Where do body fat metabolism products go?
April 18, 2012 7:39 AM   Subscribe

If I, say, go on a very long run and burn enough calories to metabolize one ounce of fat, how does that weight leave my body? That is, do the products come out in my breath, pee, poop, or some combination? Everything I can find online is either too complicated or too simple to help me find an answer.
posted by ftm to Health & Fitness (13 answers total) 20 users marked this as a favorite
The weight leaves your body in pretty much the same way fuel leaves a car. It gets burned, and you exhale carbon dioxide and water vapour, while the energy released in burning goes into making you move. The 'burning' happens on a minute scale within your cells, and is called (cellular) respiration.
posted by pipeski at 7:43 AM on April 18, 2012 [2 favorites]

Well, no, NP, the weight is not converted to energy; we don't have nuclear furnaces inside us. The actual fat is converted into chemicals the muscles can directly use (more or less). If we assume "fat" to be just the fatty substance, not the cells, for simplicity, it's mostly hydrocarbons. So we exhale the product of oxidizing hydrocarbons - water and carbon dioxide. We also pee water. The other stuff - that we ignored earlier - is re-used in the short term, and in the long term is shat or pissed out.
posted by notsnot at 7:45 AM on April 18, 2012

(NP said the weight leaves the body as heat...now the first part of my comment makes no sense)
posted by notsnot at 7:45 AM on April 18, 2012

Yes, it leaves in your breath and also marginally in your pee. This is true, incidentally, whether you are burning fat or glycogen, which is mostly what you are burning for the first many miles of running.
posted by OmieWise at 7:45 AM on April 18, 2012 [1 favorite]

Some small fraction of the water molecules that were created in the process of metabolizing the fat probably leave your body via the sweat glands as well.
posted by Johnny Assay at 7:47 AM on April 18, 2012

It never even occurred to me to make it a simple chemistry problem but now it's clear (oxidation products->water and CO2). Thanks, folks!
posted by ftm at 7:49 AM on April 18, 2012 [1 favorite]

Yeah, you do get water and carbon dioxide, but you also get waste products. Glycolysis isn't entirely efficient, and once you exercise long enough, you run out of certain products and switch to anerobic processes which are even less efficient. So you do wind up peeing out waste products.

Read up on ketosis for information on the metabolism of triglycerides.
posted by valkyryn at 8:10 AM on April 18, 2012

Also, keep in mind the heat is a big output of the body due to cellular metabolism, as well.
posted by spladoodlekeint at 8:14 AM on April 18, 2012

It's worth noting that the concept of "run x miles to burn y calories and you'll 'burn' z lbs of fat over time" is pretty much bull crap. Aerobic exercise burns far fewer calories than machines at the gym or tables of exercise data claim and the process is much, much more complex than weight change = calories in - calories out. The composition of your diet affects how your body utilizes energy and how you behave (i.e. if you eat a lot of healthy food you will *want* to go out and do stuff because your body is telling your brain that it needs to)
posted by imagineerit at 8:32 AM on April 18, 2012

Fat (mass) is transformed into heat energy that keeps your body at an even temperature, and chemical energy that catalyzes the production of metabolites, which themselves have potential energy, and direct products of respiration (water and carbon dioxide). However, the body cannot process all metabolites, and those are neutralized and excreted. So fat weight is transformed into heat and, directly and indirectly, waste products. Excretion itself takes up energy, so you burn some fat reserves there, too.
posted by Blazecock Pileon at 9:26 AM on April 18, 2012

It's worth noting that the concept of "run x miles to burn y calories and you'll 'burn' z lbs of fat over time" is pretty much bull crap.

Yep. The body will always burn available sugar before resorting to burning fat, and it'll burn fat you've recently eaten that it's still in the process of digesting before resorting to fat reserves.

So yes, there are something like 220 Calories available in a single ounce of fat. But even if you go on a run which burns 440 Calories, it's unlikely that your body will actually look to your fat deposits to supply that energy. You're pretty likely to have two ounces of sugar in the pipe somewhere it can look to first.
posted by valkyryn at 10:27 AM on April 18, 2012 [1 favorite]

NPR Science Friday show on this very topic:

"Okay, so the fat's got a bunch of carbon and hydrogen and oxygen atoms in it. What's happening to those carbon and hydrogen and oxygen atoms? What they tend to say is that they are burned up or burned off or something like that, or they'll say that they're converted into energy.

And both of those are absolutely wrong, that in fact when you lose weight what you're doing is you're breathing it out. The fat's being converted to carbon dioxide and water, and so it's leaving your body mostly with your breath."
posted by gingerbeer at 2:25 PM on April 18, 2012

Oh, great addition, thank you!
posted by ftm at 2:56 PM on April 18, 2012

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