Glycogen is a carbohydrate made up of glucose. Glycogen is the principal form in which carbohydrate is stored in the body, the same way the starch is stored in plants. Glycogen is stored in liver and the muscles and is readily broken down to glucose, the main source of energy. During physical activities the blood sugar is burned as fuel. If the level drops too low between meals, the body draws glycogen from its reserves in the muscles and liver. This has a detrimental effect on muscle and on muscle performance in horses. Fatiguing exercise substantially decreases muscle glycogen concentration in horses, impairing athletic performance in subsequent exercise bouts. Over a longer period the improvement in fitness of the horse may decelerate or even come to a halt.
After substantial exercise-induced muscle glycogen depletion, ingestion of starch-rich meals only minimally affects net muscle glycogen concentrations after exercise, despite marked differences in soluble carbohydrate ingestion and availability of glucose to skeletal muscle. "Dietary supplements may actually inhibit repletion", says Hyyppä, licentiate in Veterinary Medicine from MTT Agrifood Research Finland. Maintaining horses in a good state of hydration seems to have a moderate positive effect on repletion of muscle glycogen stores. Providing horses with an isotonic glucose-electrolyte rehydration solution soon after exercise helps to overcome dehydration significantly better than providing them with plain water.
High starch intakes increase the risk for metabolic disorders and therefore alternative feedstuffs are of interest. High-fat oat varieties have a lower starch and higher energy content than regular oats. High-fat oats can replace regular oats in the diet of athletic horses without any adverse effects on metabolism and exercise response. Due to the high energy content and a high digestibility of dietary components in high-fat oats the daily allowance of oats can be reduced and thus the intake of starch.
Muscle glycogen synthesis depends on glucose availability. Dietary carbohydrate composition may influence the rate of repletion of glycogen and support the hypothesis that molassed sugar beet pulp can replace oats in a hay based diet without impairing nutrient utilisation and metabolic response in exercising horses.
Muscle disorders are a common cause of disability in horses. For many years, clinical manifestations such as muscle pain, exercise intolerance, weakness, and stiffness were believed to be caused by a single syndrome. However, in the past years a broad spectrum of muscle disorders have been recognized including glycogen and polysaccharide storage myopathies, malignant hyperthermia, mitochondrial myopathy, hyperkalemic periodic paralysis and others.
Canadian researchers have found that muscle glycogen storage in horses during a 6 h period after exercise was enhanced by IV glucose administration (3 g/kg) but not by an equivalent glucose dose administered by mouth. While oral administration of glucose achieved a level of hyperglycaemia and hyperinsulinaemia that markedly accelerates glycogen storage in other species, the rate of glycogen storage following oral supplementation was not different to control conditions. Glucose supplementation via the IV route should be considered when rapid replenishment of muscle glycogen stores is desired.