How intensity shifts the fuel mix
The body draws on both fat and carbohydrate during exercise, and the balance between them shifts with intensity. At low intensities, fat supplies a larger share of energy because there is enough oxygen and time to oxidize it. As intensity rises, the mix tilts toward carbohydrate, which can be broken down faster and yields more energy per liter of oxygen, making it the preferred fuel for hard efforts.
The intensity at which fat oxidation peaks in absolute terms is sometimes called maximal fat oxidation, after which carbohydrate increasingly dominates.
Why this matters for training and fueling
Understanding the crossover helps explain why long, easy sessions rely heavily on fat while race-pace efforts depend on carbohydrate availability, and why carbohydrate intake becomes important for sustained high-intensity work. Training status also raises the ability to oxidize fat at a given intensity.
Individual crossover points vary with fitness, diet, and the activity. This is general educational information about exercise metabolism, not personalized nutrition advice.