No, Coffee and Tea Aren't Actually Dehydrating. Here's Why
Instead, it's quite the opposite
It’s true that caffeine is a mild diuretic, which means that it causes your kidneys to flush extra sodium and water from the body through urine. If you’re peeing frequently, and thus losing lots of liquid, it’s logical to think you could become dehydrated — but it actually doesn’t work that way, explains Dr. Daniel Vigil, an associate clinical professor of family medicine at the David Geffen School of Medicine at the University of California Los Angeles.
“When you drink a cup of coffee or you drink a glass of iced tea, you are necessarily taking in a volume of fluid along with that dose of [caffeine],” Vigil says. Even though caffeine is a mild diuretic, Vigil says, you won’t lose more fluid through urine than you take in by drinking a caffeinated beverage. Your body is able to absorb as much fluid as it needs and expel the rest, he says.
For that reason, your morning pick-me-up actually helps hydrate you, not the opposite. Vigil says that coffee and tea “can and should” count toward your daily eight-or-so cups of water per day.
And if you find yourself with headaches or other symptoms after your morning java, Vigil says dehydration likely is not the culprit. You may just be sensitive to caffeine — or dehydrated for reasons that have nothing to do with your coffee or tea habit.