Like much of the established wisdom on diet and health, this conclusion was based on very little experimental evidence. In this case, its only supporting evidence came from Sims’s overfeeding studies. These began in the mid-1960s with four small trials that led to the observation that some people will gain weight easily and others won’t, even when consuming the same quantity of excess calories. Another half dozen trials followed, each with only a handful of subjects, intending to shed light on what Sims and his collaborators called the “obvious question” of whether a carbohydrate-rich diet, independent of the calories consumed, could raise insulin levels, cause obesity, and induce hyperinsulinemia and insulin resistance. Sims and his collaborators varied the composition of the diets that their volunteers would then eat to excess. Some diets were “fixed carbohydrate” regimens, in which the amount of fat was increased as much as possible but the carbohydrates were limited to what the subjects would have normally consumed in their pre-experiment lives; others were “variable carbohydrate” regimens, in which both fat and carbohydrates were added to excess.

In the mid-1970s shortly after finishing their research, Sims and Danforth believed that obesity was most likely caused by chronically elevated levels of insulin, and that the elevated levels of insulin were likely the product of carbohydrate-rich diets. In the 1980s, their opinions changed and fell into step with the prevailing consensus on the evils of dietary fat. Sims and Dansforth now found in their decade-old results an observation that supported Flatt’s argument that it was thermodynamically more efficient to fatten on fat than on carbohydrates. When excess calories were provided in the form of fat alone, they now explained, the subjects converted a greater proportion of the excess into body fat than when the excess calories included both fat and carbohydrates. “Simply stated, when taken in excess, fat is more fattening than carbohydrate,” Danforth wrote in 1985. “Therefore, if one is destined to overeat and desires to suffer the least obesity, overindulgence in carbohydrate rather than fat should be recommended.” “In view of these considerations and the tendency toward overnutrition in most affluent societies,” he added, “main attention should be toward reducing both caloric and fat intake.”

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.”