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Response of Rats to Diets of Equal Chemical Score: Effect of Lysine or Threonine as the Limiting Amino Acid and of an Amino Acid Excess¹

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

The effect of lysine or threonine deficiency with or without excesses of all other amino acids was evaluated in a 21-d feeding study with male rats. Four amino acid mixtures were designed to be first limiting for the rat in lysine or threonine and contained either 0 or 50% excess of nonlimiting amino acids. These mixtures were incorporated into purified diets to provide seven levels [20–140% of the National Research Council (NRC) requirement] of the limiting amino acid. Food intake, body weight gain and carcass composition were measured for each rat to determine the effects of the identity of the limiting amino acid and of amino acid excess on the response to dietary chemical score. Significant effects and/or interactions of the identity of the limiting amino acid (i.e., Lys or Thr) and the presence of excess amino acids were seen for each of the measured responses. At equivalent dietary percentages of the NRC requirement, threonine deficiency supported greater body weight gain than did lysine deficiency. At equivalent deficiencies (Lys vs. Thr) threonine-deficient rats were more susceptible to adverse effects of excess amino acids. When the limiting amino acids were incorporated into the diet through incremental addition of the deficient amino acid mixture, rats responded to levels of lysine or threonine in excess of the NRC requirement. These results suggest that the current NRC requirements for these amino acids are too low and that aspects of the dietary amino acid composition other than the percentage deficit of the limiting amino acid can be important determinants of animal response.

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

¹ Research supported by the College of Agricultural and Life Sciences and Hatch Project 5115. This is paper 852 from the Department of Meat and Animal Science.

² Present address: Cargill Research Farm, 10383 165th Avenue, 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 9 May 1985. Revision accepted 3 February 1986.