Dehydration occurs when there is a fluid-electrolyte imbalance. It has long been agreed upon by health experts that caffeine acts as a diuretic. Thus, coffee and other caffeine-containing beverages are thought to have mild dehydrating effects. Beliefs about the extent of this dehydrating effect are continually changing with emerging research.
Coffee Doesn't Dehydrate
Numerous sources confirm that there is no die-hard scientific evidence that caffeine-containing beverages increase 24-hour urine volume any more than other beverages. In fact, research points to the possibility that both caffeinated and noncaffeinated beverages may actually contribute to the body's total fluid intake during the day.
Dietitian Chris Chandler states that if you are concerned you are not getting enough fluids, there is a simple formula to follow: Aim for 1 milliliter of water per kilo calorie. Based on a 2,000 calorie diet, that is roughly 2 liters of water daily. Chandler says, "If you are concerned that your caffeine consumption is dehydrating you, drink an 8 oz. glass of water for every cup of coffee you drink."