But a new study in PLoS One suggests that's not the case. The authors involved 50 healthy men in an experiment to test the hypothesis that coffee causes dehydration, separating the men into two groups, one that drank coffee and one that drank water. After three days, each man switched to the other beverage. The authors measured the men's body-water content using blood and urine samples.
After the weeklong study, neither group showed significant declines in total body water content, suggesting that coffee isn't as dehydrating as we've all been led to believe. The coffee drinkers in the study were drinking about four cups of coffee per day, too, which is more than you might be accustomed to. "These data suggest that coffee, when consumed in moderation…contributes to daily fluid requirement and does not pose a detrimental effect to fluid balance."
The belief that coffee causes dehydration is one of those dietary old wives' tales that took root in the late 1920s. One study found that coffee could act as a mild diuretic, and although no studies since then were able to confirm that finding, the idea has held fast ever since.
What about decaf? Although the authors didn't include decaffeinated coffee in their tests, they do note that "as we found minimal differences between coffee and water, we believe that it is unlikely that we would have found any significant differences if we had included a decaffeinated coffee condition." So if caffeine gives you the jitters, you can indulge in your decaf with little worry that it will cause you get dehydrated.