This is a myth. I have been told this growing up, and as a result avoided caffeine in most instances. “When it is really hot outside, don’t drink a bottle of coke, that will dehydrate you” I would hear often all over the place, from family to the radio news.
There was a point in my life where I would research claims I was told that seemed either too good to be true, or just to find out the validity of it, just to find out that most of what I was told was completely wrong. I grew up in a world of old wives tales and myths, and nobody ever bothered to see if they were even true. That led me to a wide variety of books, of which I might add to a Recommended Reading list somewhere down the road. But one of these books happened to be a book called Don’t Swallow Your Gum by Dr. Aaron Carroll and Dr Rachel Vreeman. I have since become hooked on all of Aaron Carroll’s books!
One of the chapters in this book was “Caffeinated Beverages are Dehydrating,” which both shocked me and didn’t shock me at this point, because by now I have simply accepted that most of what I was told was a fabrication.
Caffeine is believed to dehydrate you because caffeine is considered a diuretic. A diuretic stimulates kidney function and causes you to pee more. Water is considered a weak diuretic, but scientific evidence seems to suggest that coffee works just as well as water when it comes to a hydration effect. Even in large effects of daily fluid intake being replaced with 6 cups of coffee, studies have shown that caffeine intake as high as 642 mg of caffeine only increases urine production a small amount, and does not dehydrate you as a result. Another study published just year studied caffeine intake on 50 males in everyday positions, and yet again, found no difference in hydration between caffeine and water.
This does not, however, mean that we should over-consume caffeine in large quantities, as it is known to cause various health issues when taken in excess. However, caffeine taken in moderation does have potential positive health effects, and can be a part of a daily diet without going overboard. Just don’t avoid caffeine on the belief that it makes you dehydrated.