Whither the Weight?

Several times a week I hop on the ol’ elliptical trainer and chug away for half an hour. When I’m done the glowing readout informs me I’ve burned off somewhere north of 500 Calories.

While recognizing that the number is little more than a wild-ass guess, I find myself wondering as I plod along, “Just how much does 500 Calories weigh?”

Well, of course it’s not that simple. Calories are a measure of energy, and while I haven’t done the math, if we apply M=E/C2 we end up with a very, very small number. But we’re not annihilating matter to produce energy, we’re oxidizing organic molecules. So, let’s think about the weight of the fuel required to produce 500 Calories.

Since we’re only dealing in wild-ass approximations anyway, let’s round things off to make the math easy. If all the molecules used to produce that energy were fat, we’re talking about 50 grams. That’s something like a shot of vegetable oil.

Not bad! If I burn off a shot-glass of fat each time I exercise, I’ll be skinny in a month!

Alas, even after many months, I’m not skinny. First problem, only a tiny fraction of those 500 Calories come from fat. Fat is long-term storage and my body hangs onto it tenaciously, saving against the month when there’s no food. For most of the history of our organism and its ancestors there have been such stretches, and the ones who held onto their fat when times were good survived when things were bad. So, when energy is needed, the first to burn is the short-term storage.

But hey! Great news! The sugars and other carbohydrates used for short-term storage weigh even more! Instead of burning 50 grams of fuel, I might be burning more like 100g.

Which are quickly and efficiently replaced by the food I eat. And while my body is at it, if I’m going to insist on burning all that energy, it’s going to build up muscle mass so I can burn the fuel more easily next time. Which increases the dense muscle tissue and makes me heavier — but healthier. Ahhh! It’s all so complicated! But I’m digressing here.

Regardless of the source of the fuel and whether I actually lose weight, I metabolize something like 100g of hydrocarbons while I exercise.

Where does it go?

The answer is pretty simple, but kind of cool. A lot of the weight I lose goes right out my nose. We breathe oxygen in, and, through a ridiculously complex system of events, that oxygen gets mixed up with hydrogen and carbon from our food. The carbon atoms, now hooked up with oxygen, are carted back to the lungs and exit each time we exhale. Food in, CO2 out.

The hydrogen atoms from the food also hook up with oxygen to form water, which leaks out of our bodies in a dozen different ways (mammals are live-fast-and-squander-water creatures). There are some other byproducts of our metabolisms, like nitrogen from burning protein, that make their way out in our pee.

But most of the mass (and hence weight) oxidized due to my time on the trainer flows out my nose. When the gentle chime tells me my travails are done, I’ve exhaled enough carbon to make a 500-carat diamond*. And each time, it’s a little bit easier.

*The mother of all wild-ass guesses!


2 thoughts on “Whither the Weight?

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