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The Effect of Amino Acid Excess on Utilization by the Rat of the Limiting Amino Acid—Lysine and Threonine at Equalized Food Intakes¹

Department of Meat and Animal Science, University of Wisconsin, Madison, WI 53706

The effect of excess amino acids on utilization of the most limiting amino acid was investigated via restricted feeding. Lysine- and threonine-deficient amino acid mixtures were incorporated into diets for young growing rats to provide 75 or 85% of the NRC requirement of the limiting amino acid in combination with zero or 50% of relative excess of the nonlimiting amino acids. The amount of food offered was equalized within treatment groups and increased according to the intake of the group with the lowest rate of consumption. Because of differences in the amount of diet refused, total food intakes were not identical. At equal percentage of the NRC requirement, threonine- and lysine-deficient amino acid mixtures supported equal body weight, dry matter, lipid and ash gains. However, crude protein retention was greater when the threonine-deficient amino acid mixture was fed. The maximum depression in performance due to 50% of amino acid excess with restricted feeding was much less than the maximum decrease previously observed in the ad libitum feeding situation. This supports the conclusion that changes in voluntary food intake are the major effectors of depressed growth due to excess amino acids. These results raise serious concern over the usefulness of amino acid requirements and the applicability of the current system of chemical scores to evaluate protein quality.

KEY WORDS: • rat • amino acid score • chemical score • protein quality • amino acid utilization • lysine • threonine

¹ Research supported by the College of Agricultural and Life Sciences and Hatch Project 5115. This is paper number 839 from the Department of Meat and Animal Science.

² Present address: Cargill Research Farm, 10383 - 165th Ave., N.W., Elk River, MN 55330.

³ Reprint requests should be sent to: N. J. Benevenga, University of Wisconsin, 1156 Animal Sciences Building, 1675 Observatory Drive, Madison, WI 53706.

Manuscript received 13 March 1984.