Amino acid supplementation improves muscle mass in aged and young horses1

The objective of the study was to evaluate the effect of supplementary AA on the ability to support muscle mass in aging horses. Sixteen horses of light horse type were used in a 2 x 2 factorial arrangement of treatments with two age groups [< or = 10 yr (average = 9.1 +/- 0.29 yr) and > or = 20 yr (average = 22.4 +/- 0.87 yr)] and two diet groups [no supplementation (N) or supplementary lysine and threonine (S; 20.0 and 15 g/d, respectively)]. Horses were fed the diets for 14 wk and received regular light exercise throughout the study. Body weight, BCS, and venous blood samples were taken every 2 wk. Plasma was analyzed for total protein, albumin, creatinine, urea N (PUN), and an AA profile, including 3-methyl histidine (3MH) and sulfur AA. Photographs of the horses taken at the start and at the end of the experiment were used to assign a subjective muscle mass score from 1 to 5 (1 = lowest to 5 = highest). There was no difference in BW caused by diet; however, the S-group horses tended (P = 0.064) to gain more weight (6.91 +/- 2.3 kg), and in fact, the N-group horses lost weight (- 11.76 +/- 5.2 kg) during the experiment. Repeated measures analysis revealed that BCS was lower for the aged vs. the young horses (P = 0.001) as well as for the S- vs. the N-group horses (P = 0.026). Subjective muscle mass scores were not different at the start of the experiment but were greater (P = 0.047) for the S-group horses (3.77 +/- 0.13) at the end of the experiment compared with the N-group horses (3.28 +/- 0.14). Plasma creatinine was greater (P = 0.032), and PUN was lower (P = 0.027), for S-group horses compared with N-group horses. Initial 3MH concentrations were not different; however, at the end of the experiment, 3MH was lower for the S-group horses (P = 0.016) compared with the N-group horses. Plasma lysine and threonine concentrations were greater for S-group horses at the end of the experiment than for N-group horses (P = 0.023 and 0.009, respectively). Both 3MH and PUN concentrations were negatively correlated to lysine (R2 = 0.57 and 0.65, respectively) and threonine intake (R2 = 0.56 and 0.60, respectively) at the end of the study. These data suggest that horses receiving supplementary AA were able to maintain muscle mass better than those without supplementation, regardless of age, as evidenced by the improvement in muscle mass scores, lower BCS with no difference in BW, greater creatinine, and lower 3MH and PUN concentrations in the S-group horses.