Performance during exercise depends, in part, on the provision of adequate fuel to working muscles. Therefore, athletes often ingest carbohydrate during intense endurance exercise to support plasma glucose concentrations and spare muscle glycogen oxidation. Ingestion of typical dietary fat is not a useful approach for providing fuel during exercise because it may take several hours for the long-chain fatty acids to be oxidized. Long-chain triglycerides are emptied slowly from the stomach, packaged into chylomicrons in the small intestine, and secreted into the lymphatic system before entering the bloodstream. Only a portion of triglycerides present in circulating chylomicrons ultimately provide fatty acids to muscle. In contrast, medium chain triglycerides (MCTs) have been proposed as a potential ergogenic fuel during exercise and are currently present in several commercially prepared sport bars. Medium-chain triglycerides are emptied rapidly from the stomach,¹¹ rapidly absorbed and hydrolyzed by the small intestine, and secreted directly into the systemic circulation. Furthermore, medium-chain fatty acids do not require the acylcarnitine transferase system to cross the inner mitochondrial membrane in liver and muscle for oxidation. However, several studies have demonstrated that oral supplementation with MCTs is unlikely to improve performance during endurance exercise. The amount of MCTs that can be given orally is limited to approximately 25-30 grams because diarrhea and other gastrointestinal side effects are common with higher doses. Furthermore, although orally administered medium-chain triglycerides are readily oxidized,^12-14 they do not spare muscle glycogen during either moderate or high-intensity exercise.^12-16

1. Hurley BF, Nemeth PM, Martin WH, Hagberg JM, Dalsky GP, Holloszy JO. Muscle triglyceride utilization during exercise: effect of training. J Appl Physiol 1986;60:562-7.

2. Klein S. Coyle EF, Wolfe RR. Effect of exercise on lipolytic sensitivity in endurance-trained athletes. J Appl Physiol 1995;78:2201-6.

3. Klein S. Coyle EF, Wolfe RR. Fat metabolism during low-intensity exercise in endurance- trained and untrained men. AM J Physiol 1994;267:E934-40

4.Klein S, Weber J-M, Coyle EF, Wolfe RR. Effect of endurance training on glycerol kinetics during strenuous exercise in humans. Metabolism 1996;45:357-61

5.. Klein S. Peters EJ, Shangraw RE, Wolfe RR. Lipolytic response to metabolic stress in patients with critical illness. Crit Care Med 1991;19:776-9.

6. Klein S. Young VR, Blackburn GL, Bistrian BR, Wolfe RR. Palmitate and glycerol kinetics during brief starvation in young adult and elderly subjects. J Clin Invest 1986;78:928-33.

7. Wolfe RR, Klein S. Carraro F. Weber J-M. Role of triglyceride-fatty acid cycle in controlling fat metabolism in humans during and after exercise. Am J Physiol 1990;258:E382-9.

8. Dohm GL, Beeker RT, Israel RG, Tapscott EB. Metabolic responses to exercise after fasting. J Appl Physiol 1986;61:1363-8.

9. Conlee RK, Hammer RL, Winder WW, Bracken ML, Nelson AG, Barnett DW. Glycogen repletion and exercise endurance in rats adapted to a high fat diet. Metabolism 1990;39:289-94.

10. Martin B. Robinson S. Robertshaw D. Influence on diet on leg uptake of glucose during heavy exercise. Am J Clin Nutr 1978;31:62-7.