Do you know whether eggs are good for you? What about coffee, red wine, or chocolate? Most people probably have a yes-or-no impulse about each of these things, thanks to the amount of media coverage given to studies looking for health benefits or detriments of individual foods. And no matter what you say, you’re probably right, according to at least some of that science—findings often reverse or contradict one another over time, even if the conflicting studies are all methodologically sound.
“Nutritional studies are extremely difficult to do, and it’s very hard to figure out what people are actually eating, even if you try your best,” said the journalist Christie Aschwanden, speaking on a panel at the Aspen Ideas Festival, co-hosted by the Aspen Institute and The Atlantic. “If I were to ask you how many times you ate tomatoes last year and what serving size tomato you ate, it’s very difficult to answer that question.”
When studies try to set a diet or even provide food themselves, studying nutrition can still be distressingly inexact for a very relatable reason: Study participants aren’t any better at sticking to a diet than anyone else. “Even if you wanted to take a large group of people and split them into two, what often ends up happening is the two groups are actually much more similar than they’re intended to be, because you have adherence problems,” Aschwanden said.