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I'm studying scientific models of endurance exercise such as the one in a paper by Rapoport (see references below, popularization here). If I'm understanding properly, fat burning is in some sense slower than burning glycogen, but I'm confused about how to quantify "slower." I was thinking that it should be quantified in terms of the amount of oxygen consumed. House and Johnston (p. 107) say,

The metabolism of the sugars that come from carbohydrates is called glycolysis. It is a faster process (about twice as fast) than the metabolism of fats.

But I'm not sure if I'm putting this together correctly, because maybe here they mean something different by "slower." Glycolysis doesn't require oxygen, but I guess after glycolysis produces ATP, we use oxygen to burn the ATP.

Can anyone clarify the following?

  1. Why does the human body produce a lower power output in aerobic, endurance exercise when burning fats rather than carbs?

  2. If you want to quantify this relative slowness, is it correct to do it in terms of stoichimetry and oxygen consumption, or is there something else that makes it slower?

  3. Is the factor of 2 something that is universal and can be made more precise (as I would expect if it's a stoichiometry thing), or is it more vaguely defined and dependent on the individual?

References

House and Johnston, Training for the new alpinism, Patagonia Books, 2014

Rapoport, Metabolic Factors Limiting Performance in Marathon Runners, PLoS Comput Biol. 2010 Oct; 6(10): e1000960, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2958805/

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