Assessing the impact of ostarine on body composition in adult horses: A longitudinal study

The integrity of horse racing is under increasing scrutiny due to the misuse of performance-enhancing drugs that compromise fair competition and animal welfare. Among these substances, ostarine is one drug of concern. Ostarine selectively targets androgen receptors in skeletal muscle and bone and was found to enhance muscle mass in people. However, the effects of ostarine on equine physiology, and especially on muscle and fat mass, are currently poorly understood, highlighting a critical gap in research that must be addressed to inform doping regulations and safeguard the welfare of racehorses. Thus, this study aimed to investigate the effects of ostarine on muscle and fat mass in horses. We hypothesized that ostarine treatment would result in a change in muscle and fat mass compared with a control group. Nine Standardbred horses (mean age ± SD = 17.2 ± 2.5 years) were used in this study and were either assigned to the treatment group (n = 4; receiving ostarine [62 mg in 1 mL of ethanol] intravenously) or the control group (n = 5; receiving 1 mL of ethanol as a vehicle control, intravenously). Horses received the treatment/control for 4 wk 4 times a week (on nonconsecutive days) and were monitored before the treatment/control (baseline), during the treatment period and after treatment for 4 wk. The ostarine treatment duration was based on a rodent study. Measurements included body mass, ultrasound-based fat-free mass (FFM) and fat mass (FM) estimation, body condition scoring, and muscle atrophy scoring. A mixed-model ANOVA (fixed effects: group, data collection time point, and their interaction) was used to analyze differences between the groups. No significant differences between treatment and control groups were found for body mass (P = 0.60), FFM (P = 0.86), FM (P = 0.45), muscle atrophy scores of the horses’ hind region (P = 0.94), back region (P = 0.52), or neck region (P = 0.92), at any of the data collection time points. A significant group by time point interaction was found for body condition scores (P = 0.03); however, a post hoc Tukey test indicated no statistically significant differences between the groups at any of the sampling time points. This small study showed no significant effects of ostarine on selected muscle and fat mass indicators in adult horses, suggesting that ostarine may not affect muscle mass in idle adult horses. As this study was of small sample size it is possible that it did not have enough power to detect smaller differences between the treatment and control groups. Larger studies may explore longer durations of ostarine administration, or administration to exercising horses.