We had an interesting talk. The nutritionist said she didn't think I was protein deficient, but that I probably need more protein than I've been getting, considering my activity level. She also says to watch the carbs, especially simple sugars. She put together a meal plan for me to follow.
One of my cycling books says, an active person's diet should be about 55% carbs, but go easy on starchy carbs, e.g. pasta. Over a few months, a diet too high in starches can actually cause the metabolism to switch from fat-burning to sugar-burning, making the athlete more bonk-prone. Not pretty.
In fact, the story has been told of a certain pro-tour rider who was found to be terrible at burning fat. His diet was gradually changed over several months, and at the end of that time he was finishing six-hour training rides without finishing all the energy bars in his jersey pockets.
Except for centuries I probably don't have to worry about six-hour rides, but the more efficient I can get at burning fat, the better off I'll be.