Marathon runners use protein for energy all the time. They actually need to eat way more protein than bodybuilders do (and generally don't.) The body consumes key amino acids from muscle tissue, leaving behind incomplete proteins that then can't be held on to. The main amino acid lost to this process is leucine--this is the single most effective amino acid to supplement for preventing muscle loss in endurance athletes. I found it interesting that the body metabolizes leucine into ketones, not glucose. Almost like it preferred ketones to glucose.