With no food, your body has nothing to burn for fuel. With no water, you're asking for dehydration which will drop your performance in it's tracks even when it's slight. No matter what those on the Atkins diet will tell you, if you're a serious athlete who does a lot of biking, running, or any other high intensity sport, you need carbs. In fact, they should be something like 60% of your daily diet. So eat up. If you're not going to eat an actual meal during the day though, the carb content in say a Powerbar will give your body enough kick for a ride. That's what I make a quick breakfast when I go for early morning rides. Your body needs to be fueled, and be hydrated for it to work. Don't make the same mistake twice or it could be trouble.