Hirsch and his colleagues interpreted their observations to mean that the human body seems surprisingly intent on maintaining its weight—resisting both weight gain and weight loss—so that the obese remain obese and the lean remain lean. As Hirsch explained it, the obese individual appears to be somehow metabolically normal in the obese state, just as Keys’s and Benedict’s young men were metabolically normal in their lean or overweight states before their semi-starvation diets. Once Hirsch’s obese subjects took to restricting their calories, however, they experienced what he called “all the physiological and psychological concomitants of starvation.” A semi-starvation diet induces precisely that—semi-starvation—whether the subject is obese or lean. “Of all the damn unsuccessful treatments,” Hirsch later said, “the treatment of weight reduction by diet for obese people just doesn’t seem to work.”

Over the course of a century, a paradox has emerged. Obesity, it has been said, is caused, with rare exceptions, by an inability to eat in moderation combined with a sedentary lifestyle. Those of us who gain excessive weight consume more calories than we expend, creating a positive caloric balance or a positive energy balance, and the difference accumulates as excessive pounds of flesh. But if this reconciles with the equally “indisputable” notion that “eating fewer calories while increasing physical activity are the keys to controlling body weight,” as the 2005 USDA Dietary Guidelines for Americans suggest, then the problems of obesity and the obesity epidemic should be easy to solve. Those few individuals for whom obesity is a preferred condition, such as sumo wrestlers, would remain obese through their voluntary program of overeating, and the rest would create a negative energy balance, lose the excess weight, and return to leanness. The catch, as Hirsch pointed out, is that this doesn’t happen.