Clenbuterol's Impact on Horses' Body Fat Percentage

With any medication comes a risk of side effects. For instance, long-term phenylbutazone administration to treat a musculoskeletal issue can result in gastrointestinal problems; pergolide to treat Cushing's disease can cause a decreased appetite; and vaccine administration to protect against disease can cause injection site swelling and muscle soreness. And recently, a research team took a closer look at clenbuterol, a drug used to treat bronchospasm in recurrent airway obstruction (RAO, or heaves), and more clearly defined its side effects on horses' body fat percentage.

At the 2014 American College of Veterinary Internal Medicine Forum, held June 4-7 in Nashville, Tennessee, Rose Nolen-Walston, DVM, Dipl. ACVIM, shared study results on some of clenbuterol's side effects in a poster presentation. The team tested whether long-term, low-dose clenbuterol use would result in decreased body fat, and if exercise affected the magnitude of that change in body fat.

Nolen-Walston, an assistant professor at the University of Pennsylvania School of Veterinary Medicine's New Bolton Center, and colleagues employed eight resting Thoroughbreds and 47 Thoroughbreds at two polo barns to test their hypothesis. The resting horses were separated into two groups and received either 0.8 micrograms per kilogram body weight clenbuterol or corn syrup (as a control) every 12 hours for 21 days before a 7-week washout period. Then, the horses switched treatment groups.

The working horses were also separated into to two groups, one receiving clenbuterol and the other receiving corn syrup in the same manner as the resting horses. Then, the horses had a 21-day washout period before switching treatment groups.

The researchers determined the horses' body fat by measuring rump fat via ultrasonography and using an equation to calculate the percentage. The baseline weight and body fat percentage did not vary significantly between the two groups, Nolen-Walston said.

Upon reviewing their results, the team found that:

  • Clenbuterol-treated resting horses' average body fat percentage decreased significantly beginning on Day 6 of treatment and continuing through Day 18.
  • Clenbuterol-treated working horses' average body fat percentage began decreasing on Day 3 of the treatment and reached its lowest point on the final day of medication administration, when the treated horses' body fat percentage was significantly lower than the control horses; and
  • Exercise did not appear to impact clenbuterol's effect on fat levels.

"The results of this study show that even at what would be considered a commonly used dose, clenbuterol reduced body fat when compared to a placebo and may have repartitioning effects that are lipolytic (fat-burning) and anabolic (muscle-building) in nature," Nolen-Walston reported. “Other studies have shown tachyphylaxis, or a reduced effect of the drug, after about two weeks, both in its effects on the airways but also on muscle and fat. However, at this lower dose, horses were still losing fat faster than controls by Day 21, when we stopped the medication.”

Although many studies have shown that clenbuterol increases muscle mass, Nolen-Walston and colleagues said researchers have consistently shown that it is not performance-enhancing in horses, and, in fact might reduce athletic performance.

Still, the drug holds the potential to be used (or abused) for purposes other than RAO treatment, Nolen-Walston said: "This and other literature suggest that clenbuterol is effective mainly to improve muscle build rather than performance and, thus, may be more likely to have abuse potential in horses where a muscular phenotype (outward appearance) is more important than speed, such as halter horses."

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.