The concentration of TYR in brain changes directly with dietary protein content in the 0-10% PE range, but not higher. The effect is large: TYR concentrations rise as much as two- to threefold between 0% and 10% dietary protein content. This increase produces a clear stimulation of the rate of catecholamine synthesis, observed both for DA and NE, and notably in the hypothalamus, a brain area involved in appetite regulation. A similar relationship to chronic dietary protein intake may also exist for tryptophan and its neurotransmitter product, 5HT. Because the natural diet of rats, the animal model most commonly used in such studies, typically contains between 6% and 14% protein, and may contain less under unfavorable environmental circumstances, rats in the wild may frequently operate on the portion of the protein intake curve producing maximal changes in brain TYR (and perhaps TRP) concentrations. If so, then the production of catecholamines and 5HT may be similarly affected. By such a scenario, the brain might receive information regarding the animal's success in acquiring adequate amounts of protein in its diet. A similar argument can also be made for monkeys in the wild, based on their dietary habits, and thus possibly for humans. From this perspective, animals are hypothesized to monitor/regulate their intake of protein based on a threshold, rather than a set-point model. This notion is not new or unique to amino acids. For example, one current notion of leptin action is that it serves as a signal for energy intake important during periods of deficiency, but not excess. More generally, given the primacy in nature of the need to acquire adequate amounts of food in order to survive and reproduce, and the difficulty in achieving this nutritional goal, it may be that appetite control mechanisms have evolved in nature to center more on attaining and exceeding adequacy than on maintaining intake around a set-point well in excess of adequacy.