Mj. Gahl et al., EFFICIENCY OF LYSINE OR THREONINE RETENTION IN GROWING RATS FED DIETSLIMITING IN EITHER LYSINE OR THREONINE, The Journal of nutrition, 126(12), 1996, pp. 3090-3099
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.