Role of dietary energy source in the expression of chronic exertional myopathies in horses

Abstract

Abstract Muscle disorders characterized by the development of pain and stiffness during and after exercise (exertional rhabdomyolysis, ER) are common in horses. Two heritable forms of chronic ER have been identified: 1) polysaccharide storage myopathy (PSSM), a condition characterized in quarter horses and related breeds, but also reported to occur in other breeds; and 2) recurrent exertional rhabdomyolysis (RER) in Thoroughbreds. Although the pathophysiology of PSSM and RER are different, there is epidemiological and experimental evidence that feeding diets rich in hydrolyzable carbohydrates (starch and simple sugars) enhances the phenotypic expression of both disorders. The PSSM is characterized by increased insulin sensitivity, excessive muscle glycogen storage, and the accumulation of amylase-resistant polysaccharide in muscle. The feeding of concentrates rich in hydrolyzable carbohydrates may enhance disease expression by increasing the quantity of glucose available for muscle glycogen synthesis. On the other hand, diets rich in starches and simples sugars may increase clinical expression of RER via enhancement of stress and anxiety, factors known to increase the risk of ER in horses with RER. A decrease in the frequency and severity of ER has been observed when horses with PSSM and RER are fed diets with reduced DE from hydrolyzable carbohydrates (<10 to 15% of total diet) and increased DE from fat (15 to 20%) and other energy sources, such as beet pulp and soybean hulls.