The endurance athlete is fueled by 2 major fuels - carbohydrate and fat.
Carbohydrate is like "high octane" fuel - a steady supply is necessary for high intensity exercise to continue. Fatigue during endurance exercise is associated with depletion or reduction of the body's carbohydrate stores. As carbohydrate supply declines, the athlete must slow down.
Fat is like "regular" fuel - it provides a good source of energy for sustained, lower intensity exercise. Well trained endurance athletes can rely more on fat for fuel which helps to conserve their carbohydrate fuel and delay exhaustion and the need to decrease exercise intensity.
The carbohydrate "fuel tank" is limited in size. Most endurance athletes can store between 2000 and 4000 Calories worth of carbohydrate in a form called glycogen. Most of this is stored right in the muscle tissue. Some also is stored in the liver.
The fat "fuel tank" is usually thought to be rather unlimited in size. However, this may not apply to all endurance athletes. Most of the fat is stored in fat cells throughout the body. However, some fat is stored right in muscle tissue in a form called muscle triglycerides.
As the the intensity of exercise increases, the the body uses proportionately more carbohydrate and less fat for fuel.
During exercise, the muscles have 2 sources of carbohydrate. The muscles are using carbohydrate for fuel in the form of glucose. This glucose fuel comes from, 1) the breakdown of glycogen right in the muscle tissue, and 2) glucose taken up from the blood into the muscle. Glucose in the blood comes from carbohydrate that is absorbed from food and drink and from glucose produced by the liver. The liver can produce glucose from the breakdown of glycogen in the liver and by making glucose from various other molecules.
During exercise, the muscles also have 2 sources of fat. The muscles are using fat for fuel in the form of fatty acids. This fatty acid fuel comes from 1) breakdown of triglycerides right in the muscle tissue, and 2) fatty acids taken up from the blood into the muscle. Fatty acids in the blood come primarily from the breakdown of tryglycerides in fat cells throughout the body. As the triglycerides are broken down in the fat cells, the fatty acids are released into the blood and they travel to the muscle where they are taken up and used for fuel.
During long duration exercise, the body can deplete the muscle glycogen "fuel tank" within 3 to 4 hours. By consuming proper mixtures of water and carbohydrate, the athlete can provide more glucose to the muscle through the blood to help decrease reliance on muscle glycogen and conserve it longer.
During long duration exercise, the body also can deplete muscle triglyceride (muscle "fat fuel tank") significantly. Thus, the muscles can become more reliant on fatty acids "shipped out" from fat cells and taken up from the blood into the muscle.
Thus, limitations in fuel supply to the muscle can be related to depleted carbohydrate (glycogen) and depleted fat (triglycerides) right in the muscle as well as a decreased supply of these fuels from the blood.
Dietary fat intake that is excessively low for an extended period of time would be expected to result in decreased fractional absorption of fat soluble vitamins, and deficient intake of essential fatty acids. It can take many months or even years for fat soluble vitamin levels and essential fatty acid levels to reach clear levels of clinical deficiency.
The appropriate level of fat intake may vary from one person to another. Amount of body fat and overall energy expenditure of training may influence the amount of fat that is appropriate for an athlete. Due to genetic differences, some athletes may perform better at higher levels of body fat than others. In other words, it is not always better to have less body fat. There is a point of diminishing returns that likely varies greatly from one person to another.
The optimal range for fat intake of an athlete is probably 15 to 25% of energy intake. Athletes with high energy expenditure may benefit most by consuming fat at the lower end of this range since 25% of a very high calorie intake can be too many grams of total fat intake and may not be conducive to long-term health in some individuals.
C. Alan Titchenal, PhD, CNS
Department of Human Nutrition, Food & Animal Sciences
CTAHR, University of Hawaii