Total body carbohydrate stores are limited, and are often less than the carbohydrate requirements of athletic training and competition. However, the availability of carbohydrate as a substrate for muscle metabolism is a critical factor in the performance of both high-intensity intermittent work and prolonged aerobic exercise. The rate of carbohydrate oxidation during exercise is tightly regulated, with glucose availability closely matching the needs of the working muscles. Both the absolute and relative work rate play important roles in the regulation of substrate metabolism: carbohydrate-based fuels predominate at moderate to high power outputs, with muscle glycogen and glucose utilization scaling exponentially to the relative work rate. As such, strategies to maintain or enhance carbohydrate availability, such as the ingestion of carbohydrate before, during and after exercise, are critical to the performance of a variety of sports events, and are a key recommendation in current sports nutrition guidelines.