Coffee is 99% water, but is it hydrating or dehydrating? Actually, it’s both. Though caffeine is a mild diuretic, the water content of the coffee balances out and essentially negates the diuretic properties of the caffeine. If this sounds surprising, you aren’t alone in your shock. For most of the twentieth century, healthcare professionals, media outlets, and fitness fanatics referenced a 1928 study that included only three men studied over two winters. As part of the study, these men had their urine tested regularly and were required to drink at least four cups of a caffeinated beverage daily — that beverage could be coffee, tea, or water laced with pure caffeine. The study found that if the men drank “caffeine-laced water after a two month period of abstinence from both coffee and tea, the volume of their urine increased by 50%, but when they drank coffee regularly again they became inured to its diuretic effects”1. In 2002 a University of Connecticut comparative review of 10 separate evaluations of caffeine’s diuretic effects concluded that coffee is a mild diuretic at best, and a 2005 study published in The International Journal of Sport Nutrition and Exercise Metabolism found coffee had no diuretic effects, concluding that there were “no significant differences in levels of excreted electrolytes or urine volume”2 between those who drank water versus those who drank coffee.
To recap, drinking coffee will make you urinate more frequently than drinking a cup of plain water because of caffeine’s mild diuretic effect (i.e., flushing sodium and water from the body) and coffee counts towards your total daily fluid intake.
Bonus factoid: Each coffee species and variety has a unique caffeine content, and coffees with a higher caffeine content have a greater diuretic effect. Consuming a cup of robusta coffee, for example, would have a greater effect than consuming a cup of arabica coffee; similarly, consuming a cup of Kenya AA would have a greater diuretic effect than consuming a cup of Yemen Mocha Matari.