Why Running Uses Both Fat and Carbs – and Why the Switch Isn’t Sudden

Many runners repeat the simple rule ‘run slow to burn fat, run fast to burn carbs’, but the human body does not work with an on‑off switch for fuel use.

When you run, the body always uses a combination of fat and carbohydrates. Fat is a massive energy storehouse that provides long‑lasting energy, but it is mobilized relatively slowly.

Carbohydrates, on the other hand, exist mainly as glycogen stored in the muscles and liver. Although the total amount is limited, glycogen can be broken down and delivered to working muscles much faster than fat.

During low‑intensity exercise the body burns a higher proportion of fat, but as you increase speed the demand for quick energy forces a greater reliance on carbohydrates. This shift is gradual; fat oxidation never stops completely, nor does carb use start abruptly.

For runs that last between one and two and a half hours, the body typically needs about 30–60 g of carbohydrate per hour. If the effort extends beyond two and a half to three hours, the requirement rises to roughly 60–90 g per hour, depending on intensity and how quickly the gut can absorb the fuel.

Understanding that both fuels are always at work helps you plan training and nutrition more realistically: choose a pace that matches your goals, and make sure to replenish carbs during longer sessions rather than relying on the myth that ‘fat‑only’ runs will melt extra pounds.

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