Growth and Food Intake of Rats Fed Tryptophan-imbalanced Diets with or without Niacin

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Abstract
The addition of a 7.8% L-indispensable amino acid mixture lacking tryptophan to an 8% casein, niacin-adequate diet supplemented with 0.3% L-methionine consistently produced a growth retardation and food intake depression in young rats during the first few days of feeding that was prevented only by supplementation with tryptophan. The growth followed a definite pattern similar to that of other imbalances: viz, an initial growth depression followed by a gradual adaptation and subsequent growth. When a tryptophan imbalance was induced in a niacin-free, low casein diet (8% casein + 0.3% L-methionine) by the addition of 0.18 to 0.8% L-threonine, results were more variable, growth depression was less during the first few days, but more severe after several weeks. The results were more predictable when 2 to 3% of an amino acid mixture lacking tryptophan was added to the niacin-free diet. When the niacin-free imbalanced diet was fed, the growth pattern was quite different from the imbalance induced in niacin-adequate diets. The weight gain was initially increased for 2 to 3 days, then suddenly stopped, and the weight of the rats remained stationary for several weeks. After 6 to 8 weeks, the rats manifested niacin deficiency signs; niacin supplementation corrected both the growth and symptoms. It is concluded that the metabolic basis for the growth depression from the two types of imbalances is different: the first caused by a metabolic lack of tryptophan whereas the second is caused by a metabolic lack of niacin.