Efficiency of lysine or threonine retention in growing rats fed diets limiting in either lysine or threonine.
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/dI, 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.
Gahl
M J
MJ
Department of Animal Sciences, University of Wisconsin, Madison 53706, USA.
Finke
M D
MD
Crenshaw
T D
TD
Benevenga
N J
NJ
D000596:Amino Acids / Q000032:analysis; D000818:Animals; D002851:Chromatography, High Pressure Liquid; D004044:Dietary Proteins / Q000008:administration & dosage* / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004526:Efficiency; D006128:Growth / Q000187:drug effects; D008239:Lysine / Q000008:administration & dosage* / Q000378:metabolism*; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D013912:Threonine / Q000008:administration & dosage* / Q000378:metabolism*
D000596:Amino Acids; D004044:Dietary Proteins; D013912:Threonine; D008239:Lysine