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　　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 sero-

　　tonin 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 secre-

　　tion. 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 tryp-

　　tophan 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. 来源:可可英语 //m.moreplr.com/sat/201208/194662.shtml