Acute Effects of Meals on Brain Tryptophan and Serotonin in Humans

In the rat, protein and carbohydrate meals have opposite effects on brain serotonin, leading to the rather surprising conclusion that a neurotransmitter, serotonin, which is involved in the control of a number of important mental functions, can be influenced by the diet.

Abstract

Under normal circumstances, tryptophan hydroxylase, the rate-limiting enzyme on the pathway from tryptophan to serotonin, is about half saturated with tryptophan. This is true for both rats (Grahame-Smith, 1971) and humans (Young and Gauthier, 1981). As a result, alterations in brain tryptophan can influence serotonin synthesis. Brain tryptophan is in some circumstances altered by food intake, leading to the rather surprising conclusion that a neurotransmitter, serotonin, which is involved in the control of a number of important mental functions, can be influenced by the diet. As discussed below, in the rat, protein and carbohydrate meals have opposite effects on brain serotonin. There is also evidence that serotonin can influence macronutrient selection, leading to the suggestion that serotonin may be a part of a system regulating dietary intake (Wurtman and Wurtman, 1986). Thus, a carbohydrate meal would raise brain serotonin which would inhibit subsequent carbohydrate selection, thereby ensuring that protein and carbohydrate intakes would stay within certain limits over the long run.