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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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