The primary energy source for the body during high-intensity exercise is carbohydrates, which are broken down into glucose. This glucose is then utilized for quick energy production through anaerobic respiration, releasing energy in the form of ATP (adenosine triphosphate). In high-energy demands, carbohydrates provide a rapid source of fuel that is essential for performance and endurance. After depleting glycogen stores, the body may also tap into fat reserves, but this occurs primarily during lower-intensity activities.