EFFICIENCY OF LYSINE OR THREONINE RETENTION IN GROWING RATS FED DIETSLIMITING IN EITHER LYSINE OR THREONINE

Over a 21-d experiment, the efficiency of lysine and threonine retenti on was determined in 80 male Sprague-Dawley rats (65.9 +/- 0.3 g, mean s +/- SE) fed purified diets containing an amino acid mix limiting in either lysine or threonine. With additional increments of the first li miting amino acid, lysine concentration in total body protein (9/16 g N) increased (P < 0.01) in rats fed lysine-limiting diets but, when fe d threonine-limiting diets, lysine concentration in body protein first increased and then decreased (P < 0.01). As increments of the first l imiting amino acid were added, the threonine concentration in total bo dy protein increased then decreased when both lysine- (P < 0.01) and t hreonine- (P < 0.06) limiting diets were fed. Lysine and threonine ret ention were calculated based on comparative slaughter. Sixteen rats we re killed on d 0 to estimate the grams of amino acid in the body. Rete ntion responses were analyzed using a logistic equation in which lysin e or threonine intake was used to predict retention. The maximum margi nal efficiency (dr/dl, retention/intake) was observed at <40% of maxim um retention. For lysine retention, it was 81% when lysine was limitin g 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 rel ative to cumulative intake) for lysine retention was 62% when lysine w as limiting or 58% when threonine was limiting. For threonine retentio n, it was 51% when threonine was limiting and 35% when lysine was limi ting. Thus, amino acid concentration in body protein is not constant, and amino acids are used with higher efficiency when first limiting.