Over the past century, numerous studies have addressed this issue of how much more easily some of us fatten than others. In these studies, volunteers are induced to overeat to considerable excess for months at a time. The most famous such study was conducted by the University of Vermont endocrinologist Ethan Sims beginning in the late 1960s. Sims first used students for his experiments, but found it difficult to get them to gain significant weight. He then used convicts at the Vermont State Prison, who initially raised their food consumption to four thousand calories a day. They gained a few pounds, but then their weights stabilized. So they ate five thousand calories a day, then seven thousand (five full meals a day), then ten thousand, while remaining sedentary.

There were “marked differences between individuals in ability to gain weight,” Sims reported. Of his eight subjects that went two hundred days on this mildly heroic regimen, two gained weight easily and six did not. One convict managed to gain less than ten pounds after thirty weeks of forced gluttony (going from 134 pounds to 143). When the experiment ended, all the subjects “lost weight readily,” Sims said, “with the same alacrity,” in fact, as that with which obese patents typically return to their usual weights after semi-starvation diets. Sims concluded that we’re all endowed with the ability to adopt our metabolism and energy expenditure “in response to both over-and undernutrition,” but some of us, as with any physiological trait, do it better than others.

Another overfeeding study, led by Claude Bouchard, who is now head of the Pennington Biomedical Research Center in Louisiana, was published in 1990. Bouchard and his colleagues overfed twenty-four young men—twelve pairs of identical twins—by a thousand calories a day, six days a week, for twelve weeks. The subsequent weight gain varied from nine to thirty pounds. The amount of body fat gained also varied by a factor of three. In 1999, James Levine from the Mayo Clinic reported that he had overfed sixteen healthy volunteers by a thousand calories a day, seven days a week, for eight weeks. The amount of fat these subjects managed to put on ranged from less than a single pound to almost nine; “fat gain varied ten-fold among our volunteers,” Levine reported.