Less easy to imagine, though, is how anyone avoids this fate, particularly if we believe that the balancing of intake and expenditure is maintained not by some finely tuned regulatory system, one honed over a few million years of evolution to accomplish its task under any circumstances, but, rather, by our conscious behavior and our perspicacity at judging the caloric value of the foods we eat. Looked at this way, as Du Bois suggested, “there is no stranger phenomenon than the maintenance of a constant body weight under marked variation in bodily activity and food consumption.”

In 1961, the Cambridge University physiologist Gordon Kennedy discussed the paradoxes of obesity and weight regulation in the context of two propositions that he described as “common sense rather than physiology.” The first was that “there must be long-term regulation of energy balance.” The second was that “there is no a priori reason why this balance should be maintained by control of appetite alone, since it depends as much on calorie expenditure as on calorie intake.”

Like Kennedy, most researchers who studied metabolism and the science of bioenergetics and growth through most of the twentieth century assumed that energy balance must be regulated involuntarily, without conscious intent, and that the mechanisms that do so adapt both intake to expenditure and expenditure to intake. Our bodies work to minimize long-term fluctuations in energy reserves and maintain a stable body weight, and they do so, as with all our homeostatic systems, via what George Cahill of Harvard and Albert Renold of the University of Geneva in 1965 called “multiple metabolic control mechanisms.” This idea evolved in the 1970s into the popular set-point hypothesis, that our bodies will defend a certain preferred amount of body fat against either an excess or a deficit of calories. It fell out of favor because it implied that neither calorie-restricted diets nor exercise would lead to long-term weight loss.

The fundamental assumption of this idea that body weight is regulated homeostatically is that energy intake and expenditure are very much dependent variables—that they are physiologically linked so that a change in one forces a corresponding change in the other—and it is energy storage that is determined biologically within a certain range set by the interaction between genetics and the environment. Now the same law of energy conservation that decrees that calories in equal calories out, tells us that any increase in energy expenditure will have to induce a compensatory increase in intake, and so hunger has to be a consequence. And any enforced decrease in intake will have to induce a compensatory decrease in expenditure—a slowing of the metabolism and/or a reduction in physical activity.