Liquid concentrates are lower in bioavailable tryptophan than powdered infant formulas, and tryptophan supplementation of formulas increases brain tryptophan and serotonin in rats

The bioavailability of tryptophan in powdered and/or liquid concentrate for ms of milk-based infant formulas was determined by studying rat growth resp onse by using a slope ratio method (food conversion efficiency: weight gain /food consumed vs, tryptophan consumed). A gelatin basal diet formulated to be adequate in all nutrients, except tryptophan (0.03%), for rat growth wa s supplemented with graded levels of crystalline L-tryptophan (0.02, 0.04, 0.06, 0.08, 0.10, 0.12 and 0.14%, standard diets) or infant formulas provid ing 0.04 and 0.08% supplemental tryptophan (test diets). These diets were f ed to weanling rats for 2 wk. Tryptophan bioavailabilities of various formu las varied from 83 to 95%, with some of the liquid concentrates having the lowest values. The levels of bioavailable tryptophan in the liquid concentr ate forms (9.7-12.6 mg/g protein) and the powdered forms (11.1-13.1 mg/g pr otein) were considerably lower than those of human milk (17-19 mg/g protein ). Supplementation of the liquid concentrates with graded levels of L-trypt ophan (0.1, 0.5 and 1.0%) had no effect on protein quality indices, based o n rat growth, but resulted in a dose-related increase in the concentrations of tryptophan in the plasma and brain and of serotonin and 5-hydroxyindole -3-acetic acid in the brains of rats. This study supports further research to investigate the influence of tryptophan supplementation of infant formul as, to more closely simulate tryptophan composition of human milk, on trypt ophan metabolites and their potential related effects on sleep latency and neurobehavioral developments in infants.