The Evolution of Obesity

8 Appetite and Satiety

186 CHAPTER EIGHT Appetite and Satiety We eat food; we need nutrients. When we eat, how much, how often, what kind of food depends on many things. But regardless of when, how much, and so on, one aspect of our eating is to satisfy our nutrient requirements. An organism has specific nutrient requirements, and evolution has produced adaptations to enhance intake of some, but not all, of those nutrients when they are scarce. The most general, hunger, motivates an animal to eat food; thirst motivates an animal to drink water. A specific appetite for salt (sodium) has been well documented in many species (Richter, 1936; Denton, 1982; Schulkin, 1991; Fitzsimons, 1998). Briefly, sodium deficiency (or significant water loss) results in the stimulation of the renin-angiotensin system in the kidney, which acts peripherally to conserve water and sodium, and to induce release of the steroid aldosterone from the adrenal gland. Aldosterone crosses the blood-brain barrier and induces central angiotensin, which, through various brain circuits, results in motivated behaviors to acquire water and salt. The effects include changing the hedonic perception of salt, so foods and solutions that taste salty become preferred (see Rozin and Schulkin, 1990; Schulkin, 1991; Fitzsimons, 1998 for reviews). Sodium appetite is a nice example of a number of important concepts in feeding biology. It is an example of communication and coordination between peripheral organs (in this case the kidney) and the brain; physiology and behavior are in synch; need is matched to motivation and behavior . It is an example of the same peptide (angiotensin) having complementary functions in periphery (regulating physiology to conserve sodium) and brain (motivating behavior to find and ingest sodium). The number APPETITE AND SATIETY 187 of information molecules is large, but it is a finite number and could even be considered small compared with the complexity of living organisms. These information molecules have varied functions in diverse tissues under diverse conditions, and it is that diversity of function that enables the complexity if life. The idea of specific appetites for nutrients has intellectual appeal, but, except for sodium, has been difficult to demonstrate (Richter, 1957; Rozin, 1976). For instance, calcium deficiency can result in an appetite for calcium , but it also induces a sodium appetite that in many ways appears more robust than the preference for calcium (Schulkin, 2001). In another example, although rats fed a thiamine-deficient diet readily chose a thiamine -replete alternative diet when offered it, the evidence suggests that rats have no inherent ability to detect thiamine. Rather, the behavior of the thiamine-deficient rats is best explained as a combination of a learned aversion to the deficient diet and a learned association with recovery from malaise for the new diet (Rozin and Schulkin, 1990). Rats that have been made thiamine deficient can learn to avoid the taste of the thiamine-deficient food and prefer the taste of the thiamine-replete food, even though the tastes are arbitrary. If the flavorings are switched then the rats will, for a time, prefer the thiamine-deficient food and avoid the thiamine-replete food (Rozin, 1976), although eventually they will once again learn to avoid the flavor of the thiamine-deficient diet. Animals have many strategies to guide their food-choice behavior, with the strategies differing among species and among nutrients (Rozin, 1976). Time scale is an important consideration. The murkiness of the calcium story may be due in part to the fact that calcium stores (bone) are large compared with need. The consequences of short-term dietary calcium deficiency are much less serious than an equal time period of sodium or water insufficiency. The acute response to calcium deficiency primarily involves conservation and mobilization of stores; only after the deficiency becomes chronic does a behavioral component appear (Schulkin, 2001). Hunger in human beings, as a response to an energy deficit, probably falls in between these two time scales: not as immediate as a response to sodium depletion or serious water loss, but more rapid than in the case of calcium deficiency. We are relatively large animals, so we can store a significant amount of our energy requirement. However, the manner in which the energy deficit is incurred will impact the physiological and be- 188 THE EVOLUTION OF OBESITY havioral response as well. The hunger...