Originally Posted by Machka

For one thing, the fitter you are, the less you need to eat on a ride because your body will efficiently use the fuel you've got in storage.

Now (very generally) suppose you've been eating well all day, and that most days you consume an adequate amount of calories so that you are maintaining your weight. If that is your situation, you should have about 2000 calories in storage in your liver and in storage areas in your muscles (note that this is different from the idea of the body consuming muscle to get energy). Chances are you are burning 500-600 calories per hour, depending on effort etc., when you ride your bicycle. Therefore, you could conceivably ride 3 or 4 hours without eating anything on a ride. However, if you are not very fit, chances are you won't be able to do it because your body won't be able to efficiently access the calories.

So this is where eating comes in. The recommendation for longer distance riders is 200-300 calories per hour, plus a good breakfast. If you've got 2000 calories in storage, and you consume 500-600 calories for breakfast, and you eat 200-300 calories per hour (i.e. oatmeal cookies plus energy drink), you can ride for a long time without having to do a serious refuel.

But if your rides are less than 2 hours, and you aren't starving yourself during the day, you should be able to do the ride on nothing more than water ... you should have enough calories in storage to get you through.