Why does the percentage of energy derived from carbohydrates increase as exercise intensity increases?

Higher intensities create greater ATP demand and lower local oxygen availability; carbohydrate oxidation produces ATP more rapidly and can proceed anaerobically, so muscles rely more on carbohydrates.

As intensity rises, body temperature drops and enzymes required for fat oxidation stop working, so only carbohydrate pathways remain active.

Carbohydrates are stored in unlimited amounts and therefore always become the preferred fuel regardless of oxygen availability or metabolic rate.

Fats cannot be mobilized during exercise because hormone-sensitive lipase is inactivated by exercise, forcing total dependence on carbohydrates even at low intensities.