The same conclusion will be reached if our obese subject undergoes bariatric surgery. “This procedure alters gastrointestinal anatomy to reduce caloric intake beyond what could be achieved volitionally,” explains Jeff Friedman of Rockefeller University in a recent issue of Nature Medicine. “Although people who undergo bariatric surgery lose a significant amount of weight, nearly all remain clinically obese.” We will now have two individuals of more similar size and weight, one of whom needs a surgically altered gastrointestinal tract to reduce calorie intake so much that he can stay at that weight, and the other who doesn’t and can eat to his heart’s content. Our surgical patient is perceived as defective in character, having had to rely on surgery to curb his appetite. Our naturally lean subject is not, despite the possession of an identical appetite. “The implication,” as Friedman noted, “is that something metabolically different about morbidly obese individuals results in obesity independently of their caloric intake.”

Whatever the accepted wisdom, making obesity a behavioral issue is endlessly problematic. “Theories that diseases are caused by mental states and can be cured by will power,” as Susan Sontag observed in her 1978 essay Illness as Metaphor, “are always an index of how much is not understood about the physical terrain of a disease.” This is certainly the case with obesity. One goal of any discussion of the cause of obesity must be a way to think about it that escapes the facile and circular reasoning of the overeating/sedentary-behavior hypothesis and permits us to proceed in a direction that leads to real progress, to find a way of discussing the condition, as the philosopher of science Thomas Kuhn might have put it, that allows for a “playable game.”

Obesity researchers over the last century have struggled with this dilemma, but they failed to escape it, which is the inevitable consequence of circular logic. Von Noorden, for instance, sought to absolve the obese of character defects by suggesting that weight was gained so imperceptibly as to go unnoticed. He inaugurated the practice, ubiquitous today, of enumerating the subtle ways in which excess calories creep into our diet, or fail to be expended in our sedentary lives. Two hundred calories a day, he suggested, the content of five pats of butter or twelve ounces of beer, could easily slip into the diet unobserved and result in a weight increase, by his calculation, of nearly seventeen pounds a year. “These 200 calories represent such a small amount of food,” he explained, “that neither eyesight nor appetite afford any indication of it, and therefore the person can say to the best of his knowledge that his food-supply has not been altered, although he has obviously become corpulent.” Any such claims that obesity is caused by the slow and imperceptible accumulation of excess calories inevitably serves to blame obesity on the behaviors of overeating and inactivity, while avoiding the explicit accusation of a character defect. Such explanations also beg the question of how the victim managed to make the transition from lean through overweight to obese without noticing and then choosing to reverse the process.