Following the administration of theanine, the brain tryptophan content significantly increased or tended to increase, but the contents of serotonin and 5-hydroxyindole acetic acid (5HIAA) decreased. The use of inhibitors of serotonin metabolism enable us to speculate that theanine reduced serotonin synthesis and also increased serotonin degradation in the brain.
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Yokogoshi et al. (1998) studied this question.
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