下面GMAT频道为大家整理了GMAT阅读练习：Passage 39，供考生们参考，以下是详细内容。
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 neuro(5) transmitter 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 in rats given a large injec(10)tion 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 find-(15) ings suggested that the production and release of serotonin in brain neurons were normally coupled with blood-tryptophan increases. In later studies we found that injecting insulin into a rat’s bloodstream also caused parallel elevations in blood and brain tryptophan levels(20) and in serotonin 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 (25) level and the concentrations of tryptophan 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 tryptophan,(30)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 with other amino acids found in protein, such as tyrosine and leucine. The consumption(35) of protein increases blood concentration of the other amino acids much more, proportionately, than it does that of tryptophan. The more protein in the meal, the lower is the ratio of the resulting blood-tryptophan concentration to the concentration of competing amino(40) acids, and the more slowly is tryptophan provided to the brain. Thus the more protein in a meal, the less serotonin subsequently produced and released.