The endurance athlete has two primary fuels sources – carbohydrates and fat. Learn more about what is the primary fuel during endurance exercise.
Fatigue during endurance training is associated with depletion or reduction of the body’s carbohydrate stores. Carbohydrate is like fuel with “high octane” – a steady supply is necessary for high-intensity training to continue. As the carbohydrate supply decreases, the athlete must slow down.
Fat provides a good energy source for continuous training with lower intensity. Well-trained endurance athletes can rely more on fat as a fuel that helps save carbohydrate fuel and delay fatigue and reduce intensity when exercising.
The carbohydrate levels are limited in size. Most endurance athletes can store carbohydrates worth between 2,000 and 4,000 calories in a form called glycogen. Most of this is stored directly in the muscle tissue. Some are also stored in the liver.
The fat reserves are usually assumed to be relatively unlimited. But, this may not apply to all athletes. Most fat is stored in cells, but some fat is stored directly in muscle tissue in muscle triglycerides.
As for exercise intensity increases, the body uses more carbohydrates and less fat as fuel.
During exercise, the muscles have two sources of carbohydrates. Muscles use carbohydrates as fuel in the form of glucose. Glucose in the blood comes from carbohydrates absorbed from food and drink and the glucose produced by the liver.
During exercise, muscles use fat as fuel in fatty acids. This fatty acid fuel comes from the breakdown of triglycerides directly into the muscle tissue and fatty acids taken up from the blood and into the muscle.
During prolonged exercise, the body can empty the muscle glycogen “fuel tank” within 3 to 4 hours. By consuming the proper mixtures of water and carbohydrates, the athlete can give more glucose to the muscle through the blood to reduce the dependence on muscle glycogen and preserve it longer.
The body can also significantly deplete muscle triglyceride (muscle “fat fuel tank”). Thus, the forces can become more dependent on fatty acids “sent out” from the fat cells and taken up from the blood into the muscle.
Thus, limitations in the fuel supply to the muscle can be related to depleted carbohydrate (glycogen) and consumed fat (triglycerides) directly in the muscle and reduced supply of these fuels from the blood.
How much fat is too little fat? For good nutrition? For endurance?
Fat intake in the diet that is too low over a long period will result in reduced fractional absorption of fat-soluble vitamins and inadequate intake of essential fatty acids. It can take many months or even years before levels of fat-soluble vitamins and levels of essential fatty acids reach precise levels of clinical deficiency.
The right level of fat intake can vary from person to person. The amount of body fat and the total energy consumption during exercise can affect the amount of fat appropriate for an athlete. Due to genetic differences, some athletes may perform better at higher body fat levels than others. In other words, it is not always better to have less body fat. A point of declining returns probably varies significantly from person to person.
The optimal range for fat intake is probably 15 to 25% of energy intake. Athletes with high energy consumption may benefit most from consuming fat at the lower end of this range. 25% of a very high-calorie intake may be too many grams of total fat intake and may not contribute to long-term health in some individuals.