The diets may seem severe in the retelling, but, in fact, fourteen to sixteen hundred calories a day for weight loss could be considered generous compared with the eight-to-twelve-hundred-calorie diets that are now commonly prescribed, what the 1998 Handbook of Obesity refers to as “conventional reducing diets.” Nonetheless, such diets were traditionally known as semi-starvation diets, a term that has fallen out of use, perhaps because it implies an unnatural and uncomfortable condition that few individuals could be expected to endure for long.

In both experiments, even after the subjects lost weight and were merely trying to maintain that loss, they were still required to eat considerably fewer calories than they would have preferred, and were still beset by what Keys and his colleagues had called the “persistent clamor of hunger.” Of equal importance, simply restraining their appetites, independent of weight loss, resulted in a dramatic reduction in energy expenditure. This could be reversed by adding calories back into the diet, but then any weight or fat lost returned as well. One lesson learned was that, for the weight reduction to be permanent, some degree of semi-starvation has to be permanent. These experiments indicated that would never be easy.

Obese patients also get hungry on semi-starvation diets. If they have to restrict their calories to lose weight, then by definition they are forcing themselves to eat less than they would otherwise prefer. Their hunger is not being satisfied. As with lean subjects, their energy expenditure on a semi-starvation diet also “diminishes proportionately much more than the weight,” as the Pittsburgh clinician Frank Evans reported in 1929 of his obese subjects. This same observation was reported in 1969 by George Bray, who was then at the Tufts University School of Medicine in Boston, and who entitled his article, for just this reason, “The Myth of Diet in the Management of Obesity.” “There is no investigator who has looked for this effect and failed to find it,” the British obesity researcher John Garrow wrote in 1978.

The latest reiteration of these experiments, using obese subjects, was conducted by Jules Hirsch at Rockefeller University, and the results were published in The New England Journal of Medicine in 1995. Calorie restriction in Hirsch’s experiment resulted in disproportionate reductions in energy expenditure and metabolic activity. Increasing calorie consumption resulted in disproportionate increases in metabolic activity.