Why Science Can't Seem to Tell Us How to Eat Right

Medically Reviewed by Michael W. Smith, MD on August 06, 2019

eggs in crate

Eggs once fell from grace, going from the sunny breakfast staple of choice to a hard pass if you wanted to avoid heart attacks. Then, like all disgraced celebrities, they seemed to make a comeback in the 2015 Dietary Guidelines for Americans. Health experts said we could stop worrying about the cholesterol or eating too many eggs. (Brunch never looked so fantastic.)

Then, earlier this year, a study seemed to say, "Hold up!" -- cholesterol in your diet and eating eggs were linked to a higher heart risk.

Instead of screams of frustration, though, the news was met with a collective sigh of boredom. Because, to be honest, there's a long list of foods, diets, and ingredients -- coconut oil, butter, avocados, low-fat foods, salt, nuts, saturated fat, and sugar -- that seem to have gone through the same good guy-bad guy cycle.

It's understandable if you yawned, looked at your fitness-tracker watch, and asked, "Remind me, where are we in the cycle on this one?" just before digging into your plate of scrambled eggs. So who is to blame for the average person's jaded palate when it comes to foods that supposedly are or are not "healthy"?

Like many things food-related, it depends on whom you ask. Some blame the science, which, you might have guessed, has some problems. (More on that later.) Others say it's the media. And people who stand to profit -- namely, the food industry as well as nutrition "gurus" hawking diet books and products -- also add to the messages.

Here's Why It's Kind of Your Fault, Too

Americans have a glorious variety when it comes to living life, eating different foods, cooking them in different ways, and making thousands of decisions every day. These can all affect the long-term risk of things like heart disease and diabetes, which makes it pretty hard to study the connection between diet and chronic disease.

But scientists, always up for a challenge, do it anyway. For decades, the Dietary Guidelines for Americans and other advisory groups have relied on observational research, which includes studies that look at what people in the real world eat and how they fare health-wise. In those studies, people filled out questionnaires about how often they ate certain foods in the past year. For example, NHANES, or the National Health and Nutrition Examination Survey, has been conducted since the 1960s and contains more than 100 questions, such as "How often did you drink coffee?" or "How often did you eat mixed vegetables?" Answers include "never" and "two to three times per month."

But some critics say this type of research is unreliable to the point of being useless -- people barely remember what they ate for breakfast, much less how many times a week they ate broccoli last January. Plus, people tend to fudge a bit to make themselves look better.

In his research published in 2013 and 2015, Edward Archer, PhD, analyzed more than 60,000 people in the NHANES databases. He found that the number of calories people said they ate were not enough to actually keep them alive.

"The dietary data that food frequency questionnaires collect is physiologically implausible, about 65% to 85%, meaning it cannot be right," says Archer, who did the research at the University of Alabama at Birmingham and is now the chief science officer at EnduringFX, a company that analyzes data from wearable devices.

"We have this data that is just ridiculous. If people can't survive on it, why is the dietary guidelines advisory committee using it as a baseline for our dietary guidelines?" he asks. "Everyone agrees that NHANES is a joke, but the federal government continues to use it." (Archer's early studies as a graduate student took place in a lab where his research assistant salary was funded by Coca-Cola, but he says he has been self-funded since 2016.)

John Ioannidis, MD, a professor at Stanford University, has argued for years that the vast majority of nutrition research is flawed to the point of being false. He says results from observational studies are often not confirmed in placebo-controlled, randomized trials, which are the gold standard when it comes to finding out if something is scientifically sound. He also says researchers cherry-pick data to support their own biases; there are too many small, low-quality trials published; and a lot of nutrition-related research is compromised by strong financial ties to the food industry.

What's more, many one-ingredient claims -- like a study that seems to suggest that one hazelnut a day will increase your lifespan by a year -- just don't sound credible. "I love hazelnuts, and I do recommend that you eat more of them, but I don't do it because I expect to live 120 years if I eat that many hazelnuts every day," he said at a conference in 2018.

In a statement, the U.S. Department of Agriculture, which publishes the Dietary Guidelines, says it includes "study designs that offer the strongest evidence for establishing a relationship between diet and health."

"When it comes to minimizing public confusion, we encourage the public to not read too much into individual studies and not to change their eating behaviors based on one study," a USDA spokesperson said. "Instead, we encourage the public to follow the key recommendations of the Dietary Guidelines since they are based on the body of scientific evidence over time and are a reliable source for consumers and practitioners."

So, What Does Science Really Know About Food?

Not all of the recommendations for healthy eating rely on observational studies, and scientists take a variety of studies into account when coming up with general guidelines, says Bonnie F. Liebman, director of nutrition at the Center for Science in the Public Interest in Washington, D.C. "The dietary guides are based on a combination of those observational studies and randomized controlled trials," she says.

In randomized trials, scientists assign people to one group or another and follow them for a specific period of time to see which treatment (or food) is better for you. These types of trials are hard to do for food -- you can't ask people to eat only, say, hot dogs for weeks, months, or years at time. But some research does look at whether some eating habits are linked to lower blood pressure, cholesterol, or other things that are associated with chronic disease. These are considered acceptable in cases where you can't follow people over a lifetime.

"Some critics don't really understand the science or have misinterpreted the science. But if you rely on the American Heart Association, the World Health Organization, the American Cancer Society, you'll find a pretty consistent message," Liebman says.

Marion Nestle, a retired professor of nutrition, food studies, and public health at New York University, says the dietary advice hasn't changed all that much in decades. "Eat vegetables; don't eat too much salt, sugar, and saturated fat; watch your body weight; and don't eat too much junk food. I mean, that hasn't changed since 1960," she says. "What seems to change is research about individual nutrients and individual foods, but that's not how people eat."

News stories about this research may not disclose the funding source, size, or quality of the study, or explain how it fits in with other research. And some journalists may rely on university or medical journal news releases that overhype or don't tell the whole story about a study's findings, Liebman and Nestle say.

Liebman says conflicting headlines take attention away from our constant exposure to "a toxic food environment." Eating habits are more heavily influenced by the 24/7 exposure to unhealthy food at malls, gas stations, and fast-food restaurants, she says, than any federal government guidelines.

Industry-funded research is a problem and has been for decades, says Nestle, who has written multiple books on the subject, including one called Unsavory Truth. It "almost invariably comes out with results that the funder can use in marketing or in stating that their products are harmless or healthy," she says

"I get letters all the time from the yogurt industry, the grape industry, the pecan industry. You name the food, and they send out letters saying we have $30,000 to $50,000 -- usually that's the range -- and we are looking for studies that will demonstrate the benefits of our products," says Nestle. "This isn't basic science."

For example, a 2016 analysis of historical documents found that the sugar industry, specifically a group called the Sugar Research Foundation, paid for research in the 1960s and 1970s that emphasized fat over sugar as the cause of heart disease. And a 2015 study of 14 overweight, postmenopausal women suggesting that a cheese- and meat-heavy diet was better for HDL, or "good" cholesterol, than a low-fat, high-carbohydrate diet was funded by dairy industry groups.

But P. Courtney Gaine, PhD, president and CEO of The Sugar Association in Washington, D.C., says that while there might be a "few bad apples that spoil the bunch," food nutrition research is getting more, not less, transparent about funding.

"Industry scientists want the population to be healthy, too, and there are a lot of really good scientists," she says. "I think people should not throw the baby out with the bathwater -- if it's a good study, it's a good study."

"Industry-funded research for a variety of well-known reasons has gotten a bad rap," says Marc Dresner, manager of marketing and communications at the American Egg Board. "All of the research that is conducted with funding from us is conducted by top institutions, universities, around the country, and we feel that the quality of the research that we have funded is second to none."

Cut Through the Noise

Nestle recommends that people "be skeptical" when it comes to new studies or findings. "If they see studies that say it's a breakthrough, it's a miracle, it will cure more than one disease, it will take care of everything that ails you, you should be really suspicious," she says. "Especially if it says everything you thought you knew about nutrition is wrong. That's not how science works."

As for those eggs, Nestle says it "makes no sense" to call a food good or bad, and it's all about the context and your overall diet. In fact, it might be better to focus on what we know are healthy eating patterns, rather than a specific food.

As for that new study about eggs that sent some people into a tailspin? The study was large, well-conducted, and funded by the American Heart Association, the National Institutes of Health, and other non-food industry sources and was published in JAMA, a respected medical journal. But it was also observational -- meaning it couldn't tell for sure if eggs were the cause of heart trouble, only that they were linked to it. "That may have nothing whatsoever to do with eggs and everything to do with what kind of lifestyle people have who typically report eating eggs," Nestle says.

Some randomized trials have indeed found that eating more eggs can raise LDL or "bad" cholesterol in the blood to some degree, with more -- as in three or four eggs a day -- being worse than one or fewer per day, but the effect can also vary from person to person. And the 2015 Dietary Guidelines for Americans did not give dietary cholesterol a free pass, which is a message that may have been lost if you were reading only the headlines. The guidelines say it's important to "eat as little dietary cholesterol as possible while consuming a healthy eating pattern."

"You are not going to die if you eat an egg," Nestle says. "For a lot of people, eating a lot of eggs is going to raise their blood cholesterol and raise their heart disease risk, and that's not going to be good ... but that doesn't mean you can't eat eggs."

One finding in nutrition research that does seem certain: It's a good idea to eat veggies. "People who eat vegetables are healthier than people who don't. There's incontrovertible evidence for that," she says.

By the Numbers

A few facts about Americans' eating habits:

  • 54%: Percentage of Americans who say people in the U.S. pay more attention to eating healthy foods today, compared to 20 years ago
  • 25%: Percentage of Americans who say they feel overwhelmed by the inconsistent findings in food research studies
  • 73%: Percentage of Americans who say they are very or fairly focused on healthy eating
  • 59%: Percentage who say that conflicting information makes them doubt the choices they make
  • 80%: Percentage of Americans who say there is a lot of conflicting information about what foods they should eat or avoid
  • 87%: Percentage of Americans who eat less than the recommended amount of vegetables daily
  • 70%: Percentage who eat more than the recommend amount of added sugars daily

Find more articles, browse back issues, and read the current issue of WebMD Magazine.

Show Sources


USDA Choose My Plate: “Dietary Guidelines for America.”

News release, Harvard School of Public Health.

The New York Times: “Are Eggs Bad for Your Heart Health? Maybe.”

National Center for Health Statistics, NHANES history: “About the National Health and Nutrition Examination Survey.”

National Cancer Institute: “NHANES Food Questionnaire.”

National Cancer Institute: “Usual Dietary Intakes: NHANES Food Frequency Questionnaire (FFQ).”

PLoS Medicine: “Why Most Published Research Findings Are False.”

YouTube: “John Ioannidis: The role of bias in nutritional research,” Swiss Re.

Marion Nestle, Paulette Goddard professor of nutrition, food studies, and public health, emerita, New York University.

JAMA online: “Associations of Dietary Cholesterol or Egg Consumption With Incident Cardiovascular Disease and Mortality”

JAMA online: “Reconsidering the Importance of the Association of Egg Consumption and Dietary Cholesterol With Cardiovascular Disease Risk.”

Arteriosclerosis, Thrombosis, and Vascular Biology: “A dose-response study of the effects of dietary cholesterol on fasting and postprandial lipid and lipoprotein metabolism in healthy young men.”

Arteriosclerosis, Thrombosis, and Vascular Biology: “Increases in dietary cholesterol are associated with modest increases in both LDL and HDL cholesterol in healthy young women.”

JAMA Internal Medicine: “Sugar Industry and Coronary Heart Disease Research: A Historical Analysis of Internal Industry Documents.”

The American Journal of Clinical Nutrition: “Diets with high-fat cheese, high-fat meat, or carbohydrate on cardiovascular risk markers in overweight postmenopausal women: a randomized crossover trial.”

Edward Archer, PhD, chief science officer, EnduringFX.

Bonnie F. Liebman, director of nutrition, Center for Science in the Public Interest.

Statement, U.S. Department of Agriculture.

P. Courtney Gaine, PhD, registered dietitian; president and CEO, The Sugar Association, Washington, D.C.

Marc Dresner, manager of marketing and communications, American Egg Board.

© 2019 WebMD, LLC. All rights reserved. View privacy policy and trust info