One of the great things about adventure cycling, notes author Tim Barnes, is that you can eat anything you want and not gain weight. You don't need special (and expensive) energy bars or scientifically formulated "power" drinks. Just adequate calories to fuel the furnace your bike-riding body has become.
Your other major requirement is to stay hydrated. You probably already know that just a few days without water for us humans will result in death. And the exertion of cycling causes you to lose water much faster than you would typing at a desk.
The thing is water weighs a lot. Therefore, how much you carry with you should depend on how easily available water is along your route. If you pass by or through multiple towns with convenience stores in a day, you'd need to carry much less water than if you were crossing Mongolia's Gobi Desert.
How much water you will need, exactly, is one of those annoyingly fuzzy "it depends" questions: it depends on the individual, on the environment, on activity levels and other factors, too. Mayo Clinic says 3 liters (about 13 cups) is an adequate daily water intake for men, while 2.2 liters (about 9 cups) is adequate for women. Throw sustained exercise into the mix, however, and that amount jumps significantly. Before you hit that long stretch of road devoid of civilization, fill up those water bottles with a bit more than you think you'll need [source: Mayo Clinic].