Fueling Performance in Endurance Horses

The main productive function of endurance horses is work. This work may vary from relatively slow exercise over long distances, common in 100-mile rides, to exercise conducted at somewhat faster speeds over the shorter (25 to 50 miles) endurance courses. Digestible energy derived from dietary nutrients is the main factor that will directly influence whether an endurance horse can go the distance. Energy is not a nutrient per se, but rather a measure of a feed’s potential to fuel body functions and muscle contraction during exercise. Muscle contraction, in turn, will move the horse across the ground during the ride.

The endurance horse takes in, via the gastrointestinal tract (GI tract), a variety of energy sources (fiber, starch, fat, protein) which can be used to fuel muscle contraction. Because horses are not able to eat continuously during a ride, energy sources must be digested and their products stored within the body to be used later as fuel during exercise. These different fuels are transferred between blood, liver, adipose tissue, and muscle cells. Stored energy in the form of muscle and liver glycogen (sugar), intramuscular and adipose triglycerides (fat), and feed taken in during the ride will provide energy for muscle contraction. For muscle contraction to occur, the chemically bound stored energy form must be converted into mechanical energy. This conversion process occurs in the muscle cell, and utilizes adenosine triphosphate (ATP) as the currency for muscle contraction.

The most direct method to form ATP is by the breakdown of another compound, creatine phosphate (CP). However, since muscle contains only a small amount each of CP and ATP, the supply of ATP is quickly depleted with the onset of exercise. For an endurance horse to exercise for a prolonged period of time, ATP must be resynthesized at the same rate at which it is being used. Two fundamental reactions resynthesize ATP: (1) oxidative phosphorylation, breaking down carbohydrates, fats, and protein in the presence of oxygen, and producing energy (ATP); the involvement of oxygen qualifies this as an aerobic reaction; and 2) glycolysis, breaking down glucose or glycogen into lactic acid; this reaction does not use oxygen and is considered anaerobic.

Several factors determine both the choice of fuel and the pathway used to generate ATP. These factors include muscle fiber type, the speed and duration of exercise, type of feed (energy sources) provided, and the horse’s fitness.

The horse has three basic types of muscle fiber: Type I, IIA and IIB. These fiber types have different contractile and metabolic characteristics. Type I fibers are slow-contracting fibers while Types IIA and IIB are fast-contracting. Type I and IIA fibers have a high oxidative capacity and thus can utilize fuels aerobically, while Type IIB fibers have a low aerobic capacity and depend on anaerobic glycolysis for energy generation. All three fiber types store glycogen, while only Types I and IIA have significant triglyceride storage.

It is not surprising that different breeds of horses will have different percentages of the muscle fiber types. For example, racing Quarter Horses typically have more Type IIA and IIB fibers and fewer Type I fibers than Arabian horses. This would help explain why one breed, the Arabian, is known for endurance. However, within a breed, the differences in muscle fiber type distribution are so small that muscle fiber typing as a predictor of performance is of limited value. The speed of muscle contraction determines how fast the animal is able to move.

Because the amount of ATP used by a muscle depends directly on how fast it is contracting, the faster an animal moves, the greater the ATP requirement. While walking, the muscles contract very slowly and expend relatively small amounts of ATP. During this type of exercise, Type I fibers are primarily recruited and energy generation is entirely aerobic. At this speed, the muscle burns predominantly fat. Fat stores are plentiful in horses with moderate or greater body condition, and they can be mobilized fast enough to regenerate the ATP used for muscle contraction. As speed increases from a walk to a trot to a canter, Type I fibers alone are no longer capable of contracting rapidly enough to propel the horse. At this point, Type IIA fibers are also recruited. These fibers are also aerobic, but they use a combination of glycogen and fat for energy generation.

Glycogen (glucose) can be metabolized twice as fast as fat for ATP generation, and as speed increases, fat becomes too slow a fuel for energy generation. As the horse increases speed to a fast gallop, Type IIB fibers are recruited and energy generation no longer remains purely aerobic. At these speeds, the requirement for ATP has exceeded the ability of the horse to deliver enough oxygen to the muscle to produce the energy by aerobic means. Anaerobic glycolysis takes over as a rapid metabolic pathway to generate ATP. Anaerobic glycolysis results, however, in lactic acid accumulation, and fatigue soon develops as the pH in the muscle begins to fall.

The speed at which endurance horses typically travel is within the range that can be maintained almost entirely through aerobic energy production. Only during the “controlled runaway” some riders use at the beginning of a ride, the end-of-race sprints, and during hill climbing would the energy production shift toward anaerobic means, and then only for a short time. Therefore, fatigue in an endurance horse is much more likely to result from depletion of glycogen and/or triglyceride stores than from the buildup of lactic acid.