Niacin-tryptophan deficiency resulting from amono acid imbalance in non-casein diets.

The suppression of rat growth observed by several investigators (l-4), who have supplemented 9 per cent casein-sucrose diets with tryptophandeficient proteins or their derivatives, has been traced to the amino acids these preparations provide (4-6). Specifically, threonine, phenylalanine, and cystine or methionine have been shown to be involved (7-9). Since 9 per cent casein provides marginal amounts of each of these amino acids as well as tryptophan, it was postulated (9) that tryptophan was spared for conversion to nicotinic acid or its nutritional equivalent when other amino acids such as threonine limited the rate of protein synthesis. As others amino acids are added, tryptophan assumes the r81e of the most limiting amino acid, with the result that its conversion to niacin is reduced and growth is decreased. If this hypothesis is correct, non-casein diets of suitable amino acid composition should give results similar to those just described. Growth should be better with tryptophan and some other essential amino acid provided at marginal levels than when only tryptophan is marginal. The data presented here demonstrate that imbalances involving valine and lysine with zein diets, and threonine with acid-hydrolyzed casein diets, also reduce the growth rate if the tryptophan is held at this critical level of approximately 0.1 per cent of the diet. This provides strong evidence for the validity of the mechanism proposed.