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Efficiency of Lysine or Threonine Retention in Growing Rats Fed Diets Limiting in either Lysine or Threonine^1,2,3,

Mark J. Gahl^*,,4, Mark D. Finke^,5, Thomas D. Crenshaw^* and N. J. Benevenga^*,,6

^* Department of Animal Sciences Department of Nutritional Sciences, University of Wisconsin, Madison, WI 53706

Over a 21-d experiment, the efficiency of lysine and threonine retention was determined in 80 male Sprague-Dawley rats (65.9 ± 0.3 g, means ± SE) fed purified diets containing an amino acid mix limiting in either lysine or threonine. With additional increments of the first limiting amino acid, lysine concentration in total body protein (g/16 g N) increased (P < 0.01) in rats fed lysine-limiting diets but, when fed threonine-limiting diets, lysine concentration in body protein first increased and then decreased (P < 0.01). As increments of the first limiting amino acid were added, the threonine concentration in total body protein increased then decreased when both lysine- (P < 0.01) and threonine- (P < 0.06) limiting diets were fed. Lysine and threonine retention were calculated based on comparative slaughter. Sixteen rats were killed on d 0 to estimate the grams of amino acid in the body. Retention responses were analyzed using a logistic equation in which lysine or threonine intake was used to predict retention. The maximum marginal efficiency (dr/dl, retention/intake) was observed at <40% of maximum retention. For lysine retention, it was 81% when lysine was limiting and 70% when threonine was limiting. For threonine retention, it was 58% when threonine was limiting and 49% when lysine was limiting. The maximum cumulative efficiency (retention adjusted for maintenance relative to cumulative intake) for lysine retention was 62% when lysine was limiting or 58% when threonine was limiting. For threonine retention, it was 51% when threonine was limiting and 35% when lysine was limiting. Thus, amino acid concentration in body protein is not constant, and amino acids are used with higher efficiency when first limiting.

KEY WORDS: • rats • lysine • threonine • retention • efficiency

¹ Presented in part at Experimental Biology 92, Anaheim, CA [Gahl, M. J., Crenshaw, T. D. & Benevenga, N. J. (1992) Amino acid composition and retention in growing rats fed diets limiting in either lysine or threonine. FASEB J. 6: A1943 (abs.)].

² Supported by University of Wisconsin College of Agricultural and Life Sciences and by Cargill, Inc., Elk River, MN.

³ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁴ Current address: Farmland Industries, Inc., 3705 N. 139th Street, Kansas City, KS 66109.

⁵ Current address: PETsMART, 10000 North 31st Avenue, Phoenix, AZ 85051.

⁶ To whom correspondence should be addressed.

Manuscript received 25 May 1995. Revision accepted 19 August 1996.

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