Effects of pure macronutrient ingestion on plasma tryptophan and large neutral amino acids

The role of tryptophan and its competitor large neutral amino acids, proposed earlier for serotonin synthesis following carbohydrate or protein ingestion, was reassessed in relation to a recent study investigating serotonin release, including the so far unknown effects of fats. In the present study, meals of either carbohydrates, casein, or lard, were supplied to rats for 30 min and blood samples collected every 15 min to follow the changes in plasma large neutral amino acids. In response to carbohydrates, amino acid levels fell and the ratio tryptophan over sum of other amino acids increased. Following casein ingestion, all amino acids were enhanced, tryptophan somewhat less, leading to a decreased ratio. The lard meal induced a slight decrease in some amino acids while the ratio remained constant. Only in response to casein, and partly to carbohydrates, did a consistent relation appear between the previously observed serotonin changes and the ratio. These data suggest that a relationship between the ratio and the previously observed serotonin changes is not always encountered because the release is not obligatorily coupled to synthesis and is subject to behavioral influences. It remains that serotonin release is affected by the composition of the meal through peripheral metabolic mechanisms.