Effects of Several Levels of Dietary Protein and Amino Acids on Nitrogen Balance of Preadolescent Girls

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Abstract
Nitrogen balance studies were conducted with fifty-eight girls between seven and nine years of age whose daily nitrogen intakes ranged from 86 to 460 mg. per kg. body weight. Nitrogen retention was linearly related to intake over the range of intakes studied and when energy intakes were adequate all subjects with intakes above 100 mg. per kg. of body weight remained in positive nitrogen balance. Nitrogen retention by subjects fed diets containing no animal protein was not significantly different from nitrogen retentions by subjects fed diets containing both animal and plant protein, whereas feeding gelatin hydrolysates between meals appeared to decrease nitrogen retention at the higher protein intakes. The lowest level of essential amino acids supplied by any of the diets in our studies which supported positive nitrogen retention in preadolescent girls were as follows: (mg. of amino acid per kg. of body weight per day) isoleucine, 24; leucine, 45; lysine, 29; methionine plus cystine, 21; phenylalanine, 28; threonine, 20; tryptophan, 4.9; and valine, 31. These values were obtained with at least six subjects whose daily nitrogen intakes averaged from 104 to 135 mg. per kg. of body weight.