Does alcohol really cause dehydration?
Surprising research about an age-old assumption
What’s the first piece of advice you heard when you started drinking alcohol? For many of us, it was the recommendation to drink water, based on the underlying belief that alcohol causes dehydration.
The belief that alcohol causes dehydration is such an ingrained, unquestioned idea: a default assumption we absorb while growing up. But is it actually true?
Turns out, the answer is an unequivocal no.
What’s perhaps more surprising, though, is that scientists have known that alcohol does not cause dehydration since as early as 1942.
But how can that be? It’s not immediately intuitive, we grow up believing otherwise, and countless pop health blogs and hydration products point to “the dehydrating effects of alcohol” like it’s gospel. To understand what’s actually going on, we have to do what any good scientist would do: look to the primary research.
The research on alcohol and dehydration
The common “alcohol causes dehydration” belief is likely based on the observation that alcohol is a diuretic agent. Or, in common English, drinking makes you pee.
But in digging through the scientific literature, we find something surprising.
Published experiments dating back as far as the 1940’s show that additional urine output due to alcohol consumption is far less significant than we’re led to believe, so much so that it undermines the assumption that alcohol causes dehydration (citation: Eggleton, 1942).
The research shows, essentially, that drinking an initial amount of alcohol will cause more urine output than drinking the same initial amount of water (or other non-alcoholic liquid). However, continuing to drink alcohol after that initial drink does not cause any more urine output than continuing to drink water.
Basically, your first alcoholic drink will make you pee more than if you were just drinking water, but for the rest of the night any peeing you’re doing is pretty much exactly the same as if you were drinking water.
To illustrate, check out this figure below (from the Eggleton citation above):
Here we see a single drink administered, followed by a single spike in blood alcohol (the solid line), and a single spike in urine flow (the black bars). One drink followed by one extra trip to the bathroom. This outcome makes sense and aligns with our common beliefs about alcohol’s effect on the body.
OK, so what about a number of drinks in succession? Given our common assumption about alcohol and dehydration, we’d probably expect to see this pattern of alcohol → urination repeat over and over, with sustained increased urine flow. Well, the author of this same paper did that experiment, and this is what they saw:
The subjects still get the initial spike in urine flow after the first drink, but then urine flow dies down. Even though the subjects take 4 more drinks over the next 4 hours, experiencing sustained high blood alcohol concentration, they don’t see a repeat of that initial spike in urine flow relative to someone just drinking water.
Basically, when you spend a night drinking alcohol, you pee just one extra time compared to if you were spending the night drinking water. These results have been repeated dozens of times in different populations, looking at all different kinds of alcohols and moderate vs. heavy alcohol doses. The results have continuously been the same.
In summary, since at least 1942 (and probably even longer, as Eggleton actually cites another study from the 1930s), scientists have known and continued to confirm that alcohol does not cause dehydration. The authors of another paper (from 1982) looking at the biochemical effects of a night of drinking drive the point home:
“At the end of the study (the ‘morning after’) all our subjects had something of a hangover, with headache, nausea, dry mouth, and malaise. They were not, however, in obvious fluid imbalance.... Even allowing for insensitive losses during the 14-hour period, they cannot have been more than 200-300 ml in deficit [of fluids]. They also had normal blood glucose concentrations, no evidence of electrolyte imbalance or liver dysfunction, and fairly low serum ethanol concentrations.” (emphasis added, citation: Gill et al., 1982)
What these authors are saying is that after a night of drinking you’re maybe 200-300mL in the hole on water loss (i.e. one glass of water). And you have normal electrolyte levels.
Shockingly, the science is unequivocal and clear – and has been so for decades. Alcohol does not create the effects of dehydration, electrolyte imbalance, and vitamin deficiency that we grew up believing it did.
The quick intuitive case
Beliefs and biases are resilient, though – even in the face of clear data. So it’s worth taking a moment to think logically about the research and to see whether we can help our intuition make sense of it.
Consider a time you experienced actual dehydration. Perhaps you dehydrated yourself while exercising or spending all day in the hot sun.
How did you feel? Was it the same way you felt the day after drinking a fair amount of alcohol?
Unless you were simultaneously sun-bathing and drinking a bottle of rosé, those feelings were probably different. One probably involved craving cold water and urinating a few shades darker than normal, while the other probably involved feeling like your head was in a vice and being unwilling to get out of bed.
OK, so what is alcohol really doing to the body?
So if alcohol isn’t causing significant dehydration, what is it doing instead?
The truth is that despite over a century of careful research, science still doesn’t completely know. Apart from intoxication – which is well characterized for obvious reasons – it turns out that alcohol’s effects on the body are quite complicated, elusive, and variably dependent on several factors. There’s been a lot of good characterization of general bodily responses, but we have yet to truly understand the whole picture – especially as it relates to the day after drinking.
A good example of this is that researchers have observed a subset of people who experience intoxication without experiencing any of alcohol’s next-day effects, regardless of what or how much they have to drink. But while we know these lucky people exist, scientists have no idea why this is the case, nor can they find any appreciable difference between these people and the general population.
But while we don’t know the whole picture, there’s a fair bit we do know. I’ll break down two general categories that we know are pretty important: (1) alcohol-related effects; and (2) acetaldehyde-related effects. While not comprehensive, these two categories cover a lot.
We all know what alcohol is, but it’s important to note that alcohol (I’m referring to the molecule ethanol specifically here, not alcoholic beverages generally) is a known carcinogen. In high concentrations it’s very bad for you; it kills your cells and disrupts bodily processes. There’s a reason alcohol is used as a disinfectant: it’s great at killing living things!
But at less-than-high concentrations, here’s the short list of general effects alcohol has on your body apart from intoxication:
- Alcohol affects GABA and glutamate signaling in your brain, which among other things negatively affects the quality of your sleep, causing grogginess and daytime sleepiness (citation).
- Alcohol irritates gastrointestinal (GI) tissues and causes inflammation, which can result in discomfort and other GI distress (citation).
- Alcohol alters and disrupts the balance of your microbiome – the massively complex and ever-changing community of microbes that live in your gut – potentially causing bloating, gut and systemic inflammation, and other GI distress (citation).
- Alcohol inhibits antidiuretic hormone vasopressin, which leads to the small spike in urination discussed above. However, more interestingly, this impact on vasopressin causes a pendulum swing that creates wider hormonal imbalances. Alcohol’s impact on vasopressin could be responsible for all kinds of interesting effects as your endocrine system rebalances, including feelings of anxiety, your tolerance for alcohol, and your cravings for alcohol (citation).
Acetaldehyde is a metabolic byproduct of alcohol consumption (we talk about it a lot in our science section). It is a known carcinogen and a highly toxic molecule (citation). The good news is that you are exposed to a lot less of it than alcohol when you drink.The bad news is that it is much worse for you.
Here are some of the general effects of acetaldehyde exposure:
- Acetaldehyde causes vasodilation (widening of your blood vessels), which is why your face might flush red and why you might feel hot if you drink a lot (citation).
- Acetaldehyde causes cell death and DNA damage, which can result in runaway inflammation as your immune system responds (citation, citation, citation).
- Acetaldehyde binds to receptors in your brain, creates oxidative stress, and generally wreaks havoc throughout your body, resulting in some seriously nasty symptoms if your body is exposed to enough, such as nausea, vomiting, tremors, dizziness, diarrhea, and malaise (citation).
Summing things up
Together, ethanol and acetaldehyde cause a tangle of issues your body has to deal with when you drink. As I said before, the issues I listed are not comprehensive, but they paint a pretty good picture.
To massively condense (and over-simplify): the biggies you’re dealing with are interrupted sleep, GI irritation, and a bunch of cellular damage resulting in inflammation. Then there’s a whole host of smaller factors such as glucose-insulin pendulum swings, psychological and neurological effects on your brain, and vasopressin and endocrine system disruption.
The takeaway is that alcohol’s impact on the body is complex.
It has very little to do with dehydration, and it’s not just one thing. To truly understand it, we need to continue investigating the full spectrum of issues our bodies actually deal with, without defaulting to long-held but, unfortunately, untrue assumptions.
A final note
One final thought on the way out...
Even though alcohol-induced dehydration is not the core problem we grew up believing it to be, that does not mean you should stop “hydrating” while drinking alcohol.
You absolutely should drink plenty of water while drinking.
But this isn’t to combat dehydration. Instead, it’s to help you pace yourself (by spreading your alcohol consumption over time) and to support your organs (e.g., your liver and kidneys), which are working overtime to deal with the alcohol you’re ingesting. But leave the rehydration aids and electrolyte powders at the door; they’re not doing anything for you.
This article is for informational purposes only and does not constitute medical advice. The information contained herein is not a substitute for and should never be relied upon for professional medical advice.