It was once believed that the brain was independent of metabolic processes occurring elsewhere in the body. In recent studies, however, we have discovered that the production and release in brain neurons of the neurotransmitter serotonin (neurotransmitters are compounds that neurons use to transmit signals to other cells) depend directly on the food that the body processes.
Our first studies sought to determine whether the increase in serotonin observed on rats given a large injection of the amino acid tryptophan might also occur after rats ate meals that change tryptophan levels in the blood. We found that, immediately after the rats began to eat, parallel elevations occurred in blood tryptophan, brain tryptophan, and brain serotonin levels. These findings suggested that the production and release of serotonin in brain neurons were normally coupled withblood-tryptophan increases. In later studies we found that injecting insulin into a rat’s blood and brain tryptophan levels. We then decided to see whether the secretion of the animal’s own insulin similarly affected serotonin production. We gave the rats a carbohydrate-containing meal that we knew would elicit insulin secretion. As we had hypothesized, the blood tryptophan level and the concentrations of tryptophan level and the concentrations of tryptophan and of serotonin in the brain increased after the meal.
Surprisingly, however, when we added a large amount of protein to the meal, brain tryptophan and serotonin levels fell. Since protein contains rtyptophan, why should it depress brain tryptophan levels? The answer lies in the mechanism that provides blood tryptophan to the brain cells. This same mechanism also provides the brain cells. This same mechanism also provides the brain cells with other amino acids found in protein, such as tyrosine and leucine. The consumption of protein increases blood concentration of the other amino acids much more, proportionately, than it does that of tryptophan. The more protein is in a meal, the lower is the ratio of the resulting blood-tryptophan concentration to the concentration of competing amino acids, and the more slowly is tryptophan provided of the brain. Thus the more protein in a meal, the less serotonin subsequently produced and released.
以前，我们认为大脑独立于身体其它部位进行新陈代谢。但是，最近研究发现大脑神经细胞递质血清素(递质是神经细胞间用来传递信息的化学物质)的产生和释放与身体消化的食物有直接关系。
我们的第一项研究是为了确定在给大鼠注射大量色氨酸后所观察到的递质增加是否同样会出现在给大鼠饲喂能引起血液色氨酸含量提高的食物后。我们发现大鼠刚开始进食，其血液中的色氨酸，大脑中的色氨酸和大脑中的血清素水平都相应提高。这些迹象表明大脑神经递质血清素的产生和释放通常是与血液色氨酸含量的增加紧密相关。随后的研究发现给大鼠注射胰岛素也会使其血液中色氨酸，大脑中色氨酸和大脑中递质血清素水平都相应提高。然后我们决定看看大鼠自己分泌的胰岛素是否会对递质血清素的产生有相似的影响。于是，我们给大鼠喂含糖的食物，知道这会引起胰岛素分泌。正如我们所推测的，血液中的色氨酸水平和大脑中色氨酸与递质血清素的浓度都在喂食后提高了。
然而，出人意料的是，当在大鼠的食物中加入大量蛋白质时，大脑中色氨酸和递质血清素的水平却降低了。既然蛋白质含有色氨酸，大脑中的色氨酸水平为什么会降低呢?原因在于由血液向脑细胞提供色氨酸的机制。这一机制还向脑细胞提供蛋白质中的其它氨基酸，如酪氨酸和亮氨酸。蛋白质的食入使血液中其它氨基酸的浓度相对色氨酸提高了许多。食入的蛋白质越多，血液中色氨酸相对其它氨基酸的浓度就越低，色氨酸向大脑的提供就越慢。所以食入的蛋白质越多，随之产生和释放的递质血清素就越少。
