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Old 08-16-2010, 05:40 AM   #19
Darryl Shaw
Senior Member
Join Date: Apr 2008
Posts: 707

Originally Posted by Derek Weaver View Post
I may very well be wrong, but I thought liver glycogen was primarily used to fuel the brain?
Correct, liver glycogen is used primarily to fuel the brain and CNS.

Once glycogen stores were depleted the brain, and other tissues, could switch to ketones for fuel?
It can take up to a week for your brain to switch over to using ketones as fuel so in the short term you're dependent on gluconeogenesis to keep your blood glucose stable. Unfortunately free glucose can't be produced fast enough by gluconeogenesis to keep up with demand during intense exercise so if glycogen stores run out during a marathon your only options are slow down significantly, which is only going to delay the inevitable, or quit.
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