Several variations on this hypothesis were published from the mid-1970s onward by Le Magnen and others. The most comprehensive account was published in 1976 by Edward Stricker at the University of Pittsburgh, and Mark Friedman, then at the University of Massachusetts and now at the Monell Chemical Senses Center in Philadelphia. Their article, “The Physiological Psychology of Hunger: A Physiological Perspective,” should be required reading for anyone seriously interested in eating behavior and weight regulation.

The hypothesis is based on three fundamental propositions. The first, as Friedman and Stricker explained, is that the supply of fuel to all body tissues must always remain “adequate for them to function during all physiological conditions and even during prolonged food deprivation.” The second proposition is Hans Krebs’s revelation from the 1940s that each of the various metabolic fuels—protein, fats, and carbohydrates—is equally capable of supplying energy to meet the demands of the body. The third is that the body has no way of telling the difference between fuels from internal sources—the fat tissue, liver glycogen, muscle protein—and fuels that come from external sources—i.e., whatever we eat that day.

With these propositions in mind, the simplest possible explanation for feeding behavior is that we eat to maintain this flow of energy to cells—to maintain “caloric homeostasis”—rather than maintain body fat stores or some preferred weight. If the cells themselves are receiving sufficient fuel to function, the size of the fat reserves is a secondary concern. As Friedman and Stricker explained, “Hunger appears and disappears according to normally occurring fluctuations in the availability of utilizable metabolic fuels, regardless of which fuels they are and how full the storage reserves.” In 1993, the Princeton physiological psychologist Bartley Hoebel described the hypothesis in terms that echoed the origins of the theory in the work of Claude Bernard: “The primitive goal of feeding behavior,” Hoebel explained, “is to maintain constancy of the nutrient concentration of the milieu intérieur.”

From this perspective, we’re not much more complicated than insects, which will seek out food and consume it until their guts are full. External taste receptors signal whether they’ve come upon something they can benefit from eating; gut receptors signal when sufficient food has been consumed to inhibit the hunger. The role of the brain is to integrate the sensory signals from the gut and the taste receptors and couple them to motor reflexes to initiate eating behavior or inhibit it. In both flies and mosquitoes, if the neural connection between gut and brain is severed, the insect loses its hunger inhibitor and continues to eat until its gut literally ruptures. As Edward Stricker explained in The New England Journal of Medicine in 1978, hunger is little more than a disturbing stimulus, like an itch, that “feeding behavior removes or attenuates.” Satiety, on the other hand, “is more than the absence of hunger; it is the active suppression of interest in food and of feeding behavior.”