This notion of obesity as a compensatory expansion of the fat tissue came as a revelation to Pennington: “It dawned on me with such clarity that I felt stupid for not having seen it before.” By working through the further consequences of this compensatory process, Pennington said, all the seemingly contradictory findings in the field suddenly fit together “like clockwork.”

This defect in fat metabolism would explain the sedentary behavior typically associated with obesity, and why all of us, fat or lean, will become easily fatigued when we restrict calories for any length of time. Rather than drawing on the fat stores for more energy, the body would compensate by expending less energy. Any attempt to create a negative energy balance, even by exercise, would be expected to have the same effect.

Clinicians who treat obese patients invariably assume that the energy or caloric requirement of these individuals is the amount of calories they can consume without gaining weight. They then treat this number as though it were fixed by some innate facet of the patients’ metabolism. Pennington explained that this wasn’t the case. As long as obese individuals have this metabolic defect and their cells are not receiving the full benefit of the calories they consume, their tissues will always be conserving energy and so expending less than they otherwise might. The cells will be semi-starved, even if the person does not appear to be. Indeed, if these individuals are restraining their desire to eat in an effort to curb, if possible, still further weight gain, this inhibition of energy expenditure will be exacerbated.