Fuels for Energy Production During Equine Exercise

Carbohydrates and lipids are the major fuels used by the muscles of working horses during exercise. Although there are alterations in protein metabolism during exercise, data from several species indicate that protein is used minimally for energy production.

The main endogenous fuel reserves are present in the horse’s skeletal muscle, liver, and adipose tissue. Glycogen present in liver and skeletal muscle represents the storage form of carbohydrate. During exercise, additional glucose is provided by hepatic gluconeogenesis. Two major sources of fat are oxidized during exercise. These are non-esterified fatty acids (NEFA) released from triacylglycerols stored in adipose tissue and transported by the bloodstream to skeletal muscle, and NEFA derived from triacylglycerol deposits located within skeletal muscle fibers.

Whereas the endogenous supply of fat is virtually inexhaustible, carbohydrate stores are more limited. In human athletes, decreases in muscle glycogen content and plasma glucose (liver glucose supply) contribute to the onset of fatigue during exercise. Similarly, glucose availability is a limiting factor for both moderate- and high-intensity exercise performance in horses. These observations highlight the need for a greater understanding of factors that govern the supply and utilization of fuel substrates in the horse during exercise.

The relative contribution of different substrates to fuel metabolism during exercise is determined by a number of factors including the intensity and duration of exercise, the fitness of the horse, and the availability of substrates in plasma and muscle. In other species, including humans, there is unequivocal evidence that the mix of substrates oxidized during exercise is also influenced by the diet that has been consumed. Furthermore, these diet-induced changes in substrate oxidation can have profound effects on exercise performance. A prime example is the combination of exercise training and a low-carbohydrate diet that results in decreased muscle glycogen concentrations. Such depletion of the carbohydrate stores severely limits exercise capacity.

Horses that tire quickly during exercise may simply be unfit, in which case the trainer needs to work out a program of gradually increasing work to allow the horse’s muscles, bones, and circulatory and respiratory systems to adapt to the greater demands. Horses that are in good physical condition may need to have their diets adjusted by an equine nutritionist to provide them with a steady stream of energy to fuel exercise.