Partial alleviation of phenylalanine toxicity in the chick by supplemental dietary tryptophan

Studies were conducted to determine the physiological effects of feeding diets containing high levels of phenylalanine to chicks. Marked reductions in weight gain and feed conversion were observed in birds fed a diet containing 2.52% L-phenylalanine and .47% L-tyrosine. Excess dietary phenylalanine caused elevations of phenylalanine and tyrosine in serum and brain, whereas cerebral concentrations of free isoleucine, leucine, and valine were generally decreased. Supraoptimal amounts of glycine, arginine, and tryptophan were fed in an attempt to alleviate the toxic symptoms observed in birds fed high phenylalanine diets. Only tryptophan supplementation proved successful in partially alleviating the growth-depressive effects of phenylalanine toxicity. Both serum and brain levels of tryptophan were increased by tryptophan supplementation. Results of two radiotracer studies with 14C-tryptophan suggested that brain uptake of tryptophan was inhibited by hyperphenylalaninemia.