But whether just a few extra pounds raise the risk of death is a surprisingly controversial and polarizing issue. Usually, nutrition scientists tell journalists hedgy things like, “this is just what my study shows,” followed by the dreaded disclaimer: “Further research is needed.” But on this question, the researchers involved are entrenched, having reached opposite conclusions and not budging an inch. Like many internecine wars, the dispute mostly comes down to one small thing: how you define the “overweight” population in the study.

Over the years, myriad side controversies—personal attacks, money from the Coca-Cola Company, and a debate over who is truly “overweight”—have deepened the divide. But they haven’t clarified things.

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It all started in 2004, when the Centers for Disease Control and Prevention scientists published a study suggesting obesity was responsible for 400,000 deaths a year, making it almost as deadly as smoking. It turned out to be a false alarm: The authors made methodological errors that skewed their number too high.

But a CDC senior scientist named Katherine Flegal was already working with a small group of her colleagues to write a different obesity paper using better data and better methods. In 2005, they published their results, and their estimate was substantially lower: Obesity was only responsible for about 112,000 excess deaths. They also found something peculiar. Being “overweight,” but not obese, was not associated with an increased risk of death at all.

Millions of despairing dieters likely sighed with relief, perhaps celebratorily pouring a SlimFast down the drain. But while Flegal’s study was praised by some researchers, others were skeptical, saying past research had already shown that the heavier you are, the greater your risk of dying. "We can't afford to be complacent about the epidemic of obesity," JoAnn Manson, the chief of preventive medicine at Brigham and Women's Hospital in Boston, told The New York Times after Flegal’s study came out.

Flegal pressed on, and in 2013 she and colleagues published a meta-analysis—a study of studies—that replicated her earlier findings. Even when adjusting for smoking, age, and sex, overweight people—those with a body mass index of between 25 and 30—had a 6 percent lower risk of dying than normal-weight individuals. Body mass index, or BMI, is a measure of a person’s weight divided by their height. Her paper found that in terms of mortality, it’s better for this number to be slightly elevated than to be normal. A 5-foot-6-inch woman, in other words, would be better off weighing 180 pounds than 120.

A “pile of rubbish” is what Walter Willett, a Harvard University professor of epidemiology and nutrition, deemed that paper. Willett has co-authored studies finding the opposite effect. He and Andrew Stokes, a demographer at Boston University, say Flegal’s work suffers from a problem they call “reverse causality.” They think that because she didn’t examine her subjects’ entire weight history, her study didn’t control for people who used to be overweight, but became normal-weight because they got sick before they died. They argue her study conflates normal-weight, healthy people with formerly overweight people who lost weight due to liver disease, cancer, or some other illness. Having those individuals in the pool of normal-weight people makes the normal-weight people seem sicker, and the overweight people seem healthier, than they actually are.