Moderate coffee intake does not dehydrate?

Does coffee cause dehydration?

It is often suggested that coffee causes dehydration. This is because coffee contains caffeine, which has a dehydrating effect. However, coffee also contains fluid which has a hydrating effect. Therefore, it can be questioned if coffee intake really causes dehydration.

This study investigated the effect of coffee intake on hydration levels in habitual coffee consumers (3-6 cups a day). Each subject participated in two 3-day experiment periods: one coffee period and one water period. During the coffee period, subjects drank 4 cups of coffee per day (containing 4 mg caffeine per kg body weight, which equaled 200-450 mg caffeine per day). During the water period, subjects drank 4 cups of water. Hydration status was measured in several ways, including the gold standard of total body water assessed by D₂O.

There was no change in hydration status during the 3-day experimental periods in either treatment and no difference between the coffee or the water period.

Note that this study used moderate amounts of coffee and habitual coffee drinkers. Previous research suggests that higher doses of caffeine may be dehydrating and that the dehydrating effect of caffeine is less in regular caffeine consumers. Therefore, it is still possible that higher doses of caffeine and/or coffee have a dehydrating effect, especially if you’re not used to caffeine consumption.

Study:
Killer et al. No Evidence of Dehydration with Moderate Daily Coffee Intake: A Counterbalanced Cross-Over Study in a Free-Living Population. PLoS One, 2014