The Why .....

The human body uses proteins, fats, and carbohydrates as sources of fuel for exercise and muscle contraction. During moderate-intensity exercise, roughly half of the energy is derived from glycogen, while the other half comes from glucose in the blood and fatty acids.

Carbohydrates (glucose/glycogen) serve as the primary source of fuel as duration and intensity increase. The human body uses proteins, fats, and carbohydrates as sources of fuel for exercise and muscle contraction. During moderate-intensity exercise, roughly half of the energy is derived from glycogen, while the other half comes from glucose in the blood and fatty acids. Carbohydrates (glucose/glycogen) serve as the primary source of fuel as duration and intensity increase. If exercise continues for a significant period of time, fatty acids will serve as the fuel source when glycogen stores are nearly depleted.

A person’s diet will influence which source of fuel is used and therefore, performance level. If a person consumes a high-carbohydrate diet, more glycogens will be used for fuel. If the diet is high in fat, fat will be used as the fuel source. A high-fat diet is not recommended as even the leanest person has plenty of stored fat for long endurance exercise. A high-fat, low-carbohydrate diet can lead to poor performance due to low glycogen stores.