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.”

To understand the path of events that leads to obesity, “the big question,” as Bruch noted, was “why the metabolism is shifted in the direction of storage away from oxidation?” Why is fat deposited in the adipose tissue to accumulate in excess of its mobilization for fuel use? Once again, this has little to do with calories consumed or expended, but addresses the questions of how the cells utilize these calories and how the body regulates its balance between fat deposition and mobilization, between lipogenesis (the creation of fat) and lipolysis (the breakdown of triglycerides into fatty acids, their escape from the fat tissue, and their subsequent use as fuel). “Since it is now assumed that the genes and enzymes are closely associated,” Bruch wrote in 1957, “it is conceivable that people with the propensity for fat accumulation have been born with enzymes that are apt to facilitate the conversion of certain reactions in that direction.”