What serves as the predominant energy source for the body as exercise intensity increases?

As exercise intensity increases, carbohydrates stored as glycogen emerge as the predominant energy source for the body. When engaging in higher intensity activities, the body requires a more readily accessible form of energy. Glycogen, which is stored in muscle and liver cells, can be quickly broken down into glucose to meet the immediate energy demands of working muscles.

During intense exercise, the rate of energy expenditure is high, and carbohydrates provide a rapid source of ATP (adenosine triphosphate) through anaerobic glycolysis when oxygen is limited, and through aerobic metabolism when there is sufficient oxygen. This is particularly important during high-intensity activities like sprinting or heavy lifting, where the body relies on these quick energy systems.

While fat can serve as an energy source, it generally becomes more prominent during lower-intensity activities when the energy demand is less immediate. Protein is usually not a primary energy source during exercise; it mainly serves for tissue repair and muscle growth. Alcohol does not provide a meaningful contribution to energy during exercise and can impair performance and recovery.