It's true that if you don't eat enough, your body will start to feed on muscle. However, that doesn't happen to people with relatively normal diets unless they're spending way more time on aerobic exercise than you say you are. But nutrition alone isn't going to build muscle, so you should consider incorporating an upper body workout into your exercise regimen two or three times a week -- but not on consecutive days, because the most important part of building muscle through exercise is rest and recovery.

I recommend something like these resistance bands or tubing for upper body training. Pushups are really good too. For pushups, if you keep your hands shoulder-width apart or closer, you work your triceps. Wide-grip pushups (hands wider than the shoulders) works chest muscles. Perhaps more importantly, in addition, if you do them right (the drill sergeant who runs the exercise class I attend says tells us to squeeze our butt muscles like we're on a first date and we have to fart), pushups build strength in your core.

Here's a web page with a number of resistance tubing exercises:
http://exercise.about.com/cs/exercis...andworkout.htm

And here are some helpful youtube videos:
http://www.youtube.com/watch?v=X72LtRcXR6g (pushups)
http://www.youtube.com/watch?v=3P18RulSIVc (tubing)
http://www.youtube.com/watch?v=Hp0VEAZMW1o (tubing)
http://www.youtube.com/watch?v=bUek79YpZGA (tubing)
http://www.youtube.com/watch?v=Niww4BMazrQ (tubing)

I also recommend the book Cycling Anatomy by Dr. Shannon Sovndal. It explains the physiological process of building muscle, and it describes each of the muscle groups used in cycling and provides strengthening exercises for each of those muscle groups.