any difference in the results will be because of the treatment.
However, many such studies are done over a period of only weeks or months, meaning they don’t reflect real life or how a particular dietary choice might impact the body over decades. There will always be a gap when research relies on short-term studies for answers on chronic, long-term issues.
It’s also not practical, and possibly unethical, to do the types of nutrition studies that would lead to the most accurate results—for example, locking people up and observing every meal they eat for 25 years, sequestering newborns for testing, or repeatedly feeding subjects unhealthy food to see how their bodies react.
Widely used observational studies are prone to inaccuracy. Instead of controlled trials, most nutrition researchers have to rely on observational studies. These studies follow groups of people who have already been eating in a certain way for long periods of time, and track specific health markers such as rates of heart disease, high blood pressure, or cancer.
These types of studies are less precise because there are plenty of factors researchers can’t control and may not be aware of. For example, let’s say vegetarians and vegans are compared on an aspect of their health. Maybe the vegetarians score higher because they make better choices, due to having higher incomes or education levels. Or maybe the vegans score higher because they smoke less than vegetarians, exercise more, or go to the doctor more often. This is called the “healthy user effect”—it’s the subjects’ overall lifestyle that makes them healthier, which makes it hard to attribute their good health to a specific dietary choice.
“There are many lifestyle factors that add up to a person’s weight, health, and risk of disease—nutrition is a major factor, but it’s not the only factor,” said nutrition expert Dan DeFigio, the bestselling author of “Beating Sugar Addiction for Dummies.”
Another issue is displacement, says Eric Feigl-Ding, nutritionist and epidemiologist at Harvard University’s School of Public Health. If someone adopts a new diet, it might be effective because of what the new foods are displacing rather than the new foods themselves.
“If you eat more avocados, you’re actually displacing other foods. What are you displacing? Carrots? Or are you displacing bacon? This displacement substitution is something that gets lost,” he said.
Food surveys rely on memory and honesty. Since it can be difficult for researchers to monitor every meal a subject eats, they often have to rely on food surveys. This is a problem because human memory is flawed.
The other issue with self-reporting is, well, lies. When we are being monitored, we tend to want to show ourselves in the best possible light and offer answers that we think other people want to hear. This may cause a subject to “forget” to report the doughnut he ate at the staff meeting (and the second one).
“These days, people aren’t going to tell you how much sugar they eat quite as honestly as they would have 10 years ago,” said Ruth Kava, a senior nutrition fellow at the American Council on Science & Health.
In addition, even if the subject is self-reporting with a high degree of accuracy, the food they report can be misrepresented. For example, not all burgers are the same: a lean grass-fed hamburger on a whole grain bun made at home is much different from a highly processed, fast food version.
People are diverse. People’s genetics and cultural background may drastically change the response they have to certain foods. For example, whether a study was done on a group of Asian children or on a group of 200-pound white males can lead to completely different outcomes with the same foods.
Another problem with some nutrition research is small sample size; just because a phenomenon can be observed in a dozen people doesn’t mean it will apply on a larger scale.
“They may [study] a particular group of people, women or men, or people who are overweight or obese. So that restricts how applicable the results of such a study would be to the general population,” said Kava.
Animals are often used in nutritional research, yet studies with animal models may not discuss the applicability of the model to humans and its inherent limitations.
Problems applying information from animal research to human health are inevitable. Differences in anatomy, organ structure and function, and metabolism—among myriad other differences between humans and other species—can lead to inadequate or erroneous information when attempting to apply research to humans.
Conflict of interest is a big problem in nutrition research. Research needs to be funded somehow, and funding usually comes from two places: the government or private industry. Since the 1970s, health research funded by industry has steadily increased. Yet if a study is funded by a food company, it is eight times more likely to turn up positive findings for that company’s product, according to researchers from the Department of Medicine at Boston’s Children’s Hospital.
Another problem is the dubious link between industry research and some organizations that claim to protect public health. For example, between 2011 and 2015, 96 U.S. national health organizations accepted money from Coca-Cola Co., PepsiCo Inc., or both, according to a study published last year in the American Journal of Preventive Medicine. Some of the organizations accepting sponsorships included the National Institutes of Health, the American Red Cross, the American Diabetes Association, and other highly influential groups that dictate public health policy.
It’s also worth considering how industry-funded research impacts which subjects get researched at all. Studies on the health benefits of fruits and vegetables aren’t as profitable as studies on a new cancer drug.
Nutritionism is the tendency of nutrition science to focus on the specific individual nutrients in food rather than looking at food and diet as a complex, broad, and interactive system.
“Nutritionism is the science of breaking down dietary components into their individual parts, such as one vitamin or one type of fat, and studying them in isolation,” wrote Dr. Mark Hyman, in his book “Eat Fat, Get Thin.”
“This approach is helpful for studying medication, where there might be a single molecule designed to target one specific pathway and one specific disease. But it is not that helpful for studying individual nutritional components. Why? Because people eat food, not single components.”
The idea that the sum of food is all about the nutrients it contains also tends to divide nutrients into two categories: the evil and the good. This often doesn’t take into account the food in the context of an overall balanced diet. Food companies also love this approach because it allows them to demonize ingredients at the top of the current hit list (i.e. fat) to boost sales, and to market cheap, nutrient-deficient food as healthy.
The Pull of the Status Quo
In an ideal world, science and scientists would be infallible and unbiased. This is what the scientific method was invented for. Yet a scientist is inevitably part of what the philosopher Ludwik Fleck called a “thought collective,” a group of people or peers who mutually exchange ideas. In time, this collective group develops a mind of its own with a thought style, language, and personality of sorts.
This also makes the group’s scientific inquiry prone to the same rules that govern human social life: dominance of the charismatic, conformity to the majority opinion, ostracism for deviation, and discomfort with admitting to error. This can make it very difficult for dissenting voices to be heard if their research goes against the status quo.
For example, in the 1950s and ’60s, saturated fat was targeted as a leading cause of heart disease, despite research that pointed to sugar as the likely culprit. The anti-fat rhetoric won and led to the ubiquitous low-fat, high-carbohydrate food pyramids that are still in place in much of the Western world. As obesity rates hit record highs, however, this theory is increasingly being debunked, and the real causes of obesity—sugar and processed carbs—are now on the hit list, while healthy fats are starting to enjoy a restored reputation.
Media Distortions & Sensationalism
Misinterpretation, sensationalism, distortions, exaggeration—the media is guilty of it all in the name of telling a good story. Unfortunately, this makes nutrition research highly confusing for the public and derails people’s sincere efforts to pursue better health.
To know whether you can trust articles about nutrition, try to find the original research, often published in medical journals, and check whether the research is of good quality. It’s also a good idea to read what else has been written on the topic and see if it’s part of a larger body of solid evidence.
How to Know What to Trust
Look at the combined weight of all evidence, how each type of study was done, and who funded it. For example, were people observed in a hospital for long periods of time, or were they asked to self-report and prepare their own meals? Was the study funded by the Sugar Association?
“Put all the pieces together like a puzzle, consider all the potential issues and conflicts, and see the story the data tells,” said Hyman.
It’s also a good idea to see if there are different types of studies on the same question—clinical trials, observational data, lab studies—and if they are all pointing toward a similar conclusion. Different methodologies that come to similar results give a fairly good indication that there’s a link between a particular diet and a certain health outcome.
And don’t forget to trust yourself; listen to your body, and see which foods make you look and feel healthy and which ones don’t.
“The main reason nutrition research is so conflicting is that it ignores the fact that we are all unique individuals,” said author and health coach Liza Baker.
“If we look at traditional food/nutrition theory, we see ancient systems—think traditional Chinese medicine or ayurveda—begin from the hypothesis that each individual is unique, different. One person’s nourishment can be another one’s toxin.”
Where does that leave us in terms of knowing what to eat? Obesity expert and author Dr. David Ludwig has come to this simple conclusion: “A diet based on whole, natural, slow-digesting foods lowers everyone’s risk for chronic disease, regardless of body weight, age, sex, race, or country of origin.”