I've been told not to drink coffee on a long plane flight because the caffeine in coffee is a diuretic. As you may recall from high school biology, the kidney removes waste products, water and salts from the bloodstream, with most of the salts and water being quickly returned to the blood. Diuretics interfere with this kidney action by reducing the reabsorption of water and salts into the blood. Diuretics can also increase blood flow through the kidney, causing more water and salt to be filtered out. In either case, the body's water and salt levels go down and urine volume goes up.
Caffeine is a diuretic.
It occurs naturally in coffee beans, tea leaves, cocoa beans, and kola nuts. But by far, the largest concentration of caffeine is in coffee. Depending on the strength of the brew, coffee contains two to five times as much caffeine as cola or tea. So caffeinated coffee is not the thing to drink on long plane flights or when exercising, particularly in hot weather. Cola and tea are okay, but water is best for both occasions.