Substrate utilization during exercise is determined by the intensity and duration of exercise as well as by training status and the availability of substrate. The major energy source at rest and during low intensity exercise is fat oxidation while oxidation of carbohydrates and anaerobic energy utilization predominates at higher intensities. Hypoxia will induce a similar change in substrate utilization as an increase of the exercise intensity and it is suggested that O2 availability is an important determinant of muscle metabolism during exercise. Muscle glucose uptake increases during exercise and can reach values that are 30-50 times higher than at rest. The increase in muscle glucose uptake is related to the work load and the duration of the exercise, and corresponds to the output of glucose from the liver. Animal experiments have shown that muscle contraction can induce an increase in glucose uptake in the absence of insulin. Studies in man have shown that insulin and exercise at low intensity have a synergistic effect on glucose uptake, possibly due to an increased insulin inflow to the muscle. Intracellular glucose concentration is low at rest but can under certain conditions such as intensive exercise and hypoxia increase to maximum values of 3-4 mmol/l of intracellular water which demonstrates that glucose uptake under certain conditions can be in excess of the utilization of glucose.