I think it totally depends on your metabolism and blood sugar levels. Personally, I can't understand how anyone can function on just a bowl of cereal in the morning...I'm starving within an hour and my blood sugar totally bottoms out.

My breakfast is always a significant amount of protein (30-40 grams) and a small amount of fruit (about 15 grams). I sometimes also add a grain carb - steel cuts oats (not sweetened!) or sprouted grain cinnamon toast or sprouted grain English muffin. My body can deal with the grain carb after a good run; on other days it just messes with my blood sugar.

I'm protein heavy...probably just protein. I have two servings of eggbeaters and a slice of turkey bacon. It sustains me pretty well.