In the past, researchers believed that caffeinated drinks had a diuretic effect. This means that you would urinate more after drinking them, which could increase your risk of becoming dehydrated. But more recent research shows that caffeine has a diuretic effect only if it's consumed in large amounts – more than 500 to 600 milligrams (the equivalent of 5 to 7 cups of coffee) a day.

Caffeine is a mild central nervous system stimulant and can make you jittery, sleepless, anxious or restless. Studies have shown that certain genetic predispositions make some people more sensitive to the effects of caffeine.

Water is your best bet to stay hydrated. It's kilojoule-free, caffeine-free, inexpensive and readily available.