Equine Glycogen Replacement After ExerciseBy Dr. Bryan Waldridge · January 24, 2010
Glycogen is a large, highly branched sugar molecule that is stored in muscle and the liver, and is used by the body as an energy source. Made of long chains and branches of glucose, glycogen is used for quick, high-intensity exercise. Depleted muscle glycogen and buildup of its end products, lactate and pyruvate, contribute to muscle fatigue.
Horses produce glycogen two to three times slower than humans and other animals. In contrast to humans, horses are not able to accelerate muscle-glycogen replacement by consuming large amounts of digestible carbohydrates or sugars. Horses that are worked frequently, therefore, can have decreased stores of muscle glycogen that cannot be replaced before the horse is asked to work or perform again.
Persistently low muscle glycogen has been found in racing Standardbreds. Even when fed highly digestible carbohydrates, horses still took two to three days to completely restore depleted glycogen. Though intravenous administration of large amounts of glucose increased muscle-glycogen replacement after exercise, smaller amounts of glucose given by nasogastric tube did not increase glycogen production.
Horses might have reduced ability to absorb carbohydrates from the intestine or decreased ability of their muscle to take up glucose from the blood. Horses have increased insulin activity, which is important to drive glucose into the muscle cells, or increased transport of glucose into cells when they are fed after using glycogen stores during exercise.
Forages such as pasture and hay are fermented by bacteria in the large colon of horses. This fermentation yields volatile fatty acids (VFAs), which are used as energy sources, along with glycogen and glucose. Horses supplemented with acetate, one of the three VFAs, have increased ability to replace glycogen and decreased use of glucose for energy. It is believed this is due to the use of acetate as an energy source, rather than glucose.
Dehydrated horses have slow replacement of muscle glucose. If horses are given intravenous saline solution after exercise, it increases muscle glycogen production in the same way feeding digestible carbohydrates does. Therefore, rehydrating horses likely helps them replace glycogen after exercise. Administration of intravenous fluids with a typical grain and hay meal has been shown to be more effective in replacing glycogen than feeding alone.
Helping horses recover and replace muscle glycogen after exercise is complex and likely involves many factors. It is important that horses are rehydrated after work, and supplementation with acetate and electrolytes is probably beneficial.
This article has been summarized from the following paper: A.P. Waller and M.I. Lindinger. 2010. Nutritional aspects of post exercise skeletal muscle glycogen synthesis in horses: A comparative review. Equine Veterinary Journal 42(3):274-281.