This hypothesis is a reformulation based on animal research of what had been considered a fact of life in pre–World War II nutrition textbooks, that “weight loss triggers the dual pressures of increased food intake and decreased caloric expenditure,” as Stunkard put it. Nonetheless, obesity authorities have typically considered it unacceptably nihilistic. “It is not appealing from the therapeutic point of view,” as Stunkard said, “because it sounds kind of…hopeless. If you’re fat and your set point is elevated, you’re in bad shape.” And, of course, if we’re fat, or very fat, it is difficult to argue that our set point is not elevated. Moreover, the hypothesis simply failed to explain how the brain manages to monitor our fat stores, and then raise or lower food intake and energy expenditure in response. Saying that we’re all endowed with a lipostat that monitors our adiposity and then regulates hunger appropriately is just another way of saying that our weight remains remarkably stable, whether we’re lean or obese, and then assigning the cause to a mysterious mechanism in the brain whose function is to achieve this stability.

The more fundamental criticism is that the concept of a set point or a lipostat has little precedent in physiology, whereas the long-term stability of body weight can be explained by a much simpler mechanism that does. Life is dependent on homeostatic systems that exhibit the same relative constancy as body weight, and none of them require a set point, like the temperature setting on a thermostat, to do so. Moreover, it is always possible to create a system that exhibits set-point-like behavior or a settling point, without actually having a set-point mechanism involved. The classic example is the water level in a lake, which might, to the naïve, appear to be regulated from day to day or year to year, but is just the end result of a balance between the flow of water into the lake and the flow out. When Claude Bernard discussed the stability of the milieu intérieur, and Walter Cannon the notion of homeostasis, it was this kind of dynamic equilibrium they had in mind, not a central thermostat like regulator in the brain that would do the job rather than the body itself.