Running on the House Money
We tend to think of aerobic exercise in terms of calories. Run or walk for 45 minutes, burn a few hundred calories, create a deficit, and eventually lose some weight.
Fair enough. But calorie arithmetic doesn’t tell the whole story, because the body has choices about where those calories come from.
At any given moment we’re burning some mixture of carbohydrate and fat. The proportions change according to what we’re doing, how hard we’re doing it, how long we’ve been doing it, what we’ve eaten, how well trained we are, and probably a host of other things.
During sustained moderate exercise, fat can become an increasingly important part of that fuel mixture. Triglycerides stored in fat cells are broken down, fatty acids are released into the bloodstream, and working muscles can take them up and oxidize them for energy.
And here’s the wonderfully counterintuitive part: when that fat is oxidized, most of its mass eventually leaves the body through the lungs as carbon dioxide. Some leaves as water. We don’t so much “melt” fat away as metabolize it and breathe much of it out.
The House Money
Years ago I started thinking of longer aerobic exercise in terms of “playing with the house money.”
When you begin moving, there’s plenty of readily available fuel in circulation and stored as glycogen. As you keep going, particularly at a sustainable intensity, stored fat makes a substantial contribution to the energy supply.
There’s no magic point at which you suddenly switch from carbohydrate to fat. Both are being used all along, and the mixture is continually changing. But there’s still something useful about the metaphor: once you’ve developed enough endurance to keep going comfortably, you’ve gained access to an enormous reserve of stored energy.
That’s the house money.
It also helps explain why harder isn’t necessarily better. As exercise intensity rises, the body tends to rely increasingly on carbohydrate, which can supply energy rapidly. At lower and moderate intensities, fat can provide a larger share.
So a long brisk walk isn’t merely a lesser version of running. Metabolically, it has virtues of its own. You can keep doing it for a long time, recover from it easily, and do it again tomorrow.
What Happens Afterward?
The effect of exercise doesn’t end when the exercise does. Fat oxidation can remain elevated during recovery, and regular endurance activity gradually changes the machinery responsible for producing energy.
Among those adaptations are changes in mitochondria — the structures inside cells that turn fuels into usable energy — and an improved ability to use fatty acids as fuel. Exercise also affects insulin sensitivity and the way muscle handles glucose.
This is why the calorie display on a treadmill seems like such an incomplete description of what you’ve accomplished. The calories aren’t imaginary, but they’re only one part of the transaction.
The more interesting thing may be that you’re training your body to become better at supplying energy for movement.
Does It Matter When You Eat?
Probably, although not in the simple way that diet rules tend to suggest.
Eating carbohydrate before exercise generally shifts the fuel mixture toward carbohydrate and away from fat during the activity. Exercising before breakfast, for example, tends to produce greater fat oxidation during the workout.
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