Chris Carmichael says:
It’s important to realize that you burn carbohydrates, protein, and fat simultaneously whenever you exercise, regardless of the intensity
http://www.cbass.com/Carbs_Athletes.htm
Also see:
DOES DOING AEROBIC EXERCISE BURN MUSCLE?
http://www.livestrong.com/article/39...e-burn-muscle/
One loses protein if one does not replace what one burns. Simple as that. My long ride drink mix is 15% whey protein for that reason. I have protein in my breakfast before a ride and about 20% protein in my recovery drink and following recovery meal. My arm and thigh measurements change very little during the season, maybe 1/2" at the most.