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Why I even think it could have any effect: maybe when the body has deficit of the oxygen, the burning reaction modifies to burn more sugar/fat/whatever (i.e. instead of burning each molecule completely it is burning them just up to a certain point and go over to the next)? (have heard something like that but would like to have some solid facts :) If there is any difference, how much is it, is it worth to think about during workouts?

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If that were true, do you expect taking deeper breaths would result in burning more calories too? –  Ivo Flipse Mar 26 '12 at 13:32
    
Well I am not expecting anything, I am asking how it is. –  Cray Mar 27 '12 at 14:30
    
Then what did you expect to do with the answer, if not change your workout? –  Ivo Flipse Mar 27 '12 at 14:34
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My point was mostly that your question is built on some kind of assumption from which you hypothesize a certain outcome. However, if the assumption itself doesn't hold, then you wouldn't have to ask the question. So I'm wondering, why did you expect this to matter (or not), because then the answer should explain you why that was wrong and teach you something :-) –  Ivo Flipse Mar 28 '12 at 14:51
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I see, well I guess you could say that the assumption was that since there are several ways to breath, presumably letting in more/less air into the system during the exercise, the body must have some ways of variating the burning processes, and that those variations could affect the efficiency of the process also in terms of consumed fuel. –  Cray Mar 28 '12 at 15:06
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Quite simply, no. In order to understand why, you have to understand the metabolic pathways used in exercise. If there are slight differences in the amount of calories burned, it is due merely to the difference in energy needed to move the lungs.

Essentially, your muscles use Adenosine Triphosphate (ATP) to perform their work. The different metabolic pathways all result in generating ATP. The three major pathways are:

  • Oxidative: The result of aerobic activity. Fatty acids are converted to ATP with lactic acid as a byproduct in the presence of oxygen. This can last for minutes to hours.
  • Glycolytic: The result of anaerobic activity. Glycogen and lactic acid combine to create ATP in the absence of oxygen. This can last for seconds to minutes.
  • Phosphagen: The result of very high intensity activity. ATP and Creatine Phosphate (CP) are quickly depleted to handle sudden high intensity activity. This can only last for seconds.

Only one of those pathways uses oxygen. It should be noted that the metabolic pathways are listed in order of the amount of Calories burned for that type of activity. While aerobic activity burns a higher percentage of fat during activity, the energy demands are fairly low and there is no "afterburn" associated with that activity. It should also be noted that the metabolic pathways are cumulative. You are still operating oxidatively even when you are predominantly operating in the glycolytic manner. The anaerobic pathways, while they burn more carbs immediately, they also require more Calories. When you finally come to rest, the body has to replenish the energy from somewhere. In the absence of food, this means the body has to burn fat to create glycogen--which is what the "afterburn" of high intensity interval training comes from.

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Wouldn't this mean that taking in less air during an exercise does indeed burn more calories because the body is forced to use the 2 latter ways, and as you wrote, they require more calories? (because the energy depleted during that has to be replenished from burning fat (i.e. calories?)) (Or do you mean that this whole "use stored energy/then replenish by breaking fats later" breaks even to almost exactly the same amount of fats burned as the oxidative pathway does, when it is using the energy from burning directly? (hope that makes sense) –  Cray Mar 27 '12 at 14:38
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Nope. It has nothing to do with the amount of oxygen in your blood (which is affected by breathing). It has everything to do with the metabolic processes to meet the demands you are placing on your body. Heavy exertion means metabolism without oxygen--even if your blood-oxygen levels are saturated. –  Berin Loritsch Mar 27 '12 at 17:39
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@Cray ventilation is more about getting rid of CO2 from your body than getting O2 into it. Therefore, the demand is driven by how much energy you burn and how much CO2 this produces as a by product. You are partially correct that creating an anearobic (without oxygen) situation, you could force your body to burn energy more inefficiently. However, this is only required for athletes who have to perform in energy deprived situations and want to push the limit. –  Ivo Flipse Mar 28 '12 at 14:54
    
This is one of the major differences between sprinting and marathoning. –  Berin Loritsch Mar 28 '12 at 15:02
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No, its rather that you can't keep doing that forever, because at some point you're no longer able to get rid of the CO2 and you'll start to built up so much lactate acid (a residual of burning energy anaerobically) that you're simply no longer able to perform. So you have to strike a balance between limiting your oxygen built-up and the acidification of your muscles, which is what the body does automagically if you perform (sub)maximally. So instead of artificially limiting your oxygen uptake, simply push yourself harder (but make sure its part of a training program) –  Ivo Flipse Mar 28 '12 at 15:16
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