Hydration. It was long thought that caffeinated beverages were diuretics, but studies reviewed last year found that people who consumed drinks with up to 550 milligrams of caffeine produced no more urine than when drinking fluids free of caffeine. Above 575 milligrams, the drug was a diuretic.
So even a Starbucks grande, with 330 milligrams of caffeine, will not send you to a bathroom any sooner than if you drank 16 ounces of pure water. Drinks containing usual doses of caffeine are hydrating and, like water, contribute to the body’s daily water needs.