All those who have insisted (and still do) that overeating and/or sedentary behavior must be the cause of obesity have done so on the basis of this same fundamental error: they will observe correctly that positive caloric balance must be associated with weight gain, but then they will assume without justification that positive caloric balance is the cause of weight gain. This simple misconception has led to a century of misguided obesity research.

When the law of energy conservation is interpreted correctly, either of two possibilities is allowed. It may be true that overeating and/or physical inactivity (positive caloric balance) can cause overweight and obesity, but the evidence and the observations, as we’ve discussed, argue otherwise. The alternative hypothesis reverses the causality: we are driven to get fat by “primary metabolic or enzymatic defects,” as Hilde Bruch phrased it, and this fattening process induces the compensatory responses of overeating and/or physical inactivity. We eat more, move less, and have less energy to expend because we are metabolically or hormonally driven to get fat.

In 1940, Hugo Rony, former chief of the endocrinology clinic at Northwestern University’s medical school, discussed this reverse-causation problem in a monograph entitled Obesity and Leanness, which is easily the most thoughtful analysis ever written in English on weight regulation in humans.*86 Rony’s goal, as he explained it, was to “separate recognized facts from suggestive evidence, and reasonable working hypothesis from mere speculations.” This set Rony apart from Louis Newburgh, Jean Mayer, and others who were more interested in convincing their colleagues in the field that their speculations were correct.