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During a whole day breathing, how many kcal or watts do we get from the oxygen we breathe?

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    $\begingroup$ Why would you want to know something like that? What possible scientific interest does it have for you? $\endgroup$
    – David
    Commented Jun 5, 2018 at 18:16
  • $\begingroup$ I'd say the only way this question would be answerable (see the comments and existing answers) is if we tried to transform it into "How much energy is produced via aerobic versus non-aerobic metabolism (in a human)?" - it's not really the same question, though, and might not really be what you were after, especially considering the content of the answer you hastily accepted. I think if you made this change it would be better to close this one and start a new question, rather than edit, given the existing answers here. $\endgroup$
    – Bryan Krause
    Commented Jun 5, 2018 at 21:06
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    $\begingroup$ No scientific interest. It was an informal conversation and I am curious. $\endgroup$
    – Javi
    Commented Jun 6, 2018 at 8:41

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I know that you already picked an answer, but I found an alternative answer for you. On average a person consumes 550 litres of oxygen in a day. Now, our body could burn carbohydrates or fats or proteins to produce energy. Each litre of oxygen can produce 5.04 kcals if it burns carbohydrates, produce 4.68 kcals if it burns fats or produce 4.48 kcals if it burns proteins (reference). That being said, if I consider that all 550 litres were used to burn carbohydrates, a total of 550 x 5.04 kcals will be produced which is 2780 kcals.

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    $\begingroup$ @David Where did I cite 6 figures? I just felt like answering it. Doesn't really matter who he picks or not $\endgroup$ Commented Jun 5, 2018 at 18:33
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    $\begingroup$ 27.7585 kcal! Four decimal places, but six significant figures. $\endgroup$
    – David
    Commented Jun 5, 2018 at 18:38
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    $\begingroup$ @David ah.. u can edit it as u seem fit. $\endgroup$ Commented Jun 5, 2018 at 18:52
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    $\begingroup$ OK. Have done that. Reduced to 3 significant figures as determined by values of 4.68 and 5.04. Anyone who doesn't understand this may be interested in my self-teaching web application on this topic at mvls.gla.ac.uk/Teaching/SLS-FTB/sigfigs.html. $\endgroup$
    – David
    Commented Jun 5, 2018 at 20:31
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    $\begingroup$ I am not a biology expert (not even familiar with biology). I was just curious so I assumed all the answers are OK. But I like now this one more :) Thanks! And come on, discussing for some decimals? $\endgroup$
    – Javi
    Commented Jun 6, 2018 at 8:44
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This question is not actually answerable.

Consider this. Take a nice campfire. It's warm and toasty. How much energy comes from the wood you piled at the bottom. It's a lot right?

Now how much energy comes from the oxygen the fire consumed?

We know that a fire needs three things to burn: fuel, oxygen, and heat. How much energy comes from the heat?

Who creates a baby, the mother or the father? Obviously the mother caries it to term, but the creation itself takes two, interacting.

These questions don't have clear answers because there's no natural division to be made. We can't state how much energy came from the fuel or how much energy comes from the oxygen, because it is not the fuel nor the oxygen which provides the energy, but the reaction between the two.

In general, we find oxygen is more accessible than fuel. We can always get oxygen from the air by breathing, but one has to find oxidizable fuel (food). As such, we find it convenient to associate all of the energy of the reaction to the fuel, for bookkeeping purposes. But that's just bookkeeping. We do this because, if we're short on oxygen, we just have to breathe a little more. If we're short on fuel, we have to go find fuel (food).

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  • $\begingroup$ But in a combustion there is a clear amount of energy taking place, I do not care where it exactly comes from. What I wanted to know is how much energy does breathing give to our body in average. $\endgroup$
    – Javi
    Commented Jun 6, 2018 at 8:46
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    $\begingroup$ @JaviV The human body burns roughly 2000 Calories a day (roughly 8.3MJ/day). Thus, in exactly the same sense that "in a combustion there is a clear amount of energy taking place," breathing gives an average of 8.3MJ/day. $\endgroup$
    – Cort Ammon
    Commented Jun 6, 2018 at 14:25
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    $\begingroup$ Of course, it's not the breathing that releases the energy. The energy is released in the cells themselves, through the Citric Acid Cycle. All breathing does is simply make sure there is oxygen available in the cells. The cells do the releasing of the energy. $\endgroup$
    – Cort Ammon
    Commented Jun 6, 2018 at 14:27
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    $\begingroup$ Also, note that that average does vary significantly based on food consumption, because the body regulates itself to avoid running out of stored fuel. If you go on a diet, consuming fewer calories, your body will adjust the amount of oxygen it brings (by breathing less and/or using less oxygen from each breath) $\endgroup$
    – Cort Ammon
    Commented Jun 6, 2018 at 14:52
  • $\begingroup$ @CortAmmon Roughly 8.4 MJ/day if you round the numbers correctly. $\endgroup$ Commented Apr 29, 2022 at 16:17
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Oxygen does not provide any calories (energy) or watts (power), just the same as a water bottle states it contains zero calories. Energy is held within glucose, or other macromolecules than can be broken down to supply electrons that generate ATP, or other similar molecules.

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    $\begingroup$ Well yes and no. The reaction of oxygen with the terminal electron acceptor of the electron transport chain provides the necessary Gibbs free energy change to allow the chain to continue, producing a chemiosmotic gradient from the oxidation of NADH and FADH2. Energy is not held within molecules. It is produced by their chemical reaction. $\endgroup$
    – David
    Commented Jun 5, 2018 at 18:25
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    $\begingroup$ I would argue energy is held between the bonds of atoms, unless you are talking about quantum effects. Thank you for correcting my knowledge ETC, but I nevertheless believe that ATP is not going to be produced with oxygen alone and in a biological sense and therefore does not biologically contain energy. However I also think this question was poorly asked. $\endgroup$
    – user42909
    Commented Jun 7, 2018 at 12:23
  • $\begingroup$ I didn't say that energy could be produced by oxygen alone — obviously it cannot. And yes, sugars etc are a potential source of energy because they have the potential to participate in reactions that are endo-ergonic. But to participate in the types of reaction we are considering — oxidation — they need a co-reactant that is also going to be changed. This is important. You can get round this to some extent by looking at the electrode potentials for the half reactions here, but it turns out that they are both +ve, so both are contributing to the –ve free energy change. $\endgroup$
    – David
    Commented Jun 7, 2018 at 13:50

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