This could all sound depressing to anyone who has been battling her own hunger cues for years, but researchers like Cummings have a different view. "If you're born with obese genes," he says, "now you know it's not your fault. A lot of overweight people realize that their bodies have fought their efforts to lose weight, but those around them have always faulted their willpower."

For people to really curb hunger, Friedman anticipates drug therapies that manipulate (and outsmart) chemical signals—therapies that are already in the animal-testing stage. "We're looking at it the way you'd look at any other medical problem, like cancer," Friedman says. "You figure out how the system normally works, then figure out what's different when you have the disorder, then develop therapies." For hunger and overeating, Cummings says, one very promising research direction is a ghrelin blocker, now in preclinical studies, that would stop ghrelin from acting on the parts of the brain that trigger hunger. Researchers are also pursuing the use of PYY as a possible long-term treatment for obesity.

Once we have "the equivalent of Prozac for obesity," Friedman says, people may finally begin to believe that big appetites and big bodies are the result of more than "simply a set of bad lifestyle choices." But if, in the view of biologists like Cummings and Friedman, hunger is primarily an unbending fact of nature, how do they explain the enormous rise in obesity over the past two decades? It's not that our genes have altered in 20 years to make so many more of us fat, Friedman says. Rather, across the population there has been an average weight gain of seven to ten pounds in the past decade that's still within the approximately ten-pound range of variation that many people's set points allow them. This overall weight gain, attributable mainly to environment, he says, has pushed a certain percentage of the population over the border into the range of obese. Those whose genes gave them a blueprint for slenderness, meanwhile, have remained at much the same weight.

So how might a Prozac for hunger change this situation? Those who have a major wiring problem will benefit from drug treatment, and the rest will have to cope on their own—the same way people with the ordinary blues rather than clinical depression come to terms with their problems and learn behavioral strategies to feel better. Mentally, that could mean shooting for a reasonable, livable weight toward the low end of your set point. Practically, it means avoiding buffets or big loads of refined carbs, becoming sensitive to your personal hunger-rating scale, and simply stopping to think before overeating. "If you're craving a half-gallon of ice cream every night," Head says, "I wouldn't trust that craving." She suggests asking yourself, What else is going on in my life? Am I feeling lonely and depressed? If so, a half-gallon of ice cream isn't going to solve that—although a dish of ice cream, eaten slowly enough so that all those little satiation nerve signals get activated, might make a tasty compromise.

More on Healthy Eating