Chronic administration of therapeutic levels of clenbuterol acts as a repartitioning agent.

The purpose of this study was to examine the effect of therapeutic levels of clenbuterol, with and without exercise training, on body composition. Twenty-three unfit Standardbred mares were divided into four experimental groups: clenbuterol (2.4 microg/kg body wt twice daily) plus exercise (ClenEx; 20 min at 50% maximal oxygen consumption 3 days/wk; n = 6), clenbuterol only (Clen; n = 6), exercise only (Ex; n = 5), and control (Con; n = 6). Rump fat thickness was measured at 2-wk intervals by using B-mode ultrasound, and percent body fat (%fat) was calculated by using previously published methods. For Ex, body fat decreased (P < 0.05) at week 4 (-9.3%), %fat at week 6 (-6.9%), and fat-free mass (FFM) increased (P < 0.05) at week 8 (+3.2%). On the other hand, Clen had significant changes in %fat (-15.4%), fat mass (-14.7%), and FFM (+4.3%) at week 2. ClenEx had significant decreases in %fat (-17.6%) and fat mass (-19.5%) at week 2, which was similar to Clen; however, this group had a different FFM response, which significantly increased (+4.4%) at week 6. Con showed no changes (P > 0.05) in any variable at any time. These results suggest that exercise training and clenbuterol have additive effects with respect to %fat and fat mass but antagonistic effects in terms of FFM. Furthermore, chronic clenbuterol administration causes significant repartitioning in the horse, even when administered in therapeutic doses.