The Effect of Exercise and Diet on Muscle and Liver Glycogen Repletion in Standardbred Horses

Muscle and liver glycogen are important substrates for energy generation in horses performing various intensities of exercise. In a review of energy considerstions during exercise, Hodgson (1985) summarized a number of experiments that measured muscle glycogen utilization in horses performing a wide variety of exercises from endurance rides covering up to 160 km to Thorougbreds racing at speeds greater than 950 m/min. In all of these exercises muscle glycogen was used to produce energy, with the amount of utilization increasing with work intensity. During long distance exercise, muscle glycogen has been shown to be almost totally depleted in horses (Snow et al., 1982). Because of the potential for fatigue that accompanies muscle glycogen depletion, a number of experiments have been conducted with horses to manipulate muscle glycogen storage (Kline and Albert, 1981; Topliff et al., 1983, Topliff et al., 1985). In these studies rather extreme dietary manipulations were used in combination with intense exercise to deplete, then replete muscle glycogen. While these methods of glycogen "loading" have been generally successful at increasing glycogen storage in the muscle, they are considered too severe to be used under practical management situations. Therefore, the following studies were conducted to determine whether muscle and liver glycogen stores can be manipulated using more acceptable diets and exercise regimes.