This was in line with the belief that we match our intake to expenditure by simple mechanisms such as those that limit the volume of food consumed in a single meal. It also led to the notion that eating fiber-rich, leafy vegetables will prevent weight gain by filling our stomachs with fewer digestible calories than if we consumed the densely packed calories of fat or refined carbohydrates. The more rigorous experiments with laboratory animals, however, suggest otherwise. The seminal experiments on this question were done by the University of Rochester physiologist Edward Adolph back in the 1940s. Adolph diluted the diets of his rats with water, fiber, and even clay, and noted that the rats would continue to eat these adulterated diets until they consumed the same amount of calories they had been eating when he had fed them unadulterated rat chow. The more Adolph diluted the chow with water, the more the rats consumed—until the meals were more than 97 percent water. At these very low dilutions, the rats apparently expended so much energy drinking that they couldn’t consume enough calories to balance the expenditure. When Adolph put 90 percent of their daily calories directly into the rats’ stomachs, “other food was practically refused for the remainder of the twenty-four hour period.” Putting water directly into their stomachs had no such effect. Adolph’s conclusion was that rats adjust their intake in response to caloric content, not volume, mass, or even taste, and this is presumably true of humans as well.

The fourth piece of evidence is thermodynamic. The idea dates to the late nineteenth century and its revival by the University of Massachusetts nutritionist J. P. Flatt was coincident with the rise of the dietary-fat/heart-disease hypothesis in the 1970s. According to Flatt’s calculations, the “metabolic cost” of storing the calories we consume in adipose tissue—the proportion of energy dissipated in the conversion-and-storage process—is only 7 percent for fat, compared with 28 percent for carbohydrates. For this reason, when carbohydrates are consumed in excess, as the University of Vermont obesity researchers Ethan Sims and Elliot Danforth explained in 1987, the considerable calories expended in converting them to fat will “blunt the effect on weight gain of high-carbohydrate, high-caloric diets.” High-fat diets, on the other hand, would lead “to a metabolically efficient and uncompensated growth of the fat stores.” Flatt’s analysis omitted all hormonal regulation of fuel utilization and fat metabolism (as well as a half-century’s worth of physiological and biochemical research that we will discuss shortly) but it has nonetheless been invoked often during the last twenty years to make the point, as Sims and Danforth did, that obesity is yet another “penalty for living off the fat of the land rather than the carbohydrate.”