Carbohydrate Nutrition and Fatigue

David L. Costill, Mark Hargreaves

Carbohydrates are important substrates for contracting muscle during prolonged, strenuous exercise, and fatigue is often associated with muscle glycogen depletion and/or hypoglycaemia. Thus, the goals of carbohydrate nutritional strategies before, during and after exercise are to optimise the availability of muscle and liver glycogen and blood glucose, with a view to maintaining carbohydrate availability and oxidation during exercise. During heavy training, the carbohydrate requirements of athletes may be as high as 8 to 10 g/kg bodyweight or 60 to 70% of total energy intake. Ingestion of a diet high in carbohydrate should be encouraged in order to maintain carbohydrate reserves and the ability to train intensely. Ingestion of a high carbohydrate meal 3 to 4 hours prior to exercise ensures adequate carbohydrate availability and enhances exercise performance. Although hyperinsulinaemia associated with carbohydrate ingestion in the hour prior to exercise may result in some metabolic alterations during exercise, it may not necessarily impair exercise performance and may, in some cases, enhance performance. Carbohydrate ingestion during prolonged, strenuous exercise, where performance is often limited by carbohydrate availability, delays fatigue. This is due to maintenance of blood glucose levels and a high rate of carbohydrate oxidation, rather than a slowing of muscle glycogen utilisation, although liver glycogen reserves may be spared. During recovery from exercise, muscle glycogen resynthesis is critically dependent upon the ingestion of carbohydrate. Factors influencing the rate of muscle glycogen resynthesis include the timing, amount and type of carbohydrate ingested and muscle damage. Adequate carbohydrate availability before, during and after exercise will maintain carbohydrate oxidation during exercise and is associated with enhanced exercise performance.