The effect of lowering plasma tryptophan on food selection in normal males

The effects of a tryptophan deficient amino acid mixture on food selection were studied using a double-blind counterbalanced crossover design in normal male subjects. The subjects ingested tryptophan deficient or nutritionally balanced amino acid mixtures in the morning after an overnight fast. Five hours after the tryptophan deficient amino acid mixture plasma tryptophan was only 19% of the level found five hours after the nutritionally balanced amino acid mixture. After both mixtures subjects were allowed to select lunch from a buffet. The tryptophan-deficient mixture was associated with a modest but significant decline in protein selection with no significant alteration in selection of carbohydrate, fat or total kcal. Our results suggest that 5-hydroxytryptamine is involved in the control of protein selection in humans.