This is especially relevant, too, at a time when many people are needlessly avoiding gluten, or simply think that carbs are bad.

“There is still some misconception about the role of carbohydrates in a healthy diet,” said Frank Hu, a professor of epidemiology and nutrition at Harvard, and one of the study’s authors. “Some people still believe that all carbohydrates are bad, and some people still promote a very low carbohydrate diet without strong scientific support.”

Hu sees this study as further evidence that the type of carbohydrate is “very important.”

Lauren Giordano / The Atlantic

Published in the American Heart Association’s journal Circulation, the new Harvard study is an analysis of 12 prior studies, as well as previously unpublished results drawn from the National Health and Nutrition Examination Survey. The combined studies involved 786,076 people and 97,867 total deaths.

This is a correlation, an epidemiological study—so people in Facebook timelines and comment threads will predictably shout that correlation is not causation. While true, the accusation is misplaced. Epidemiology may well be the most important type of research at our disposal to understand the role of food in chronic disease.

In many areas of science, the gold-standard approach is a randomized controlled trial. This works very well, for example, when testing drugs for short-term effectiveness and side effects. The effects of our food, though, tend to be too expansive to lend themselves well to the same studies. Chronic diseases (as the name implies) manifest not over weeks or months, but decades—longer than most research institutions can keep thousands of subjects adhering to any particular diet. And longer than most people would be willing to take part.

(Would you help us please by using only white bread for the rest of your life, and see what diseases you get or don’t get? Actually, wait, you can’t know that it’s white bread or that will ruin the experiment. Wear these dark sunglasses always? And let us cauterize your tongue?)

So it is reliable and worthwhile to know that long-lived, healthy people tend to eat a lot of whole grains.

The study did not, however, differentiate between milled grains and whole grains that tend to be eaten whole—quinoa, farro, amaranth, and the like. I asked Hu what the deal was there.

“That's a really good question,” he said. “We don't have a sufficient amount of data to address it.” But, like any good scientist, he was willing to speculate: “When whole grain is milled and becomes whole-wheat flour, the digestion and absorption process is still fast. And that can induce higher insulin responses. Theoretically, that kind of product is less beneficial compared to whole grains that are minimally processed, or not processed at all.”

Those insulin responses correspond with a measure known as glycemic index, essentially the speed at which glucose enters our blood upon eating. Pixy Stix are high, and broccoli is low. Eating a lot of foods with high glycemic indices is known to be associated with diabetes, obesity, heart disease, and even liver damage. (A newsy randomized clinical trial in JAMA suggested otherwise in 2014, but that study lasted just five weeks.) It’s not a perfect metric, but an interesting one.