Where Does Fat Go When You Exercise? (A Molecular Monday Post)

Exercise is good for you, I guess. It’s probably one of the better options for anyone who’s trying to lose weight. But when you exercise, where does the weight go, physically speaking?

The first time someone asked me this question, my best guess was that it had something to do with Einstein’s E = mc2 equation, the equation that allows matter to be converted into energy. But I knew that couldn’t be right. That’s more of a nuclear physics thing, and the human body is not a nuclear reactor.

The actual answer has to do with chemistry. Rather simple chemistry. This is a triglyceride molecule:

Okay, it is sort of a complicated-looking molecule. Don’t worry. Your body knows what to do with it, even if your brain doesn’t.

The important thing, in relation to today’s question, is that triglyceride is composed almost entirely out of carbon and hydrogen atoms, with a few oxygen atoms sprinkled in.

Now when your body exposes triglyceride to the oxygen you breathe in, that highly reactive oxygen starts breaking the triglyceride molecule apart. With each chemical bond that breaks, a little bit of energy is released (allowing you to keep exercising), and the broken pieces of triglyceride recombine with oxygen to make carbon dioxide (CO2) and water (H20).

It’s worth noting that chemical bonds do contribute marginally to the total mass of a molecule, so when you break them and turn them into energy, E = mc2 does apply, sort of. But that’s nowhere close to being a significant factor in terms of weight loss.

The vast majority of the weight you lose comes in the form of carbon dioxide, which you breathe out through your lungs, and water, which you sweat out or pee out or breathe out as water vapor. (If you want to get into the math and find out how many kilograms of oxygen you need to burn how many kilograms of triglyceride, producing how many kilograms of water and CO2, click here.)

When I started studying chemistry, this was not the kind of thing I was hoping to learn. I’m a science fiction writer. I’m interested in the type of chemistry that makes rocket engines go, or drives weather patterns on other worlds, or could make alien life possible.

But still, it’s exciting to me when I can connect all that outer space science to some of the mundane aspects of life here on Earth.

* * *

Today’s post is part of a special series here on Planet Pailly called Molecular Mondays.

On the first Monday of the month, we take a closer look at the atoms and molecules that make up our physical universe, both in reality and in science fiction.

15 thoughts on “Where Does Fat Go When You Exercise? (A Molecular Monday Post)

  1. Interesting. Fat is broken down constantly I guess, not just when we exercise – hence the need to breathe continuously. Glucose and alcohol get oxidized in a similar way, but I don’t understand why we can’t just eat oil and other hydrocarbons. They also consist of C and H chains, but turn out to be toxic. Chemistry! Bah!

    Liked by 1 person

    1. Definitely some questions there beyond my skill level, and I think I may have oversimplified the biological side of this by focusing purely on the chemistry. I’m under the impression that our bodies don’t start burning fat until they’ve used up their supply of glucose.

      As for why some hydrocarbons give us energy while others kill us, I’m guessing—and this is just me guessing—that it has to do with chemical interactions that happen before the hydrocarbon can be processed in the mitochondria.

      But I should probably do more research on that and stop guessing about these things. Maybe I’ll return to that in a future Molecular Monday post.

      Liked by 1 person

  2. My brain, you mean I’ve got one? C02 and H20 Duh what? I always thought carbon dioxide was the bad stuff we exhaled as a result of breathing in the good stuff like air – C02 see I know stuff, but then you get all high brow with really long wordy explanations like triglyceride which combines other stuff and thats like really cool stuff to know also. So now I know lots and lots of really cool super stuff, thanks dude.

    Liked by 1 person

    1. You have a brain, my friend. I know you do!

      And you weren’t wrong: you breathe oxygen (O2) in, and carbon dioxide (CO2) comes out as a waste product. Except now you know where the O2 picks up that C from!


  3. I remember being somewhat awed when I realized that the raw inputs trees use to build themselves are sunlight and carbon dioxide, with only trace minerals from the soil needed in addition to that. So it makes sense that animals (including humans) using excess fuel reserves produces carbon dioxide and water.

    From a sci-fi author perspective, the interesting question is can we build machines that do those sorts of things? The answer might be yes.

    Liked by 1 person

    1. Thanks for the link! That feeds into something else I’ve been researching.

      It’s sort of interesting to me that plants basically put water and CO2 together, and then we take them apart.

      Liked by 1 person

      1. Me too. It’s a reminder that we’re an integral part of an ecology, a biosphere, and not just any biosphere, but the one of the current epoch. If we could time travel, we’d have trouble breathing before about 850 million years ago, and couldn’t survive the temperatures from many other periods. (I learned today that the Earth’s surface temperature 3 billion years ago may have been 167F.)

        Liked by 1 person

    1. I think by focusing solely on the chemistry, I may have glossed over too much of the biological complexity of this. You still have to do some kind of physical activity to make your body start burning fat. It’s just that once the fat burns, the weight comes out (mostly) in your breath.

      As for the fat cells, I think you had it right. They’re still there. It’s just that the triglyceride inside them goes away.


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