The two main fuels for muscle metabolism are carbohydrate and fat. There is a limited store of carbohydrate in the body but this is not the case with fat. The average lean man has about 15% of his body weight as fat, whereas the average lean women has about 25% of her body weight as fat. Male and female endurance athletes have only about 7-10% of their body weights as fat. Sedentary people consume diets which contain about 35-40% of their energy intake as fat. The recommended intake of fat in the diet of active and sedentary people is less than that percentage. Although there is a need to increase carbohydrate intake as part of the preparation for heavy training and competition there is no need to supplement the normal diet with additional fat. Fat is mobilized from adipose tissue in response to stimulation of an intracellular lipase by the catecholamines. The products of the hydrolysis of triglycerides, the storage form of fat, are fatty acids and glycerol. The 'free' fatty acids are transported to muscle in loose combination with plasma albumin where they are released and taken up and oxidized. Glycerol is not used directly as a substrate but undergoes gluconeogenesis in the liver. This process helps restock liver glycogen stores which, in turn, provides glucose as a fuel for the central nervous system and for muscle metabolism. Training increases the capacity of skeletal muscles to use fat as an energy source. An increase in fat metabolism during prolonged exercise has a glycogen sparing effect and as such improves endurance capacity.(ABSTRACT TRUNCATED AT 250 WORDS)