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The primary source of energy for high-intensity exercise is carbohydrates. During such exercise, the body relies heavily on glucose, which is derived from carbohydrates, to quickly generate ATP (adenosine triphosphate). This process occurs through anaerobic glycolysis, allowing for rapid bursts of energy required for high-intensity activities such as sprinting or heavy lifting.

Carbohydrates can be stored in the muscles and liver as glycogen, ready to be converted to glucose when energy is needed, particularly during short-duration, high-intensity workouts. As the intensity of the exercise increases, the body shifts from burning fats to predominantly using carbohydrates, as they provide a faster source of energy.

While fats and proteins can serve as energy sources under different circumstances, they are not the primary energy substrates during high-intensity efforts. Fats are utilized more during prolonged, moderate-intensity exercise, and proteins generally serve as a last-resort energy source when carbohydrates and fats are not adequate. Vitamins, while essential for metabolic processes and overall health, do not provide energy directly. Therefore, carbohydrates stand out as the preferred fuel for high-intensity exercise.