Nutrients Used During Muscle Activity
In addition to the large usage of carbohydrates by the muscles during exercise, especially during the early stages of exercise, muscles use large amounts of fat for energy in the form of fatty acids and acetoacetic acid, and they use to a much less extent proteins in the form of amino acids. In fact, even under the best conditions, in those endurance athletic events that last longer than 4 to 5 hours, the glycogen stores of the muscle become almost totally depleted and are of little further use for energizing muscle contraction. Instead, the muscle now depends on energy from other sources, mainly from fats.
Figure 84–4 shows the approximate relative usage of carbohydrates and fat for energy during prolonged exhaustive exercise under three dietary conditions:high-carbohydrate diet, mixed diet, and high-fat diet. Note that most of the energy is derived from carbohy-drates during the first few seconds or minutes of the exercise, but at the time of exhaustion, as much as 60 to 85 per cent of the energy is being derived from fats, rather than carbohydrates.
Not all the energy from carbohydrates comes from the stored muscle glycogen. In fact, almost as much glycogen is stored in the liveras in the muscles, and this can be released into the blood in the form of glucose and then taken up by the muscles as an energy source. In addition, glucose solutions given to an athlete to drink during the course of an athletic event can provide as much as 30 to 40 per cent of the energy required during prolonged events such as marathon races.
Therefore, if muscle glycogen and blood glucose are available, they are the energy nutrients of choice for intense muscle activity. Even so, for a long-term endurance event, one can expect fat to supply more than 50 per cent of the required energy after about the first 3 to 4 hours.