The controlling factors in this movement of fat to and from the fat tissue have little to do with the amount of fat present in the blood, thus little to do with the quantity of calories consumed at the time. Rather, they must be controlled, Wertheimer wrote, by “a factor acting directly on the cell,” the kind of hormonal and neurological factors that Julius Bauer had discussed. Over the next decade, investigators would begin to refer to these factors that increase the synthesis of fat from carbohydrates and the deposition of fat in the adipose tissue as lipogenic, and those that induce the breakdown of fat in the adipose tissue and its subsequent release into the circulation as lipolytic.

The second phase of this revolution began in the 1930s, with the work of Hans Krebs, who showed how our cells convert nutrients in the bloodstream into usable energy. The Krebs cycle, for which Krebs shared the Nobel Prize in Medicine in 1953, is a series of chemical reactions that generate energy in the mitochondria of cells, which are those compartments commonly referred to as the “power plants” of the cells. The Krebs cycle starts with the breakdown products of fat, carbohydrates, and protein and then transforms them into a molecule known as adenosine triphosphate, or ATP, which can be thought of as a kind of “energy currency,” in that it carries energy that can be used at a later time.*113 This cycle of reactions will generate energy whether the initial fuel is fat, carbohydrates, or protein. Indeed, Krebs had initiated his research assuming, as was common at the time, that carbohydrate was “the main energy source of muscle tissue.” But he came to realize that fat and protein also supply fuel for muscle tissue, and that there was no reason why carbohydrates should be the preferred fuel. “All three major constituents of food supply carbon atoms…for combustion,” he wrote.