Fernstrom J D, Fernstrom M H
Department of Psychiatry, UPMC Health System Weight Management Center, University of Pittsburgh School of Medicine, UPMC/Western Psychiatric Institute & Clinic, PA 15213, USA.
Nutr Rev. 2001 Aug;59(8 Pt 2):S60-5; discussion S66-8. doi: 10.1111/j.1753-4887.2001.tb05502.x.
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.
在0 - 10%蛋白质能量（PE）范围内，大脑中酪氨酸（TYR）的浓度随饮食蛋白质含量直接变化，但超过该范围则不然。这种影响很大：饮食蛋白质含量从0%增加到10%时，酪氨酸浓度可升高两到三倍。这种增加对儿茶酚胺合成速率有明显刺激作用，多巴胺（DA）和去甲肾上腺素（NE）均如此，尤其在下丘脑，这是一个参与食欲调节的脑区。色氨酸及其神经递质产物5 - 羟色胺（5HT）与长期饮食蛋白质摄入可能也存在类似关系。由于大鼠是此类研究中最常用的动物模型，其天然饮食通常含有6% - 14%的蛋白质，在不利环境条件下可能更少，野生大鼠可能经常处于蛋白质摄入曲线中使大脑酪氨酸（可能还有色氨酸）浓度产生最大变化的部分。如果是这样，那么儿茶酚胺和5HT的产生可能会受到类似影响。通过这种情况，大脑可能会接收到关于动物在饮食中获取足够蛋白质成功与否的信息。基于野生猴子的饮食习惯，也可以对它们提出类似观点，人类可能也是如此。从这个角度来看，假设动物根据阈值而非设定点模型来监测/调节蛋白质摄入量。这个概念并非全新的，也不是氨基酸所独有的。例如，目前关于瘦素作用的一种观点是，它在能量缺乏而非过剩时期作为能量摄入的信号发挥作用。更一般地说，鉴于在自然界中为了生存和繁殖获取足够食物的需求至关重要，而实现这一营养目标存在困难，食欲控制机制可能在自然界中进化，更多地围绕达到并超过充足水平，而不是将摄入量维持在远超充足水平的设定点附近。
