Busting Myths: Caffeine & Hydration

Disclaimer: I am not a doctor and this is not medical advice. Everyone's body is different. Each of us have different habits, activity, pre-existing conditions, and so on that can complicate what is otherwise observed in very controlled situations. For instance, if your body has a problem processing caffeine, or if you have existing conditions that affect your hydration, the following could affect you differently, and it's always best to know your body and what you're putting into it before making any assumptions about whether something is harmless.
 

Let's begin with how this happens

There are always a set of myths or half-truths that get passed around and down between generations. Whether it's the idea that gum takes years to digest, that shaved hair grows back thicker, that wet hair in cold weather can give you a cold, or that cracking your knuckles can give you arthritis, these often take the form of some short quip of supposed general knowledge that often people accept without thinking twice. The sneaky bit about these pieces of information is they're often so widespread that they aren't challenged or carry a weight of trustworthiness they don't deserve.

"Caffeine dehydrates you" is the one we're tackling today. I hear this time and time again, whether it's someone on a crusade against energy drinks, or wanting to appear smart when someone brings up coffee or tea, or fraught conversations about fitness, wellness, and nutrition that plague us today with all sorts of garbage ideas. I remember being scolded as a teen, taking a relatively low-dose caffeine pill while on a trip to offset some jet lag I was feeling, that it would contribute to dehydration and that I should stop. I hear conversations on video streams and in communities where food and drink come up often and it's inevitable that this myth eventually sneaks its way in by way of someone mindlessly passing along this assumed-true line of thinking.
 

So does it dehydrate you?

Well, no, but also sometimes, and there are many factors to consider. Nuance is absolutely necessary for this discussion. The reality is much more complicated as is often the case for many of these types of discussions, so let's dig in. There are multiple factors to consider here, and the biggest one that seems to bear out relates to the caffeine dose- and it's worth remembering that caffeine gets metabolized over a period of time, so time is a big factor in dosing.

One study found that it was dose-dependent, at least when all administered at once (caffeine was dissolved in 200ml of water and drank all at one time). While the study was small and only tested a few things, it points to the idea that reasonable-to-even-slightly-high caffeine intake (in this case 3mg of caffeine per kg of body weight, so 300mg of caffeine for someone weighing 100kg/220lbs) did not induce diuresis or induce any significant change in fluid balance (output was roughly the same as the group drinking just water), whereas double that dose did induce it acutely.

Regardless of diruetic effect, I wouldn't exactly recommend anyone consume 6mg of caffeine per kg of body weight at any time, as you'll experience plenty of other unpleasant effects and hydration will be the least of your worries. Even at the non-diuretic level of 3mg/kg, for a lot of people that will probably be too much caffeine at once anyway. I am over 220lbs but 300mg makes me jittery and uncomfortable.

However, even at the high 6mg/kg level, another study that looked at effects over two weeks, and even used capsules instead of beverages, found no significant effects on the fluid-electrolyte balance or renal function, and found that the following markers stayed within normal ranges: body mass, urineosmolality, urine specific gravity, urine color, 24-h urine volume, 24-h Na+ and K+ excretion, 24-h creatinine, blood urea nitrogen, serum Na+ and K+, serum osmolality, hematocrit, and total plasma protein.

Another angle to consider is caffeine tolerance, looked at in this review. In individuals that had no caffeine for days or weeks prior, a significant (but common) dose of 250-300mg (think 2-3 cups of coffee, a can of Bang Energy, or two Monster Energy drinks) did stimulate urine output in the short-term. But those that regularly consumed caffeinated beverages did not experience the same. This study seems to bear that out, in that those that regularly consume 3-6 cups of coffee a day on average were given similar amounts of caffeine over three days in a controlled way, and coffee was found to be just as hydrating as water.
 

What about those energy drinks though?

Keep in mind all of this has been mostly looking at caffeine as an isolated ingredient. None of these studies looked at any other compounds being consumed simultaneously. A number of them did use coffee, so it's relatively safe to say no other compounds in coffee had any significant effect. However, once you bring energy drinks and preworkout mixes and other sources of caffeine in- it's possible some of these could be dehydrating, but due to their other ingredients- but this is an unknown. There are so many other ingredients in some of these drinks, all at varying doses, that they need their own individual investigations. However, it's worth noting that the yellow color your urine takes on after consuming drinks full of high-dose B-vitamins is just excess B-vitamins like Riboflavin being excreted by the kidneys- it's not a sign of a change in hydration levels. See how confusing this can get?
 

It's complicated and that's okay

In conclusion, many factors matter: pre-existing conditions, dose, tolerance, form taken, time, body weight, and more. However, I think it's relatively safe to say overall, in general based on the existing studies and reviews of these studies, large acute doses of caffeine can induce short-term diuresis, but more common doses that people intake on a regular basis generally do not, unless they have not had caffeine for days-to-weeks prior. Longer-term (at least a couple of weeks vs a couple of days), even the larger doses seem to have minimal overall effect on fluid balance and renal function, at least in one particular study where this was monitored. As far as true "long-term" (months/years), it's possible there are other effects, but dehydration doesn't seem to be one of them.

While the capsules did not have any worse of an effect, it is worth considering that if you consume your caffeine along with a beverage, or in your beverage directly, you're taking in more water in the process, and are contributing more to your hydration levels regardless. So there is some level of offset happening there, but it doesn't seem any studies looked too closely at that particular factor.

Ultimately, some of these studies are small, some of them have different methodology, and different timeframes and types of individuals participating. So while we may have a decent general picture of caffeine and its effects on hydration that counteract the myth, the truth is it's complicated, but that is part of the point I wanted to make with this post- it's better that we admit the complexity than mindlessly pass around a myth in its place.

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