Chocolate is bad for you. No, wait, studies show chocolate boasts health-promoting antioxidants! Eggs are rich in cholesterol, which is bad for your heart. Oh, but they also offer protein, vitamin D, and choline, so you’d do well to have one a day.
Red meat may be carcinogenic, so don’t eat it — unless you’re going paleo. And what about red wine? Wheat? The list goes on and on.
Sifting through all this conflicting research in an effort to keep up with health trends can lead to a kind of nutritional whiplash. How on earth can we know what’s best for us? And why is it so hard for scientists to come to a consensus on matters of nutrition?
Understand the Process
For starters, science is constantly changing and evolving. So when you read a headline stating that a once-demonized food may actually be OK after all, it shouldn’t undermine your faith in nutrition research altogether.
On the contrary, it’s evidence of the scientific method in action: Knowledge is built slowly over time by seeking evidence to support or counter a particular theory. And when we know better, as the saying goes, we do better.
The name of the game in nutrition science is to find a causal relationship between a nutrient and an illness or health benefit. It’s extremely hard to do, in part because the research methods themselves are replete with limitations.
In medical research, the gold standard is the randomized controlled trial (RCT). Researchers test a drug on one group of people, give a placebo to another group, and compare the results. It’s “random” because the researchers decide randomly who gets the drug and who gets the placebo. That way there’s no selection bias, which is why the method is so well regarded.
“Nutrition researchers would love to only do randomized controlled trials,” says Bonnie Liebman, MS, director of nutrition at the Center for Science in the Public Interest. Ideally, researchers would keep the participants in a research facility, where they could control confounding factors like exercise and sleep habits. Subjects would be fed a particular diet over a period of months or even years.
Obviously, Liebman adds, “You just can’t keep people under those controlled conditions for months. There’s just a limit.”
That’s one reason that, in the field of nutrition, RCTs aren’t usually a practical option. While they afford greater control over participants, they typically involve small samples and last only a few weeks or months.
That’s a problem in nutrition research, explains Walter Willett, PhD, professor of epidemiology at the Harvard T. H. Chan School of Public Health. “In most instances, it takes decades of a good or bad diet before you see the impact on disease risk.”
A more practical research method is an observational study in which researchers ask participants to record what they’ve eaten over a set time period. This generally provides reliable and detailed data. But it also presents possibilities for human error: It’s easy to forget about that second cup of coffee or underestimate the serving of pasta you ate for dinner. Those little inaccuracies can add up over time.
What’s more, these types of studies can’t control for innumerable confounding factors that influence health outcomes. In an observational study about eggs, for example, even if every person in a group eats the same number of eggs over a period of weeks, perhaps one eats more sugar than the rest. Perhaps one is a smoker, while another is an avid triathlete. It can be difficult to separate the effect of the food in question from the effect of lifestyle choices.
Ultimately, the best way to study nutrition is probably a combination of the two research methods. “We want as high a level of certainty as we can get,” Willett explains, “and in general, we can do that by combining different kinds of studies.”
Consider the Source
Many nutrition studies are actually funded by food-industry associations, which is worth considering when you’re evaluating their validity, says renowned nutrition expert Marion Nestle, MPH, PhD, author of Unsavory Truth: How Food Companies Skew the Science of What We Eat.
“As government funding for nutrition science has declined, food companies have moved in to fill the gap,” Nestle explains. “This is unfortunate because industry-funded research almost invariably comes out with results that favor the sponsor’s interest, and the researchers do not even realize that this is happening.”
Indeed, a 2007 study found that the majority of studies funded by the related industry generally published their findings only if they were favorable to their industry.
The commonly cited research stating that chocolate is good for you? Much of it was cosponsored by Mars, Inc., maker of Snickers and M&M’s.
The study released in 2019 that found that eating processed and red meat isn’t so bad for you after all? Its leader had financial ties to the International Life Sciences Institute, a trade group whose members have included executives from Cargill, one of North America’s largest beef processors. (For more on the food industry’s outsize influence in nutrition science, see “Is Dark Chocolate Actually Good for Us?”and “Decoding Health Media”.)
Not all industry-funded studies are influenced by their backing, however. In one 2019 Stanford University study comparing the health effects of meat with those of plant-based alternatives, the funder — in this case, Beyond Meat — agreed to remain unaware of the results before they were published.
Do Your Homework
So how can we cut through all the confusion and determine what information is reliable? It comes down to doing a careful read of the study and following your nose. Experts offer these guidelines.
Don’t believe the hype. “A healthy dose of skepticism is always a good idea, especially when a study claims to be a breakthrough or to overturn everything you thought you knew,” Nestle explains. “That’s not how science works.”
Articles that fly in the face of conventional wisdom may be headline-grabbing, but according to Liebman, one study alone does not constitute enough evidence to refute what previous studies have found. “The media wants what we call ‘man bites dog’ headlines because that’s what gets readers’ attention — clickbait,” she says.
Go straight to the source. If you’re trying to evaluate recent nutrition research, the best practice is to read the study itself. Don’t be intimidated by the jargon and data — if you read the abstract and conclusion, you should be able to get a sense of the research methods and results.
The conclusion will also often place the findings in the context of established knowledge in the field and point to further research that needs to be done. Plus, studies are supposed to list any possible conflicts of interest due to funding.
Find outlets you trust. The impulse for media to publish clickbait comes from the pressure to survive in an incredibly competitive environment, Willett explains — but there’s still responsible reporting to be found. Liebman says, “A good reporter will a) disclose any industry funding or industry ties to the researcher, and b) try to get an opinion from a researcher who wasn’t involved in the study or a researcher who can comment on the evidence as a whole.”
Check the prevailing wisdom. Liebman recommends looking for endorsements of the research findings from major health authorities, like the American Heart Association, the American Cancer Society, or the American Diabetes Association. This isn’t a completely foolproof method — these associations are often hampered with limitations of their own (see “Rethinking Heart Health”). But as part of your larger research strategy, seeing what the authorities have to say can offer you some clues.
Dig deeper. Most studies will list funding sources in the acknowledgments section, which can reveal who’s sponsoring the research. “If it’s a study on breakfast cereal and weight gain, for example, did the NIH [National Institutes of Health] fund it or did Kellogg’s pay for it?” writes Mark Hyman, MD, in his book Food Fix: How to Save Our Health, Our Economy, Our Communities, and Our Planet — One Bite at a Time. “Google the organization. See who is behind it. Be a sleuth.”
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