I’m interested in knowing more about the way metabolism proceeds after one has exhausted glycogen stores. I wish I’d paid more attention in my biology classes, but here we go:

Figures 2.4 and 2.5 of TfUA (a truly great purchase) show an interaction between the anaerobic and aerobic metabolic pathways. In particular, it shows pyruvate being produced through the anaerobic pathway, which then helps fuel the aerobic pathway. Note that on the left, glucose is still an input to the full system.

The point of these figures is to show how aerobic capacity to use pyruvate determines the maximum capacity at which one can run the anaerobic pathway without accumulating lactate (anaerobic threshold). I’m interested to know if this same cartoon applies when using the anaerobic system *minimally*. If so, this implies that even if you are relying on your fat-burning aerobic system to the maximum extent possible, you still need some amount of glucose to “keep the engine running”.

Alpinists do things like continue moving as fast as possible for a day or more after running out of food (and if I remember correctly, Mark Twight even suggested deliberately training this, in “Extreme Alpinism” :D). My assumption is that in this situation, one has essentially zero glycogen stores yet is somehow continuing to use fat to produce energy, but I’m not sure how. It also seems conceivable that at this point, one burns protein.

If not the one shown in Figures 2.4 and 2.5, which pathway or pathways are used after glycogen stores are completely exhausted? Is there another way to produce glucose,
another way to produce pyruvate, or another pathway that doesn’t require pyruvate at all?

I understand that these processes are very complex, these particular figures are simplifications to make a point about pyruvate balance, and the full set of metabolic pathways wouldn’t fit in a book on training. However, the practically interesting part of these questions, for me, is how they might inform decisions around nutrition and performance, in those situations in alpinism and un- or self-supported running, when one doesn’t have enough to eat.

The sorts of questions this might raise:

– Is having a tiny trickle of food (that can be converted to glucose) intake far superior to having absolutely zero, because it allows you to do a little bit of anaerobic work to “kick start the aerobic system” (which is a term I’ve heard people use in the context of fasted training)?
– Is there a difference between fasted training (probably a little bit of glycogen left) and continuing after bonking (really none left)?
– On a purely metabolic basis, how much is running out of food completely going to slow me down?
– If I’m bonking on a long route and I only have one gel to last me several hours, should I ration it, or just eat it now and avoid any potential sticky mess?