The primary source of energy for the body during high-intensity exercise is carbohydrates, primarily stored as glycogen in muscles and the liver. During intense activities, such as sprinting or heavy lifting, ATP (adenosine triphosphate) is produced quickly from glycogen stores through anaerobic glycolysis. This rapid energy release is crucial for sustaining peak performance. As the intensity decreases or duration increases, the body may also tap into fats as a secondary energy source, but carbohydrates remain the most effective fuel for high-intensity efforts.