Is Coffee Dehydrating?

This causes a lot of confusion and debate. Some people say that because caffeine is a diuretic, coffee dehydrates you. Others say this is a myth and that coffee counts toward your daily hydration. Both can’t be true. Does coffee dehydrate you or mess with your fluid balance? Or, does the water in coffee help to hydrate you just like any other water?

What is a Diuretic?

First, we have to understand the claim. A diuretic is a substance, or drug, that increases the body’s excretion of water and salts. In other words, it makes your kidneys output more fluid by helping your kidneys release more sodium into the urine, taking water from your blood along with it.

Diuretic drugs are often called ‘water pills.’ These are useful in medicine as they can help control buildups of fluid in the body such as in the lungs and can help reduce the load on the heart by decreasing blood pressure, etc. Not all diuretics are the same. They affect different parts of your kidneys and some may also cause your body to shed potassium. While doctors prescribe diuretic drugs, many herbs, such as dandelion, can also have a diuretic effect. And, of course, so can caffeine.

Since caffeine is a diuretic and coffee contains a fairly high amount of it, then drinking coffee must cause you to put out more fluids, leading to dehydration. This has been the prevailing wisdom for many years, and most people still believe it: Coffee is dehydrating.

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What Science Says About Coffee and Hydration

Recently, I came across a food-myths article from a popular site being shared on Facebook. It mentioned that coffee was not dehydrating. A shocked response followed in the comments:

How can you say this. As a scientist I know that caffeine is a diuretic. The more you partake the more your urine output so, yes it is dehydrating. Please stop telling lies.

Science isn’t about explanation alone. Science is about evidence. Knowing something is a diuretic and having evidence that coffee is dehydrating (in general) are two different things. Certainly, a scientist would know that, as well.

Just because you can explain why something must be the case doesn’t mean you have proven it to be the case. So, what is the case?

In the few studies that have occurred, there has been no evidence that moderate coffee intake is dehydrating. Obviously, people drink a combination of different beverages all day. And a cup of coffee is 98% water.

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So, the reason people are repeating that coffee is not really dehydrating is that this is what the evidence seems to indicate. Obviously, if all you drink is coffee all day long, you would probably be less hydrated. However, we still don’t know that you would be chronically dehydrated.

For example, in one study, which was a counterbalanced cross-over study of coffee consumption and dehydration, 50 male coffee drinkers who habitually consumed 3 to 6 cups of coffee a day participated in two trials of three days, with controlled physical activity, food, and fluid intake, and consumption of either four 200ml coffees or water.

Researchers kept track of their total body water and other markers of hydration. No significant differences were found between the coffee trials and the water trials. The researchers concluded that coffee, when consumed in moderation by caffeine habituated males, is pretty much as hydrating as water. 1

Assuming the same would be true of females, the important part is ‘caffeine habituated.’ The researchers were not indicating that they think that the opposite is true of novice coffee drinkers. They were simply pointing out that their results are only valid for regular coffee drinkers.

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While it may be true that a novice coffee drinker will experience a stronger diuretic effect and may output more urine than an experienced coffee drinker, the evidence we have generally indicates that the mild diuretic effect of coffee doesn’t negate the water we get from the coffee.

Coffee is 98% water and, even though it does cause you to pee more, you still get more water from it than you lose, especially if you drink all sorts of other beverages, including water, throughout the day. 2

And for those lifelong habitual coffee drinkers like myself, coffee is even less dehydrating. A fairly profound tolerance to the diuretic effect of caffeine occurs through habitual consumption. If you go off of caffeine for a couple of weeks and then return to it, your urine ouptut will increase for a limited time, after which you will again become more tolerant.

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The same is true of the other effects of caffeine. We become more tolerant to them over time.

When the available (good) studies are taken in aggregate, there is no support for the notion that coffee drinking or the consumption of other caffeine-containing beverages leads to fluid imbalance or dehydration. 3

Yes, you probably do need to pee after your first few cups of coffee, but it’s because you just drank a lot of water as much as anything else.

Keep in mind that some people are caffeine sensitive and may never be able to tolerate caffeine (nervousness, jitters, etc) no matter how long they consume it. Furthermore, it is possible to become more sensitive to caffeine even after having consumed coffee for many years. This may be dose and timing-dependent.