Second, laboratory rats will become obese on a high-fat diet. This is the evidence that convinced George Bray that excessive dietary fat would cause obesity in humans, too, and Bray has been among the most influential obesity authorities and the foremost proponent of this dietary-fat/obesity hypothesis. According to Bray, the rats used in his laboratory experiments would grow reliably obese on high-fat diets. “I could feed them any kind of composition of carbohydrates I want,” Bray said, “and in the presence of low fat, they don’t get fat. If I raised the fat content, particularly saturated fat, in susceptible [my italics] strains I would get obesity regularly.”

But some strains of rats, perhaps most of them, will not grow obese on high-fat diets, and even those that do will grow fatter on a high-fat, high-carbohydrate diet than a high-fat, low-carbohydrate diet. Moreover, to induce obesity even in susceptible rodents, the percentage of fat in the diet has to be greater than 30 percent, and usually closer to 40 or even 60 percent (which still makes only some strains of rats fat). Though 30 percent sounds like a low-fat or moderate-fat diet for humans, it’s far greater than anything rats would normally consume, either in the wild or in the laboratory. It’s what researchers will call a pharmaceutical dosage of fat. Rat chow is typically 2–6 percent fat calories. Rats will also fatten when fed large amounts of carbohydrates in the form of sugar. Moreover, other animals fatten on carbohydrates, including pigs—whose digestive apparatus is most like that of humans among experimental animals—cattle, and monkeys.

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.