There’s been a lot of confusion over the years about what constitutes a healthy breakfast. In the 1970s, carbs were in, fat was out, and the only omelet considered healthy was one made with egg whites and asparagus. In the 1990s, carbs became public enemy number one, and suddenly a cheese-filled egg scramble was considered optimal for health and energy. Now plant-based diets are on the rise, and a healthy breakfast might skip eggs altogether in favor of an oatmeal bowl or a green smoothie with plenty of nuts and seeds.
Eggs are the subject of one of the longest-standing and most contentious debates in nutrition, thanks to dietary cholesterol: one large egg yolk has 186 milligrams, making it one of the richest sources of the nutrient out there. In 1977, the government started recommending low-fat, low-cholesterol diets, because research at the time indicated that dietary cholesterol elevated bad blood cholesterol (LDL). Eggs were dubbed indulgent and unhealthy.
Then in 2013, the American Heart Association announced that limiting dietary cholesterol didn’t lower a person’s LDL cholesterol after all, changing its official stance. The 2015–20 U.S. dietary guidelines followed suit, stating that dietary cholesterol was “not a nutrient of concern for overconsumption.” A 2018 meta-analysis in Nutrients went further, explaining that saturated fat had been responsible all along for elevated LDL cholesterol and increased heart-disease risk. Eggs have just over one gram of saturated fat and thus were back in favor.
But in March 2019, a study in the Journal of the American Medical Association flip-flopped again, concluding that eating an egg a day is linked to a significantly higher risk of heart disease after all.
Each of these pivots was based on legitimate nutrition research, which raises the question: How is it possible that nutrition experts—doctors, dietitians, and researchers alike—disagree so often on the basic tenets of nutrition science? How do they study the same questions but come up with such different answers? How can an everyday person make sense of it all? Here’s a primer on the ever changing world of nutrition science and why you should take most of it with a grain of salt.
Data Collection Is Flawed
For that 2019 JAMA study, researchers analyzed data from six previous studies for a total of 29,615 adults monitored for an average of 17.5 years. All of the studies used self-reported baseline-diet data, meaning subjects recorded what they ate on a daily or weekly basis at the start of a study. Researchers then drew conclusions based on the assumption that subjects ate roughly this way every day for the remainder of the study.
There are, of course, some major issues here. It’s a stretch to assume that someone’s eating habits will stay the same over the course of several years (or decades). Beyond that, people tend to misrepresent what they eat. “Self-reported diet data is fraught with errors,” says Connie Weaver, a nutrition professor at Purdue University. “People do not report what they eat accurately, because they don’t remember, don’t know how to judge portion size, don’t know the ingredients, or don’t want to admit the snacking they do.” And the majority of nutritional studies rely on self-reported data.
In an ideal world, Weaver says, there would be controlled-feeding studies, where researchers prepare every meal and know exactly what subjects are eating at all times. These do exist, but they’re time intensive and expensive. Realistically, they can only be used for short-term smaller studies, which can’t asses long-term chronic-disease risk and are too small to be applicable to the general population.
Food Doesn’t Exist in a Vacuum
When a study sets out to evaluate a single nutrient, like dietary cholesterol, it’s impossible to know for sure that the cholesterol is causing an effect on its own. Nutrients don’t act alone, and the presence of one can affect the impact of another. “For example, calcium absorption is influenced by vitamin-D status,” Weaver says. Likewise, eating a lot of fiber likely has a positive influence on heart-disease risk, while eating a lot of saturated fat likely has a negative influence—so it would be hard to study the effects of either of these nutrients in someone who eats fiber-rich whole grains and vegetables daily but also regularly eats saturated fat-laden red meat.
Even if scientists could engineer single-nutrient foods and measure and track diet with precision, the research would still be complicated by the fact that food is just one of many factors that impact health. “The biggest problem in nutrition science is that we can’t reduce whatever effect we’re looking at to one component,” says Linda Bacon, researcher and author of Health at Every Size. “Exercise, relationships, sleep, stress, and a long list of other things will affect health beyond nutrition. And yet when you’re doing a study on nutrients, all you’re looking at is nutrition.” Maybe someone is eating eggs for breakfast every day but also has a stressful job and never gets more than six hours of sleep. If this person has heart-health issues down the line, it’s impossible to say that eggs are the culprit.
Researchers do often take into account factors like race, socioeconomic status, and gender. It’s widely accepted that these things can be stronger determinants of health than diet, Bacon says. But other influencing factors are nearly impossible to control for, like chronic mental-health problems, food anxiety, and genetics.
Everyone’s Needs Are Different
Every five years, the U.S. Departments of Agriculture and Health and Human Services’ dietary guidelines committee examines the existing body of research to determine which eating pattern is the most health promoting for most people. It’s the closest thing we have to a consensus on a “perfect” diet. But these guidelines—which currently recommend limiting your sugar intake, eating plenty of plants, and getting at least part of your protein intake from eggs—are generalized public-health advice and won’t necessarily work for everyone.
Our bodies all react differently to various foods, dietary cholesterol among them. “I would never tell someone that reducing their dietary cholesterol would reduce their blood cholesterol without first knowing their medical history,” says Kevin Klatt, a molecular-nutrition scientist. Individuals have varying tolerance for dietary cholesterol, based on genetics or chronic diseases like diabetes. Blood work is the only way to find out how you, individually, might be impacted. Overall, Klatt considers eggs a neutral food. He explains that dietary cholesterol is fine for most people in moderate amounts—about an egg a day, maybe two.
Contradictory nutrition science isn’t going anywhere. Nor are the debates about eggs or the endless Twitter threads about low-fat versus low-carb diets. But that doesn’t mean that you should live by general nutrition advice.
“Your body can give you a lot of great information,” Bacon says. She uses fiber as an example: if you don’t eat enough, you’ll likely feel sluggish and constipated. That’s a signal to eat more fiber in the forms of whole foods. The same goes for sugar. It’s fine to eat it, but too much too often might make you feel lethargic. Instead of trying to keep up with changing nutrition science, pay attention to how food makes you feel, and you’ll likely find that you end up eating in a way that supports good health.
If you have chronic health problems that you suspect are related to food, a more personalized approach to nutrition science might help. Talk to a doctor or dietitian about your symptoms, and work with them (through blood work, allergy tests, and other diagnostic tools) to figure out if a specific dietary change might be what’s best for you. Whatever you do, though, try and ignore the headlines.