Nitrogen Retention Studies on Rats, Dogs and Man; The Effect of Adding Methionine to an Enzymic Casein Hydrolysate

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Abstract
The addition of cystine or methionine to a casein hydrolysate given as the sole source of nitrogenous food increased the rate of growth in rats. When the casein hydrolysate was given by vein to dogs, the addition of methionine increased nitrogen retention, whereas the addition singly of the other 9 essential amino acids had no such effect. In 4 groups of human subjects, the addition of methionine to a casein hydrolysate given as the sole source of nitrogenous food, did not increase nitrogen retention. This was true in surgical patients to whom the material was given intravenously, in infants fed a luxury level of nitrogen, and in well adults fed a maintenance nitrogen level. When 4 human subjects were protein-depleted for 21 days, the subsequent administration of minimal amounts of an enzymic casein hydrolysate, with and without extra methionine, gave identical nitrogen retention. These data are interpreted to mean that there is a species difference in the requirement for sulfur-containing amino acids, and that the human requirement for them is less than the requirement of the rat and dog, due presumably to a greater need for cystine in the building of hair. The generally recognized nutritive difference between casein and lactalbumin is valid for the rat and for the dog, but not for man.