Starch and Fat as Energy Sources for Endurance Horses

Dietary energy is usually expressed in terms of kilocalories (Kcal) or megacalories (Mcal) of digestible energy. Digestible energy (DE) refers to the amount of total energy in the diet that is actually absorbed by the horse. The DE requirements for different types of horses are calculated based on the horse’s maintenance DE requirements plus the additional energy expended during exercise. Varying amounts of DE are required above maintenance at various speeds. For example, a 450 kg (990 lb) endurance horse would have a maintenance DE requirement of 14.9 Mcal/day. This same endurance horse, if ridden at a medium trot (250 meters/min) by a 75 kg (165 lb) rider for 3 hours, would have an additional energy requirement of 14.9 Mcal/day. The total energy requirement would be nearly 30 Mcal of DE/day, a value seen as intense work. The total DE requirement (maintenance + exercise) can be provided by several dietary energy sources including starch and fat.

Starch, a carbohydrate composed of a large number of glucose (sugar) molecules, is the primary component of cereal grains, making up 50 to 70% of the grain’s dry matter. Of the grains commonly fed to endurance horses, corn has the highest starch content, followed by barley and then oats. Horses break down starch into glucose units in the small intestine, where it is absorbed into the blood. Once in the blood, these glucose units can be used for a number of different purposes including being oxidized to produce ATP or being used to make muscle glycogen, liver glycogen, or body fat.

Starch is the dietary energy source of choice for glycogen synthesis. Starch digestion results in a direct rise in blood glucose and insulin, two of the most important factors involved in glycogen synthesis. Muscle glycogen is a versatile fuel for energy generation during endurance exercise, since glycogen can be metabolized either aerobically (with oxygen) or anaerobically (in the absence of oxygen). In addition, glycogen stored in the liver is available for the production and release of glucose into the blood during exercise. Maintaining blood glucose levels during exercise is of prime importance since glucose is the only fuel that is available to the central nervous system. In endurance horses, hypoglycemia (low blood sugar) as a result of prolonged exercise can be a cause of fatigue.

Corn oil and soybean oil along with animal fat are the most common sources of fat in the horse’s diet. These fat products contain roughly 2.25 times as much DE as an equal weight of corn, oats, or barley. Numerous digestion studies have confirmed that fat is both very palatable and extremely well digested. Fat is a less versatile energy source than starch since it can only be stored as body fat or oxidized aerobically to produce energy. Fatty acids derived from fat metabolism cannot be converted to glucose or be used to synthesize glycogen. Fat is, however, an extremely useful dietary energy source.

Research studies have concluded that feeding fat to horses resulted in a greater mobilization and utilization of fat during long-distance exercise. In essence, it appears horses trained their enzyme systems to utilize fat, thereby sparing the use of muscle and liver glycogen. Further, endurance horses in heavy training have a very high daily DE requirement. Often these endurance horses cannot or will not eat enough feed to meet their energy requirements. The result is a steady decrease in body condition. In these instances, adding fat will increase the energy density of the diet so that less feed is required to maintain body weight. Research has shown that endurance horses consuming fat-supplemented diets required less feed to maintain body weight.