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Yesterday, I saw TV Game Show.

There is question like this: Body loses more fat in which form? It gave 4 options:

1) CO2

2) sweating

3) xxx

4) xyz

sorry i didn't remember 3 & 4 options. so right answer is CO2. anchor explaining the reason behind it, suddenly my brother switch the channel. all gone.....

Everybody thinks sweating is right answer, i have reason for it, if we work out our body whole fat comes out in sweating form.

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  • $\begingroup$ Sweat is the release of water that evaporates, allowing the body to cool itself under exertion. When fats are converted to energy, they are used as an input into the Citric acid cycle. The molecules are eventually broken up and the byproduct is CO2. $\endgroup$
    – AMR
    Dec 14 '15 at 2:21
  • $\begingroup$ Fatty Acid Metabolism and The Citric Acid Cycle. And for perspiration $\endgroup$
    – AMR
    Dec 14 '15 at 2:28
  • $\begingroup$ This is a curious case of a rather poorly stated question that actually has something pretty interesting underneath --- where does the atoms from fatty acid oxidation go? CO2? Water? Now I don't know how to vote :) $\endgroup$
    – Roland
    Dec 14 '15 at 7:17
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If you exercise heavily, you can sweat enough where you have to drink to replace it. But on a normal day, you don't sweat all that much.

On the other hand, when you metabolize food, you essentially burn it. According to the Merck Manual,

Carbohydrates, proteins, and fats supply 90% of the dry weight of the diet and 100% of its energy. All three provide energy (measured in calories), but the amount of energy in 1 gram (1/28 ounce) differs:

4 calories in a gram of carbohydrate or protein

9 calories in a gram of fat

A typical diet is 2000 calories per day. If you get 50% of your calories from fat (shame on you!), that is 1000 calories of carbohydrate or protein, or 250 grams. It is 1000 calories of fat, or 111 grams. 361 grams total.

We will simplify. CH bonds contain little energy compared to CC bonds. H weighs little compared to C. We don't change much by ignoring H. There is some O and other stuff, but we will ignore that too. It will still be roughly right. So you burn about 361 grams of C.

For every C atoms you metabolize, you must breathe in 2 O atoms, and breathe out all 3 atoms as CO2. C atoms have an atomic weight of 12. O is 16. So CO2 is 44. Every 12 grams of C becomes 44 grams of CO2. So to burn 261 g of C, you breathe out 1324 g of CO2.

1324 g of water would be 1.324 liters. It is easily possible to sweat that much in a day, but you typically don't. You typically breathe out more CO2 than you sweat water.

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  • $\begingroup$ I think this is rather misleading. The CO2 produced in fatty acid oxidation is not derived from O2 but from water, via hydration reactions. O2 is the terminal electron acceptor in the respiratory chain. You must take this into account when estimating water production. Also, C-H bonds are as important as C-C; many high energy electrons are extracted in fatty acid oxidation by replacing C-H with C-O bonds. $\endgroup$
    – Roland
    Dec 14 '15 at 7:14
  • $\begingroup$ @Roland - I am sorry. I did not see this until today when someone upvoted. I thought I was supposed to be notified. You clearly know more than I about this. How should I improve the energy estimate of CO2 production? The O in CO2 can originate from water in a hydration reaction, but C cannot. Also this was about sweating, not the total water produced by metabolism. Some water is breathed out, some goes in urine. $\endgroup$
    – mmesser314
    Dec 22 '15 at 20:43
  • $\begingroup$ Yeah the questions was pretty vague to being with. I would just ignore the effect of oxygen since it doesn't originate from fat anyway, and say that since fat is mostly (CH2) units with mass ~14, then by mass 12/14 = 85% of fat is exhaled as CO2 and 2/14 = 15% leaves as water. Of course we're tallying up parts of molecules here but it's the way the question was posed I think. The energy yield is not relevant I think. $\endgroup$
    – Roland
    Dec 25 '15 at 18:07

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