What the Vermont investigators failed to take into account, however, was their own previous observation that the nutrient composition of the diet seemed to affect profoundly the desire to consume calories to excess. One potentially relevant observation that Sims and his colleagues neglected to publish, for example, was that it seemed impossible to fatten up their subjects on high-fat, high-protein diets, in which the food to be eaten in excess was meat. According to Sims’s collaborator Edward Horton, now a professor of medicine at Harvard and director of clinical research at the Joslin Diabetes Center, the volunteers would sit staring at “plates of pork chops a mile high,” and they would refuse to eat enough of this meat to constitute the excess thousand calories a day that the Vermont investigators were asking of them. Danforth later described this regimen as the experimental equivalent of the diet prescribed by Robert Atkins in his 1973 diet book, Dr. Atkins’ Diet Revolution. “The bottom line,” Danforth said, “is that you cannot gain weight on the Atkins diet. It’s just too hard. I challenge anyone to do an overfeeding study with just meat. You can’t do it. I think it’s a physical impossibility.”

Getting their volunteers to add a thousand calories of fat to their daily diet also proved surprisingly difficult. Throughout their numerous publications, Sims and his colleagues comment on the “difficult assignment of gaining weight by increasing only the fat.” Those fattening upon both carbohydrates and fat, on the other hand, easily added two thousand calories a day to their typical diet. Indeed, subjects in some of his studies, Sims and his colleagues reported, experienced “hunger late in the day…while taking much greater caloric excesses of a mixed diet”—as much as ten thousand calories a day.

Sims and his collaborators evidently did not wonder why anyone would lose appetite—develop “marked anorexia,” as they put it—on a diet that includes eight hundred to a thousand excess fat calories a day, and yet feel “hunger late in the day” on a diet that includes six to seven thousand excess calories of fat and carbohydrates together. It would seem there is something about carbohydrates that allows the consumption of such enormous quantities of food and yet still induces hunger as the night approaches.