My reading of the passage Kaj_Sotala quoted is that the brain is decreasingly likely to encourage exerting will toward a thing the longer it goes without reward. In a somewhat meta way, that could be seen as will power as a depletable resource, but the reward need not adjust glucose levels directly.
I never suspected it had anything to do with glucose. I'd guess that it's something where people with more willpower didn't do as well in the ancestral environment, since they did more work than strictly necessary, so we evolved to have it as a depletable resource.
r/Fitness does a weekly "Moronic Monday", a judgment-free thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. I thought this seemed like a useful thing to have here - after all, the concepts discussed on LessWrong are probably at least a little harder to grasp than those of weightlifting. Plus, I have a few stupid questions of my own, so it doesn't seem unreasonable that other people might as well.