What is the main source of energy for the body during prolonged exercise?

During prolonged exercise, the primary source of energy shifts significantly from carbohydrates to fats, specifically lipids. Initially, the body relies heavily on glucose stored in muscles and liver as glycogen, but as exercise continues and glycogen stores diminish, lipid metabolism becomes the dominant source of energy.

Lipids provide a more energy-dense fuel than carbohydrates, yielding more ATP per gram when metabolized. This is particularly advantageous for sustaining energy during extended physical activity when glycogen resources are limited. The body engages in a metabolic process called lipolysis, where fatty acids are released from adipose tissue and transported to the muscles for oxidation, allowing for prolonged endurance.

While glucose is vital for short bursts of high-intensity activity and proteins can serve as a last resort energy source under starvation or extreme conditions, lipids are the essential and sustainable energy source during longer durations of exercise. Thus, lipids are indeed the main fuel that supports the body through prolonged physical activity.