The tryptophan requirement of the rat as affected by niacin and level of dietary nitrogen

Abstract

The effects of niacin and level of dietary nitrogen upon the tryptophan requirement of the rat were investigated. Niacin had a marked tryptophan-sparing effect. With adequate niacin in the diet, the tryptophan required for normal growth at dietary protein levels of 9, 10, or 10.8% of protein were respectively 0.10, 0.12, and 0.13% of the diet. When niacin was omitted from the diet, the tryptophan required for normal growth at a 10.8% level of dietary protein increased to 0.19% of the diet. When the level of dietary protein was increased to 19–20% by the addition of gelatin or casein hydrolyzate, the tryptophan requirement was increased to 0.17% of the diet in the presence of adequate niacin. Without supplementary niacin and with casein used as the source of tryptophan, the tryptophan requirement for normal growth was 0.30% of the diet. A marked depression of growth was consistently demonstrated as a result of adding tryptophan-deficient protein or protein hydrolyzate to diets, even in the presence of excess niacin. The growth depression was corrected by the addition of dl-tryptophan. The primary effect of such tryptophan-deficient materials was to decrease the efficiency of utilization of tryptophan and hence to increase the requirement for this amino acid. The requirement of the rat for tryptophan is not a constant value. It increases as the level of dietary nitrogen increases, but the relationship does not appear to be linear. The implications of these results in practical nutrition are discussed.