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Amino Acid Retention and Balance in the Young Rat Fed Varying Levels of Casein¹

Department of Biochemistry and Nutrition, Virginia Polytechnic Institute, Blacksburg, Virginia

Amino acid balance and retention efficiency were determined in rats as a function of the level of dietary protein (casein). Twelve diets were prepared ranging in total protein from 4.8 to 39.1%. It was anticipated that the most limiting amino acids would show the greatest retention efficiency as long as protein synthesis was the dominant influence on utilization. The expected pattern was observed for the first three limiting amino acids but not valine and isoleucine. Protein efficiency ratio (PER) and individual amino acid retention efficiencies declined rapidly below 8% dietary protein despite continued weight gain. The decline in PER was accompanied by a change in the relative proportions of several amino acids in the carcass gain. It was proposed that the latter change resulted from a shift in the rate of protein synthesis favoring proteins essential for maintenance of life at the expense of more expandable proteins such as hair or muscle. The data are essentially in agreement with a previous study employing lactalbumin as the protein source and presenting a better balanced array of amino acids.

² Research Corporation, 405 Lexington Avenue, New York, N. Y. 10017.

Manuscript received 6 March 1969.