ON FEBRUARY 7, 2003, THE EDITORS OF Science published a special issue dedicated to the critical concerns of obesity research. It included four essays written by prominent authorities, all communicating the message of the toxic-environment hypothesis of the obesity epidemic and the belief that obesity is caused by “consuming more food energy than is expended in activity.” The one article that offered a potential solution to the national and global problem of burgeoning waistlines—other than the promise of future obesity-fighting drugs—was written by James Hill of the University of Colorado, John Peters of Procter & Gamble, and two colleagues. Hill and Peters introduced the concept of an “energy gap” that could purportedly explain the existence of the obesity epidemic and illuminate a path of action by which it might be halted or reversed. By their calculation, the obesity epidemic represented an energy gap of a hundred calories per person among the American public per day that had been consumed but not expended. To undo the epidemic, Hill and Peters suggested, Americans would have to make either comparable increases in daily energy expenditure—walking one extra mile, perhaps—or decreases in energy consumption, such as “eating 15% less (about three bites) of a typical premium fast-food hamburger.” Two years later, when the U.S. Department of Agriculture released the sixth edition of its Dietary Guidelines for Americans, it offered similar advice based on the identical logic: “For most adults a reduction of 50 to 100 calories per day may prevent gradual weight gain.”

This proposition should evoke a distinct sensation of déjà vu, because it is the precise argument that Carl von Noorden made over a century ago. Hill, Peters, and the USDA authorities, like von Noorden, were treating the regulation of body weight as though it were a purely arithmetical process, in which a small excess of calories consumed, day in and day out, accumulates into pounds of flesh and then tens of pounds, and a small deficit, day in and day out, does the opposite. That this argument is now the cornerstone of the official U.S. government recommendations for obesity prevention made the single caveat in Hill and Peters’s Science article all that much more remarkable. Speaking of the hundred-calorie energy gap, they said that their “estimate is theoretical and involves several assumptions”—in particular, “Whether increasing energy expenditure or reducing energy intake by 100 kcal/day would prevent weight gain remains to be empirically tested.”