Determination of the optimum dietary proportions of lysine and tryptophan for growing rats based on growth, food intake and plasma metabolites.

Weanling rats were divided into 24 groups and fed one of three levels of lysine and one of eight levels of tryptophan in a 3 X 8 factorial design to determine the optimum dietary proportions of lysine and tryptophan. The tryptophan "requirement" and the optimum lysine:tryptophan ratio were estimated for both adequate and subadequate lysine levels on the basis of weight gain, feed efficiency and changes in plasma levels of lysine, tryptophan and urea. The various criteria yielded similar estimates and indicated a tryptophan requirement of approximately 0.15% when lysine was adequate. The optimum lysine:tryptophan ratio was between 4 and 5 for the rapidly growing 3- to 6-week old rat.