The primary difference between humans and insects, by this logic, is that we have two primary fuel tanks (three if we include glycogen stored in the liver, and four if we include protein in the muscles), and they effectively have one. In our case, fuel is stored initially in the gut for the short term, and then in the fat tissue for the medium to longer term. The fat tissue extends the time we can go between meals by hours, days, or more. The fuel supply to the cells is maintained by the filling and emptying of both these energy reserves. “Energy metabolism,” Friedman and Stricker wrote, “is maintained by alternating tides of nutrients that sweep in from the intestines or adipose tissue at regular intervals depending on when food consumption occurs.” The fat tissue participates actively in metabolism by acting as an energy buffer: it provides storage for nutrients that arrive with the meal but are not immediately necessary for energy, and then it releases them back into the circulation as this absorptive phase is coming to an end. In effect, the fat tissue prevents dramatic shifts in the energy supply, which would otherwise be unavoidable considering the fact that, unlike cattle or sheep, we don’t graze continually but, rather, eat episodically in discrete meals.

We can think of eating and satiety as a cycle that begins with the meal and fills the gastrointestinal reserve—the gut. As nutrients are absorbed into the circulation, some are used for fuel immediately, and the rest restock the fat reserves, the glycogen reserves in the liver, and the protein in the muscles. As the gut empties, and this dietary fuel is either stored or oxidized, the fat reserves become the primary source of fuel. As the fat reserves begin to empty and the fuel flow shows signs of faltering, the inhibition of hunger is lifted, we are motivated again to fill the gut, and the cycle begins anew.

This “harmony of tissue metabolisms” is orchestrated by the hypothalamus, via the central nervous system and the endocrine system of hormones. These regulate the filling and emptying of the various storage depots in response to an environment that might require that we suddenly expend more or less energy, or store more or less fat, to accommodate seasonal variations. The hypothalamus does what the brains of insects do: it integrates sensory signals from the body and the rest of the brain, and couples them to motor reflexes that permit or restrain eating behavior. It also adjusts this filling and emptying of the fuel reserves to accommodate the immediate need for fuel and the anticipated need for fuel.

According to this hypothesis, weight stability is nothing more than an equilibrium between the fatty acids flowing into the energy buffer of the fat tissue and the fatty acids flowing out. What the body regulates, as Le Magnen suggested, is the fuel flow to the cells; the amount of body fat we accumulate is a secondary effect of the fuel partitioning that accomplishes this regulation.