like mrzap said; let's say your metabolic rate is like 3000 calories/day on average...
you can eat 200 grams of protein (800 calories) now let's say you eat 100 grams of fat, that's 900 calories, so we're at 1700 now, and then you eat 200 grams of carbs another 800, we are now at a total of 2500 calories, but our metabolism burns 3000 for the day, so -500 calories, where do you think the calories will come from? it has to come from somewhere... soo....
either your fat stores or protein stores (muscle), or you could have pre-existing glycogen stores, but they will eventually deplete..
I think they key lies in supplying your body with the right amount of protein and protein catalysts at times where anabolism can occur, and maitaning good ratios at other times.. even with catabolic side of protein turnover; if you've gained small amounts you could still keep the same amount of muscle.
there's also the whole supplement side of the story... it gets quite elaborate.. but experimentation is key;