Nutrition and fuel utilization in the athletic horse.

Substrate depletion and end product accumulation are two important factors in exercise fatigue. Fatigue during long-term exercise results from a depletion of muscle and liver glycogen and coincides with an inability to maintain blood glucose levels. During high intensity exercise, the rapid catabolism of carbohydrate and the resultant production of lactate and hydrogen ions cause a reduction in muscle pH that inhibits maximum force generation. Dietary manipulations that can influence carbohydrate status or lactate accumulation may be beneficial to performance. In human athletes, carbohydrate loading and carbohydrate supplementation can enhance endurance time during long-term exercise. These practices have not been explored extensively in the equine athlete, although glycogen loading does not enhance the performance of horses during short-term intense work. Short-term work can be detrimentally affected if glycogen levels are inadequate. The most marked effect of exercise on nutrient requirements is in the energy requirement. Horses in heavy training may require more energy than they can consume on a conventional diet. Fat has been added to horse diets to increase energy density, usually at levels between 6% and 12% of the total diet. Although protein requirements may be slightly increased in the working horse, supplementing protein as a means of adding calories is not an efficient practice. In addition, although studies with horses are not available, human studies indicate that there are no benefits to vitamin supplementation above required levels. At this point, more is unknown than is known about feeding performance horses. Most information on fuel utilization is extrapolated from studies with rats and humans. Areas that have received little attention but are critical to optimizing feeding practices are the timing of pre-event feeding and the determination of ideal body composition in equine athletes of different types.