Utilization of Amino Acids from Foods by the Rat

Abstract

Experiments were conducted to provide a test procedure for the quantitative evaluation of the utilization of tryptophan from foods using the male weanling rat. The method developed is of short duration and provides an adequate range of response to tryptophan supplementation of the diet. A tryptophan-deficient basal ration composed of 10% of oxidized casein + 4% of untreated casein, supplemented with cystine, methionine, and tyrosine, was used. The weight gain of rats fed the basal ration plus graded levels of L-tryptophan or known amounts of tryptophan in foods, was used as the criterion for determining the quantitative utilization of tryptophan from those foods. The repeatability of the results using different levels of the test foods, after varying experimental periods, and in independently repeated experiments gives support to the validity of the methods employed. Attempts to obtain improved growth response with supplements of niacin and threonine resulted in decreasing tryptophan utilization values as the amount of test food was increased. The percentage of tryptophan utilized for growth of the male weanling rat when fed raw and roast beef, ham and lamb, nonfat dry milk, overheated soybean meal, and split peas ranged from 75 to 107%. Tryptophan utilization results for unheated and normally-processed soybean meals, and rolled oats ranged from 117 to 132%. The results suggest that cooking or heat processing may result in reductions in the availability of tryptophan to support growth. The percentage of ingested tryptophan excreted in the feces ranged from 1 to 10%. The high values observed for tryptophan utilization in these studies gain added significance when compared to those observed in the earlier studies on lysine and methionine. The ranges in the observed amino acid utilization values for the foods tested were 49 to 98% for lysine, 48 to 83% for methionine, and 75 to 132% for tryptophan.