Effect of balanced vs. standard protein on muscle mass development in exercising horses.

Twelve horses weighing 570 ± 35 kg were split into two groups (standard protein, STD, and balanced protein, BAL) to evaluate the effect of balanced vs. unbalanced protein on muscle mass. Groups were fed grass hay and a 12% CP feed (one formulated for balanced protein). Horses were out of work for 14 wk prior and participated in 1 to 2 h of light-to-moderate exercise 5 d/wk for 12 wk. A 4-d total urine and feces collection period was conducted at the start and end of the study. Urine and feces samples were analyzed for nitrogen (N) to calculate N balance. Muscle mass status was evaluated using histochemistry, muscle scoring, and body measurements at the start and end of the study. Muscle biopsies were analyzed for free amino acid concentrations. An ANOVA was done using the Proc MIXED function of SAS (2006). Nitrogen retention was greater and urine nitrogen as a percent of intake less for the STD group at the end of the study (P = 0.01). Muscle free concentrations of methionine, leucine, phenylalanine, and arginine were greater in the BAL group at the end of the study compared to the STD group (P < 0.04) while muscle free histidine, threonine, and valine were less for the STD group at the end of the study compared to the start of the study (P < 0.05). Muscle fiber diameters were greater at the start of the study for the STD group (P = 0.004) but greater for the BAL group at the end of the study (P = 0.001). There is some evidence of improved muscle mass for the BAL group with greater muscle free amino acids (arginine, leucine, methionine, phenylalanine, and valine) as well as greater type II muscle fiber areas and diameters. While changes did not appear evident between groups at the "whole body" level, changes at the tissue level appear more evident.