But some strains of rats, perhaps most of them, will
In the 1970s, Anthony Sclafani of Brooklyn College demonstrated that rats get “super obese” if allowed to freely consume a selection of foods from the local supermarket. This made their eating habits and subsequent obesity seem particularly like ours in character. But, as Sclafani explained, his rats fattened preferentially on sweetened condensed milk, chocolate-chip cookies, and bananas. Among the foods they didn’t eat to excess were cheese, pastrami, and peanut butter—the items that were high in fat and low in carbohydrates.
The third supporting leg of the hypothesis that fat is particularly fattening is an assumption that the density of fat calories fools people into eating too many. Density was originally invoked to explain why some rats would eat fat to excess and become obese. Because the fat used in these experiments is typically an oil—Crisco cooking oil poured over the rat chow—it was hard to imagine that palatability was the deciding factor. As a result, researchers suggested that the density of the fat calories—nine calories per gram, compared with four for protein and carbohydrates—fooled the rats into consuming too many.
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.”
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,
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
