The heavier you are and the harder you exercise, the more calories you burn. The body can store roughly an hour-and-a-half to two-hours worth of glycogen (musclefuel). That's all. So, if you are riding longer, you need to carry (or stop to purchase) food and consume& enough calories to keep from developing a glycogen deficit.

Beat The Bonk
This glycogen deficit causes a miserable condition that's known as the bonk or hitting the wall, which feels like you have run out of gas. Your legs feel incredibly weak and small hills become Mt. Everest. You may experience a pins-and-needles feeling in your arms and lightheadedness, even nausea. If you stop for a while, you may get back on the bike and feel fine, only to have the bonk return in just a few minutes. You can even become disoriented and dizzy, which can lead to a crash.

Food To Go
Jersey pockets are designed to carry the energy bars, fig bars, fruit or energy gels you need to prevent the bonk. Stashed like this, the grub is easily reached while riding, too. Some people use electrical tape to stick packets of energy gel to their top tube or stem for even easier access (good for racing). For high-intensity events or rides, energy gels and drinks work better than energy bars. They can be swallowed in seconds (chewing an energy bar can interfere with breathing) and the ingredients enter your system almost as quickly.

Taste Test
Be sure to experiment in training or on rides that are not as important as your big event to make sure that your food and drink choices are right for you. What works for one person will not necessarily work for others. And, twenty miles into a century is no time to find out that the energy drink your training partner recommended upsets your stomach.