Caffeine: Is it dehydrating or not?

I've been seeing ads that say cola and coffee drinks hydrate you as well as water does. Is this true?
- Albert / Ohio
Mayo Clinic dietitian Katherine Zeratsky, R.D., L.D., and colleagues answer select questions from readers.

It is true. Researchers used to believe that caffeinated drinks, even consumed in moderate amounts, had a diuretic effect. This means that you would urinate more after drinking them and become dehydrated. Recent research shows that this is not true, and caffeine only has a diuretic effect if you consume large amounts of it — more than 500 to 600 milligrams a day.