“Unessential” Nitrogen: A Limiting Factor for Human Growth

Abstract

Four infants were fed decreasing quantities of milk protein with constant calories, in order to ascertain the most limiting amino acid of the diet. It was found that when weight gain and nitrogen retention eventually suffered, this could be restored to normal by the administration of unessential nitrogen in the from of glycine or urea. Thus unessential nitrogen appears to be the most limiting factor. The incorporation of unessential nitrogen into the hemoglobin and plasma proteins of infants fed a low-protein intake was demonstrated by studies in which N¹⁵ urea and N¹⁵ ammonium chloride were given. In one instance a further reduction of protein intake was attempted in order to discover the second most limiting nitrogenous factor. Some evidence was obtained that this was methionine. The implications of these observations for the determination of protein requirements, for the evaluation of protein quality and for the practical problem of supplementation are discussed.