What is the primary source of energy that is used during prolonged exercise when glycogen stores become depleted?

During prolonged exercise, the body's energy needs shift as glycogen stores become depleted. Once these insulin-sensitive carbohydrate reserves are low, the body primarily turns to adipose tissue, which stores fat. Fatty acids are mobilized from adipose tissue and utilized for energy through a process called beta-oxidation. This transition is crucial because it allows the body to sustain activity even when carbohydrate sources are limited.

Adipose tissue serves as a significant energy reservoir, providing a more abundant and sustainable source of energy for prolonged efforts. While dietary carbohydrates may support energy needs during shorter or less intense activities, they are not a primary source when glycogen becomes low. Similarly, proteins can be used for energy, but this is not the preferred or main pathway during prolonged exercise, as protein metabolism typically occurs in situations of severe energy shortage or for tissue repair rather than primarily for fueling exercise. Muscle glycogen, on the other hand, is effectively what the body uses first, before the transition to fat stores occurs as the exercise continues and glycogen depletes.