Nitrogen Retention in Men Fed Varying Levels of Amino Acids from Soy Protein with or without Added L-Methionine

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Abstract
A metabolic study was undertaken to determine methionine requirements of six young men fed soy protein isolate (SB) for 9-day periods. Formula diets contained 9 g of nitrogen from intact proteins with glycine and alanine added. Nitrogen derived from intact protein was, g per day: reference eggwhite (EW), 3.0, 4.5, and 6.0; and SB, 3.0, 4.5, 6.0, and 7.5. SB diets were fed with and without addition of L-methionine to raise the total amount of sulfur amino acids (SAA) to 900 mg. Mean nitrogen balance was positive with diets containing at least 4.5 g EW, 6.0 g SB, and 4.5 g SB with methionine added. Individual requirements for SAA were: for one man, between 320 and 480 mg; for three, between 480 and 640 mg; for one man, between 640 and 800 mg; and for the sixth man, between 640 and 900 mg. Thus 900 mg, stipulated by the 1973 FAO/WHO Committee as being necessary for the 70 kg reference man, was sufficient for all participants. Nitrogen balance was equally improved by increasing the SB nitrogen level by 1.5 g or adding methionine at each level of SB fed.