Why Nutrition Advice Keeps Changing

Saturated fat is in, red wine is out. But you still need to eat your veggies.

Posted Sep 03, 2020

Once upon a time, the American Heart Association (AHA) told us to stick to three eggs a week. The fear was that the cholesterol in eggs would raise the cholesterol in your blood and increase your risk of heart disease. Now we read that eating three eggs a day could actually help your lipid profile, according to one study.

Diet advice changes, but that doesn’t mean it’s silly. Instead, think of nutrition as like a detective story, with twists and turns. Over time, scientists are developing a better understanding of how food affects us. 

Nutrition is a fairly young science; the first studies tracking what people ate came in the late 1800s. We’re watching it grow up. 

Remember that studies tend to focus on a single food or nutrient in order to reach a specific conclusion—but it’s rare that your health will be dramatically affected by one food item. So for example, a 2017 study found that over about 20 years, eating red hot chili pepper lowered the chance of dying by 12 percent. That doesn’t mean you’ll live into your 100s if you eat red hot chili pepper at every meal and otherwise eat junk. It does suggest that if you like red hot chili pepper, it might be a health plus.

Judge the source. But first, you need to judge the quality of a study. This one drew upon data from a government survey, pulling out 16,179 U.S. adults who ate red hot chili peppers. That’s much better than a study based on say, 500 people’s responses online on a private website. It was published in PLOS One, a top peer-reviewed journal (peer review means other scientists not involved in the research evaluated the study). The authors are affiliated with a medical school, not a red-pepper industry group.

It’s a lot of work to read studies carefully, so a better bet is to trust news from reputable health organizations or journalism groups that maintain standards for reliable sources. Don’t trust a statement because someone on Facebook sent it to you or you saw it on Reddit. Your friend might be giving you a good summary of a good study, but you’d have to read the study.

Also, no one study is the final word. Remember that surprising or hopeful results, even in a small study, will attract attention. Scientists need to see many studies of high quality, especially large ones, to draw conclusions. They also need to look at the many studies you never hear about because their conclusions weren't exciting.  

Although we see the zigzags, mainstream advice actually changes slowly. In 2015, a panel working on the official U.S. Guidelines dropped warnings against high-cholesterol food like eggs only after the evidence accumulated over decades.

Epidemiology is just the start. One common reason diet advice changes: It was based on early comparisons between large populations. In the 1980s, for example, French epidemiologists noticed that people in France had low heart disease rates despite eating foods high in cholesterol and saturated fat—thought to raise your heart risk. They also drank red wine at dinner. Could red wine be protecting them? A series of studies of red wine followed and we read stories urging us to drink red wine or take supplements of resveratrol, potentially wine’s magic ingredient. Research about resveratrol in mice was exciting, too.

But as it turned out, you’d have to drink enormous amounts of red wine to get an equivalent dose to the amount that helped rats. Studies of the effects of resveratrol supplements or high resveratrol diets in humans didn’t show the expected heart benefits.

Meanwhile, the evidence turned against the original premise about saturated fat. This year scientists reported the results of a 10-year study, which found no link between heart attacks and eating saturated fat. Eating polyunsaturated oils instead actually bumped up your heart risk. Polyunsaturated oils are common in processed food. The French may fare better because they eat less of the junk based on soybean oil filling U.S. supermarkets.

Oh, but maybe red wine is okay anyway? Wouldn’t we see the French dying of something else if it was bad? Although that’s not bad reasoning, a huge international study concluded in 2018 that the best amount of alcohol to drink is zero. 

Don’t listen to the nutrition industry. Business is business. Anyone who has built a career around a specific diet or supplement can’t avoid a conflict of interest if he gives any individual diet advice. Never become an ambassador for a product or you’ll be in the same position, urging people to follow your program even though you don’t know much about that person’s nutritional status or risks.

See the big picture. Americans would be healthier if we ate more fruits and vegetables and fewer sweets, carbs and processed foods. Follow those basic rules, keeping your weight low and your exercise up, and you don’t need to be glued to nutrition news. But you might find it fascinating. When advice evolves, we know that the system is working. Scientists are testing each other’s ideas and giving doctors new information.

A version of this story appears on Your Care Everywhere.