I grew up around serial dieters filled with rage, at themselves and their predicament. I can hardly remember a meal at which someone didn’t blow up about something, leaving the rest of us to tackle our brisket in silence. I flashed back to those depth-charged dinners when I read Lionel Shriver’s new novel, Big Brother, in which a man who has grown suddenly obese visits his younger sister. He shovels in the Cinnabons while her family watches with increasing disgust. Finally, her husband lets loose a zinger: “It’s not my fault that to snap a family photo I have to take three steps back to fit you in the frame."

The impulse to castigate the plus-sized—even when, especially when, they are us—runs very deep. And that’s sad for two reasons. First, by 2010, more than 40 percent of American adults and 17 percent of youths were obese or morbidly obese. By 2030, more than half of America’s adults and a third of its young people may fit that description. Second, we’re learning ever more reasons not to regard the corpulent as pathetic losers. Obesity clearly amounts to more than a failure of willpower. Even the staid American Medical Association agrees; it just upgraded obesity from a “condition” to a “disease.” Our knee-jerk irritation at the overweight, it turns out, may be the twenty-first-century equivalent of the nineteenth-century sniff at the tubercular (too neurasthenic to live) and the 1980s view of the HIV-positive (obviously depraved). “This is one of the larger trends in the history of medicine,” said Rudolph Leibel, a pioneer in obesity research at Columbia University. “First we blame it on the patient.”

If you’re the reformer type, you’ve stopped blaming patients and started blaming society—the food industry, the chemicals industry, globalism. But what if even this enlightened analysis of the obesity epidemic distracts us from a more pressing question? What if Michelle Obama’s “Let’s Move!” campaign and Michael Bloomberg’s sugary-drink ban divert attention from the problem that most needs solving?

The biggest mystery when it comes to obesity is not how to prevent it. It’s how to treat it. Don’t get me wrong. We need to know what expands our girth so that millions more don’t suffer the type 2 diabetes and heart disease that follow. But millions are obese, right now, and the medical establishment doesn’t really know how to help them. I learned that to my dismay when I tried to find a program for a relative who seemed too young for stomach bands and gastric bypasses. The problem is simple and well known. It’s hard but not impossible to lose weight. But it’s nearly impossible to keep it off.

An evolutionary theory exists (one always does) to explain why even heroic dieters swell back up. In the days when food was scarce, rapid weight loss signified starvation. To defend against that, our bodies still fix on a size and fight to keep it. So when we shed pounds, levels of an appetite-suppressing hormone called leptin drop. Levels of an appetite-enhancing hormone called ghrelin rise. The hypothalamus slows down our metabolism. Our muscles burn fewer calories, even when we’re sitting still. Another obesity pioneer, Jules Hirsch (a former collaborator of Leibel’s), once demonstrated how this works. He produced a spring and pulled it taut and thin. That, he said, is the body right after a diet. Then he let it go, and it sprang back into place. That was the body a little later on.