None would be forthcoming, and a century of empirical evidence would be rendered irrelevant, as the AMA’s spin on Banting’s low-carbohydrate diet as fad was quickly adopted as the conventional wisdom, one that has been adhered to faithfully ever since. Dietary fat had been identified as a probable cause of heart disease, and low-fat diets were now being advocated by the American Heart Association as the means of prevention. At the same time, the low-fat diet as the ideal treatment for weight loss was adopted as well, even though a low-fat diet was, by definition, high in the very carbohydrates that were once considered fattening.

This transformation is all the more remarkable because the medical authorities behind it were concerned with heart disease, not obesity. They presented no dramatic scientific data to support their beliefs, only ambiguous evidence, none of which addressed the efficacy of low-fat diets in weight loss. What they did have was the diet-heart hypothesis, which proposed that the excessive consumption of fat in our diets—particularly saturated fats—raises cholesterol levels and so causes atherosclerosis, heart disease, and untimely death. The proponents of this theory believed that Americans—and later the entire developed world—had become gluttons. Americans ate too much of everything—particularly fat—because we could afford to, and because we could not or would not say no. This overnutrition was certainly the cause of obesity. Eating too many calories was the problem, and since fat contains more than twice as many calories per gram as either protein or carbohydrates, “people who cut down on fat usually lose weight,” as the Washington Post reported in 1985.

A healthy diet, by definition, had suddenly become a low-fat diet. Beginning in the late 1980s with publication of The Surgeon General’s Report on Nutrition and Health, an entire research industry arose to create palatable nonfat fat substitutes, while the food industry spent billions of dollars marketing the less-fat-is-good-health message. The U.S. Department of Agriculture’s (USDA’s) booklet on dietary guidelines, and its ubiquitous Food Guide Pyramid, recommended that fats and oils be eaten “sparingly,” while we were now to eat six to eleven servings per day of the pasta, potatoes, rice, and bread once considered uniquely fattening.