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Recently, I have started working on my weight and, having a curious mind, I looked deeper into this whole exercise/metabolism/energy production thing. And I became curious how much water is needed to burn 1000 calories while doing aerobic exercise.

At first I looked into glucose oxidation and it looked pretty straightforward - fully oxidizing 1 glucose molecule generates so much energy and releases 6 water molecules. So unless a lot of water is involved in breaking down glycogen to individual glucose molecules, it's a no-brainer.

Then however I realized that glycogen is used in moderate-to-intensive exercise, while I am curious about slow energy burning exercise like walking. And apparently in this case, especially after some time walking, the body starts burning fat. This is where I am completely lost, as it is way above my high school level of biochemistry, most of which I have forgotten.

So my question is, is water consumed or released while burning fat? And how much would it be in order to generate 1000 kcalories?

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    $\begingroup$ As biochemical, metabolic and tissue structure processes are involved, it is a huge simplification to reduce it to direct chemical fat–water equivalence. // It is known fact that as fat has lower density than water and in initial stages is being replaced by the same water volume, so one can even gain weight. This does not related to the chemical equivalence. $\endgroup$
    – Poutnik
    Commented Jun 14, 2023 at 13:51

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There are 9 (dietary) calories in a gram of fat, as a rule of thumb. So for 1000 kcal (or 1000 dietary calories), you would metabolize about 110 g of fat. To estimate how much water is made, we assume the fat is made exclusively from oleic acid. The net reaction for complete oxidation is:

$$\ce{C18H34O2 + 51/2 O2 -> 17 H2O + 18 CO2}$$

To get the mass of water, we take the mass of fat and multiply it by the ratio of stoichiometric coefficients and the ratio of molar masses:

$$m_\mathrm{water} = m_\mathrm{fat} \cdot \frac{\pu{18 g mol-1}}{\pu{282.47 g mol-1}} \cdot \frac{17}{1} = \pu{119 g}$$

This is just the core reaction. There are lots of other things that happen, including the hydrolysis of the triglyceride (which uses up one water for every fatty acid released). As the fatty acid is oxidized, you might be making ATP, which has an effect of the water balance as well. You would have to specify exactly what you mean with "burning" fat to give an idea what you want to consider as the net effect. The net reaction I used looks like a combustion reaction, but the set of reactions going on in the body is more elaborate and more controlled.

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  • $\begingroup$ How do you produce ATP from fatty acid oxidation? $\endgroup$ Commented Jun 15, 2023 at 3:32
  • $\begingroup$ This is exactly what I wanted, including the bit about triglyceride hydrolysis. Thank you, Karsten. I am aware of subsequent ATP hydrolysis to release the stored energy, but the reverse also happens in order to make ATP from ADP. What I meant by "burning" was the ultimate use of released energy by muscles. I guess I have one follow-up question - I read that energy released in the process of oxidation is used to make ATP molecules. My question is can the released energy be directly used in muscles right away, or is the ATP creation a necessary step? $\endgroup$
    – Renat
    Commented Jun 15, 2023 at 11:15
  • $\begingroup$ @Renat The creation of ATP is a necessary step in the way metabolism currently works. There might be some extinct "primitive" species with a more direct path, or some future species. ATP is genius because you can take different chemical reactions that all make ATP, and you have a choice what to do with the ATP, so it is a very flexible system. Just like having money instead of 1:1 bartering where you trade your chemistry skills against some reputation points but really you need some music instruction. $\endgroup$
    – Karsten
    Commented Jun 15, 2023 at 13:21
  • $\begingroup$ @JiaoCtagon It is a three-step process, and some students spend an entire semester on learning it. Briefly, fatty acids are oxidized while reducing NAD+ to NADH. NADH is oxidized with O2 while pumping protons across the mitochondrial matrix. Protons flowing back through ATPsynthase is coupled with turning ADP and phosphate into ATP and water. Water, again. Here is a textbook chapter (also Ch. 19). $\endgroup$
    – Karsten
    Commented Jun 15, 2023 at 13:25

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