When i look at my performance diagnostic for cycling, from a certain point on (~after fat max zone) the amount of fat burned decreases to zero (with increasing wattage) and only carbs are used to produce ATP.


Why is the aerobic system not used anymore?

What i understood / assume so far:

  • the aerobic system is limited by the vo2max and the absolute usage of this capacity --> from a certain point on the total amount of required ATP can not be produced by the aerobic system

  • the anaerobic system can produce ATP much faster than the aerobic system

  • anaerobic and aerobic system can be used in parallel(?) since the aerobic system requires pyruvate.

My best guess:

Both systems share a limiting factor, so from a certain point on the production shifts to the anaerobic system(?)

1 Answer 1


This is another area of metabolism where we don't fully understand everything, and what we do understand can have some misconceptions. Just like the "fat burning zones" (When in actuality we utilize fat in all areas, just to higher or lower degrees), it appears that the energy cycles may also be subject to similar misconceptions. So your assertion that fat burning drops to zero is not quite factual.

That being said, the main reason that aerobic pathways are utilized less in higher, anaerobic type efforts is simply speed. The body needs energy faster than the slower pathways can produce it. This is why creatine monohydrate works as well as it does. ATP breaks a bond to produce ADP (Tri phosphate to di phosphate) which produces energy to drive the muscle. That has to be replenished from ADP to ATP, and creatine monohydrate in the cells provides a source that can be used immediately. After that, you start moving into the ATP/PC, Lactic acid and glycolytic, which are slower. This is why pure anaerobic effort is short lived, usually no more than 30-60ish seconds.

There is a nice chart on this page a few slides down, showing the overall performance against the energy systems and how long each energy system is in use.

  • Thank you for the answer! Two followups: 1) "So your assertion that fat burning drops to zero is not quite factual." --> It is the info one gets from all the spirometry reports i have seen so far. So you would argue the provided infos are wrong? 2) " The body needs energy faster than the slower pathways can produce it." This would also be my assumption as listed in the question. But in theory both systems could work in parallel - and they actually do, no? Why would the aerobic pathway be used less and not used both in paralell - do they share a limiting input factor?
    – Tlatwork
    Commented Mar 8, 2022 at 22:37
  • @Tlatwork - All the energy pathways are working all the time, but in differing amounts. The oxidative ("fat burning") pathway is slower and really won't contribute in short burst, high intensity. So while it doesn't totally stop, it doesn't really contribute either. It's a very fine point, but... And yes, all systems work in parallel, but the aerobic pathways generally don't get used hardly at all until the anaerobic are used up. Look at that chart and read through that page, it's got some really good info (2nd link).
    – JohnP
    Commented Mar 9, 2022 at 15:33

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