By Jan Helgerud, Professor of Medicine The more you move, the more energy your body uses. But how exactly does the body burn energy — and what types of exercise lead to the highest energy expenditure? Understanding the mechanisms behind energy conversion can be complex, but here’s a clear explanation of how the body turns food into fuel.
Oxygen Uptake = Energy Conversion
Oxygen uptake — a key measure of fitness — is measured in liters per minute (L/min). For every liter of oxygen your body consumes, it produces about 5 kilocalories (kcal) of energy. In other words, when we measure your oxygen uptake, we’re actually measuring your energy metabolism.
Once we know your maximum oxygen uptake, we know the size of your body’s “engine.” But a large engine alone isn’t everything — if you're “driving a heavy truck,” it takes a lot of energy just to move your own body. That’s why efficiency matters too.
Your Engine Needs Fuel
The food you eat provides the fuel your body needs to function. The two primary sources of fuel are carbohydrates (sugars) and fats.
However, there's a catch: each gram of carbohydrate binds to 3 grams of water, which means you carry 4 grams of weight for every gram of carbohydrate energy stored. That translates to just 1 kcal per gram of body weight. When carbohydrates are converted to fat in the body, the energy density increases — and the weight decreases by a factor of 8.
Carbohydrates Are Limited — But Essential
Because carbohydrate storage is "heavy," the body keeps its stores limited. Even when fully topped up, you only carry around 1 to 1.5 kg of carbohydrates, equal to 4,000–6,000 kcal — enough for about 90 minutes of high-intensity work. After that, you "hit the wall" unless you refuel or significantly reduce your intensity.
That said, without carbs, you couldn’t sustain high-intensity work at all. You’d be grateful for their limited but powerful presence.
Carbs vs. Fat: What’s Burning?
Your body typically burns a mix of carbohydrates and fat, depending on intensity and duration:
Interval Training vs. Long Runs
High-intensity interval training relies almost entirely on carbohydrates. A long, slow-distance run at around 50% of your max capacity allows you to burn both carbs and fat — and continue for hours.
Interestingly, fat burning peaks at 40–60% of maximum oxygen uptake, but even then, you're still burning carbohydrates at the same time. It’s never just one or the other.
How Duration Affects Fuel Use
The longer you exercise, the more the body shifts toward fat as fuel. For example:
Still, duration alone doesn’t give you the same training effect as intensity.
Moderate Training Isn’t Enough
Prolonged moderate-intensity training is not our recommended approach for improving fitness. For example, you’ll burn the same amount of energy in a 30-minute 4x4 interval session as you would in 50 minutes of moderate training — even though the interval workout is 40% shorter.
When it comes to building maximum oxygen uptake, intensity is key. Duration simply can’t replace it.
Here’s the Paradox
New research shows that the single most important factor for improving fat metabolism is... maximum oxygen uptake.
And the best way to increase that?
High-intensity interval training — which relies entirely on carbohydrate metabolism.
The paradox is this: while interval training burns mostly carbs in the moment, it significantly improves your aerobic capacity. As a result, you become a far better fat-burner in all other activities.
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