Good Calories, Bad Calories

Among their discoveries is that both dietary fat and a considerable portion of the carbohydrates we consume are stored as fat—or, technically, triglycerides—in the adipose tissue before being used for fuel by the cells. These triglycerides are then continuously broken down into their component fatty acids, released into the bloodstream, moved to and from organs and tissues, regenerated, and merged with fatty acids from the diet to reform a mixture of triglycerides in the fat cells that is, as Schoenheimer put it, “indistinguishable as to their origin.” Fat stored as triglycerides in the adipose tissue, and the fatty acids and triglycerides moving through the bloodstream are both part of the same perpetual cycle of fat metabolism. “Mobilization and deposition of fat go on continuously, without regard to the nutritional state of the animal,” as the Israeli biochemist Ernst Wertheimer explained in 1948, in a seminal review of this new science of fat metabolism.^†112 “The ‘classical theory’ that fat is deposited in the adipose tissue only when given in excess of the caloric requirement has been finally disproved,” Wertheimer wrote. Fat accumulates in the adipose tissue when these forces of deposition exceed those of mobilization, he explained, and “the lowering of the fat content of the tissue during hunger is the result of mobilization exceeding deposition.”

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.

By 1950, the addition of the Krebs cycle to the revelations about fat metabolism from Schoenheimer and others provided the foundation for understanding the fundamental mechanisms that assure a constant supply of energy to our tissues and organs, regardless of how the demand might change in response to the environment and over the course of seconds, hours, days, or seasons. It is based on a generator—the Krebs cycle—that burns fat, carbohydrates, and protein with equal facility, and then a supply chain from the adipose tissue that ensures the circulation of fuels at a level that will always be more than adequate for the needs at hand. “The high degree of metabolic activities present in the fat tissues,” as Hilde Bruch explained, “becomes understandable as necessary for a continuous reserve for energy requirements. Instead of a savings account for unneeded surplus, as fat deposits have commonly been described, a coin purse would be a far closer analogy. Fat tissues contain the ready cash for all the expenditures of the organism. Only when the organism does not or cannot draw on the ready cash for its daily business is it put into depots, and excessive replenishment, through overeating, takes place.”