8. Muscular people inflate the survival rate of the fat group. BMI “doesn't differentiate between fat and muscle mass,” observes the Wall Street Journal. So people who exercise and gain muscle are counted as fatter, when really they’re just stronger.
9. Sick people depress the survival rate of the “normal” group. The JAMA analysis controlled for smoking and pre-existing disease, and it didn’t include anyone in a hospital or hospice. But critics aren’t mollified. They insist, according to the Associated Press, that the normal-weight sample unduly “included people too thin to fit what some consider to be normal weight, which could have taken in people emaciated by cancer or other diseases.” It would take further, more precise studies to falsify this assertion.
10. Overweight gets you more medical attention and intervention. Doctors’ belief that fat signals a health risk makes them more likely to scrutinize heavier patients for disease symptoms or risk factors. Lots of evidence suggests doctors treat these patients more aggressively, thereby reducing mortality. In this way, the medical profession’s assumption that weight correlates with illness makes that correlation less visible in mortality data.
11. Medicine has made fat less harmful. “New pharmacological therapies and invasive treatments for existing disease may prolong survival,” the JAMA editorial points out. These and other advances, particularly those that reduce cholesterol and blood pressure, “may account for the weakening of associations between obesity and mortality.”
12. Overweight doesn’t mean you’re getting fat. It means you’re resisting obesity. "In a society prone to both epidemic and increasingly severe obesity, it may be that those who manage to remain in the 'overweight' class are, in fact, those who are actually doing quite well," says David Katz, director of Yale’s Prevention Research Center. So instead of seeing these people as formerly thin folks with bad habits, we should see them as fat-prone folks with good habits. How this squares with the notion of overweight as a gateway to obesity—a warning echoed by Katz—isn’t clear.
On one level, these explanations sound weak and weaselly. Dogmas, even in science, don’t surrender easily to contrary evidence. Experts who think weight gain is dangerous will find ways to reaffirm that belief, explaining away data that don’t fit it. But science, in its grudging way, does evolve. The explanations offered today in defense of the fat-is-bad doctrine are actually modifications of it. They’re taking us beyond crude categories such as BMI, overweight, and fat. A decade from now, we’ll still believe fat is bad for you, but we’ll be far more sophisticated in what we mean by “bad” and “fat.” And the JAMA study’s critics, like its defenders, will take the credit.