What Fuels Are Used For Exercise?
While carbohydrate and fat are the two main sources of energy for muscular work, each of these fuels is comprised of two components, one within skeletal muscles and one in the blood. Thus, four primary energy substrates fuel muscle contraction and physical activity. Values for the average man are listed below.
While you probably do not think of your muscles as being fatty, fat exists as droplets of triglyceride within skeletal muscles. Muscle is the primary tissue that oxidizes fat, and intramuscular fat is readily available as a fuel source for muscle work. Additionally, as shown above, the total amount of calories available from intramuscular triglycerides is greater than that available from muscle glycogen. Remember that while fat is more dense in calories than either carbohydrate or protein, fat is light in weight. Thus, 2,500 calories of fat weighs only about nine ounces.
What about protein as a fuel source? Amino acids from protein breakdown can undergo oxidative metabolism to be used to fuel muscle contraction. Although not primary energy substrates, amino acids are used as auxiliary fuels during muscular work. Exercise duration, carbohydrate content of the diet, and muscle glycogen levels affect protein use for fuel. Oxidation of branched-chain amino acids (leucine, isoleucine, and valine) within skeletal muscles is increased during prolonged exercise and when muscle glycogen stores are low. Protein may contribute up to 10-15% of the energy needs in endurance exercise, particularly when the athlete is carbohydrate depleted.
So far, the focus has been on providing energy for muscle contraction. However, energy to fuel the brain is essential for alertness and mental concentration in any sport. Normally, the brain is fueled almost exclusively by carbohydrates, glucose from the blood or from the breakdown of liver glycogen.
