Most horse owners appreciate the sight of a well-muscled horse, along with the time and effort riders or trainers must commit to helping that animal fill out. But chances are, fewer owners consider the factors within a horse's body that allow him to build—or lose—muscle mass.

At the 2013 Alltech Symposium, held May 19-22 in Lexington, Ky., Kristine Urschel, PhD, an assistant professor of equine science at the University of Kentucky, delivered a lecture about muscle metabolism and how equine muscle mass is regulated and reviewed research on the topic.

Skeletal Muscle

Urschel began by describing the basics of equine skeletal muscle and the pathways that regulate muscle mass. Forty to 55% of a horse's mature body weight is comprised of muscle, she explained. Muscle itself is comprised of roughly 70% water; 20% protein; and 10% fat, vitamins, and minerals, and has both structural and metabolic functions,.

Muscle mass is determined by protein synthesis and breakdown, both of which occur simultaneously, Urschel said; essentially, greater protein synthesis than breakdown results in increasing muscle mass, while increased protein breakdown compared to synthesis results in decreasing muscle mass. Factors known to affect muscle mass—either positively or negatively—include age, medications, physical activity level, nutrition, illness or disease, and hormones, she said.

Next, Urschel described the signaling pathway responsible for regulating protein synthesis, commonly known as mTOR. She explained that three main factors—insulin, amino acids, and exercise—act through independant pathways to activate mTOR, thus initiating protein synthesis.

Previous Research

Limited research on protein synthesis signaling pathways has been carried out in horses. But Urschel described several studies on the topic she's worked on during her career.

First, research she carried out with colleagues at Virginia Tech's Middleburg Agricultural Research and Extension Center showed that muscle protein synthesis increases following meals in horses, possibly due to post-feeding elevations in either amino acid or insulin concentrations, she said.

Based on the results of that study, Urschel wanted to find out more about mTOR signaling activated by insulin alone. In the following study, she infused different rates of insulin, while keeping plasma glucose concentrations constant through the simultaneous infusion of a glucose solution, to see whether increased circulating insulin concentrations would result in increased mTOR signaling. While mTOR signaling did increase with increasing insulin infusion rate, the greatest changes were seen in the ranges of insulin that approximated physiological concentrations.

Urschel has also evaluated the effects of age on mTOR signaling in horses. In one study, she found that mTOR was influenced more after feeding in yearling horses compared to 2-year-olds, which she said indicates that the muscle protein signaling pathway is more sensitive in younger growing horses and declines with age. In another study, she learned that one part of the pathway—S6K1—was less active in older horses' muscle; further research is needed to understand why that pathway is less responsive to stimuli, she said.

Recent Research

More recently, Urschel has investigated the effects of insulin resistance on muscle protein metabolism. In one study on the topic, she induced insulin resistance by administering the steroid dexamethsone to a population of horses. In that study, she found a "profound decrease" in all muscle metabolism signaling proteins when horses received dexamethasone. From that study, she concluded that insulin resistance likely does impair muscle protein synthesis and probably reduces affected horses' ability to make and maintain muscle mass.

She also recently evaluated the effects of exercise on muscle protein synthesis. She concluded that exercise increases protein synthesis and has variable results on protein breakdown.

In another study Urschel described, researchers evaluating the synergistic effects of exercise and nutrition on muscle protein balance showed that providing amino acids and glucose to horses following exercise has a beneficial effect on rates of muscle protein synthesis and break down, likely resulting in a potential increase in muscle mass. Essentially, supplementing the exercising horse's diet with both amino acids and carbohydrates has a "really" positive effect on building muscle, she said.

Supplementing with amino acids and protein is also positive, but not to the same degree, she said.

Supplementing with carbohydrates alone is unlikely to increase muscle mass, she said, as is the case with exercise alone (with no amino acid feeding in the proximity of exercise). There has been little research in horses, she noted; however, there is ample research from human athletes that can be used as a guide for what might be expected in the equine athlete, Urschel added.

Future Research

"There are many opportunities for future research," Urschel said, such as in the field of aging horses' muscles and muscle metabolism.

One area of particular interest is the effects of exercise on a growing animal's muscle: "If growing muscle is more sensitive to feeding stimuli, will it also be more sensitive to exercise? It's a great opportunity for equine research," she said.

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

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