Obese patients who try to reduce their weight by semi-starvation, as Rony noted, will always be fighting what he called their “spontaneous impulses of eating and activity.” Once they give in to these impulses, which is effectively preordained, they will get fat again. This is exactly what we would expect to see if obesity were merely a consequence of an underlying disorder, much as high blood sugar and glycosuria—i.e., sugar in the urine—are symptoms and consequences of diabetes. Consuming fewer calories can serve only to address the symptoms temporarily, just as with diabetes. It does not remove the underlying abnormality.

This is why the long-term failure of semi-starvation diets is significantly more informative about the true nature of obesity than is the short-term weight loss. This failure is an important “clue to the puzzle,” as Bruch suggested in 1955. The obese, Bruch noted, “react exactly like normal people after starvation. They continue overeating.” This drive to become fat can be inhibited or even temporarily reversed by restricting calories—just as a child’s growth can be stunted by starvation or malnutrition—but in neither case will the caloric deprivation address the metabolic and hormonal forces at work.

Just as we will decrease energy expenditure in response to caloric deprivation, we will also increase expenditure in response to caloric surplus. This compensatory effect of overeating was also demonstrated in the late nineteenth century by Carl von Voit and Max Rubner, although they disagreed about the mechanisms at work. It has since been encapsulated in a German word, Luxuskonsumption, which means a spendthrift metabolism that wastes excess calories as heat or superfluous physical activity. The term was first used in this context in 1902 by the German physiologist R. O. Neumann, who spent three years studying how his own body weight responded to extended fluctuations in caloric intake. Luxuskonsumption was Neumann’s explanation for the apparent disassociation between the calories he consumed and the ease with which he maintained his weight.

Through the first half of the twentieth century, this capacity for Luxuskonsumption was assumed to be a critical factor in the genesis of obesity or leanness. To borrow Gordon Kennedy’s phrase, this seemed like common sense rather than physiology. “Food in excess of immediate requirements and not needed to replenish stores can be readily disposed of, being burnt up and dissipated as heat,” wrote David Lyon and Sir Derrick Dunlop, clinicians at the Royal Infirmary of Edinburgh, in 1932. “Did this capacity not exist, obesity would be almost universal.” And so the ability to burn up small excesses, they observed, on the order of a few hundred calories a day, is “well within the capacity of the ordinary person, but in the obese individual the power of flexibility is much less evident.”