Diuretic Effects of Coffee

The truthful answer is "it depends". Here's the scoop.

Caffeine is the culprit, because of the effect it has on your kidneys. Scientifically, caffeine increases the rate at which kidneys filter waste and excess fluids from the blood. It also reduces the reabsorption of water in the kidneys. Also, caffeine increases the production of a specific hormone that promotes the excretion of sodium and water in urine. All of these lead to an increase in urine production. 

Practically, this is why you may feel the need to urinate more frequently after drinking coffee. But can this lead to dehydration?

An older 2014 study asked that exact question, and concluded that, after their trials, "These data suggest that coffee, when consumed in moderation by caffeine habituated males provides similar hydrating qualities to water." Let's break that down a bit.

• "consumed in moderation" to the study authors was defined as consuming 3-6 cups per day.
• "habituated males" meant a group of 50 men who regularly consumed coffee in moderation of 3-6 cups per day.
• "similar hydrating qualities to water" meant that that the control group in the study consumed water, and the test group consumed the same volume of water. And at the conclusion of the study, it was determined that there were no significant differences across a "wide range of haematological and urinary markers of hydration status between trials".

So for males who regularly consume up to 6 cups of coffee a day, the above statement  is FALSE - that caffeine intake does not cause dehydration under these conditions. 

Let's be clear, though, this is cups - 8 ounces - not 3-6 refills of a 32-ounce Stanley or Yeti thermos! The key here and in other studies like this one is the total caffeine consumption. They suggest 500-600 mg caffeine is the limit before dehydration becomes an issue. At a typical 100-200mg per cup of coffee, you should do the math.

A daily drinker like me (two 12-ounce mugs of brewed coffee, or around 300-600mg caffeine) will not need to be concerned. Drinking more? Consider upping your water intake to compensate, or switch to a great decaf for part of your total coffee intake.

Additionally, a dietetic expert recommends, "Don't let your caffeine intake impair your intake of straight fluids like water." Coffee may not dehydrate you detrimentally, and it can even count toward your daily hydration—but it's still imperative to drink enough water (and other more hydrating drinks) throughout the day.