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There's a widespread belief that caffeinated drinks will make you dehydrated because the caffeine itself makes you pee.

But is caffeine really a diuretic?

Technically, yes. But it's not affecting you as much as you might think.

You might find you need to pee after your morning coffee.

But you may also need to pee after drinking a similar amount of water.

Still, is the caffeine making you pee more?

It may seem like this would be easy to figure out.

You just need to know how much liquid goes in and how much comes out, right?

Actually, it's trickier than that.

First of all, people vary a lot in how much liquid they take in, and in how much they pee out.

So rather than giving some test subjects coffee and others water, it's best to measure the same people on different days, plus or minus caffeine.

And it may matter when you take a measurement.

Kidney output changes with sleep-wake cycles.

They slow way down overnight, then crank up again during the day.

So it's probably a good idea to monitor subjects over at least a 24 hour period.

Plus, we don't know exactly how much liquid a person needs in a day, it depends on who they are and what they're doing.

Which makes a difference when you're monitoring them for hydration.

How much water we lose can also change, based on things like how much we exercise or the temperature outside.

You can get around some of this by establishing a baseline fluid intake level for each subject, and making sure they do basically the same things on their caffeine and non-caffeine days.

So it's actually quite a challenge to design an experiment that really demonstrates people are gaining or losing water.

That said, researchers over the years have attempted it, just not always successfully.

The idea that caffeine makes you dehydrated dates back at least several decades, possibly to a study from 1928.

This study wasn't ideal, since it included just three subjects, and the only measure was urine output.

I don't know about you, but I'm not going to quit drinking caffeine based on ninety-year-old data from three dudes.

Lucky for all of us, plenty more studies have been done since then, looking at the effects of everything from caffeine pills to energy drinks to coffee.

When you look at all the data together, some trends emerge.

A moderate amount of caffeine, in the neighborhood of 300 to 500 milligrams, does not seem to lead to water loss in most studies.

That's the equivalent of 2 to 3 cups of barista-brewed coffee, or 6 cups of tea, or more than 2.5 liters of cola.

When you go above those amounts, caffeine can have a minor diuretic effect.

But, as even those researchers back in 1928 recognized, if you have caffeine every day, you quickly become resistant to its dehydrating effects.

In one 1997 study, the diuretic effect of even a relatively high dose of caffeine also disappeared when the subjects were exercising.

This myth has a surprising amount of staying power.

It's everywhere, and even some medical professionals continue to spread outdated guidelines.

But take comfort.

The research is clear: drinking a few servings of coffee, tea, or caffeinated soda will not make you dehydrated.

There may be other reasons to stay away from sugary, caffeinated drinks, but hydration isn't one of them.

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