What is a hangover?
Surprising research unearthed about an age-old problem
A hangover is not dehydration
What is a hangover?
To answer this question, I think it’s critical to start with what science knows extremely well a hangover is not. A hangover is not dehydration or electrolyte imbalance.
I’m constantly surprised that pop-science and blog write-ups about hangovers still completely focus on dehydration, when we have decades of peer-reviewed scientific research that debunk this myth over and over again.
It’s such an ingrained, unquestioned idea: a default assumption we absorb from our friends and family when we first start drinking. But if we really think about it, the idea falls apart...
If a hangover really was just dehydration, no one would ever get a hangover.
How hard is it to drink a glass of water? Do you feel hungover when you dehydrate yourself while working out? Or while hanging out in the sun? When’s the last time you spent all day on the couch feeling like your head was in a vice because you ate too much beef jerky and didn’t hydrate? We continue to believe this collective myth that dehydration caused by alcohol creates a hangover, despite countless real world examples that dehydration caused by anything else does not.
But it’s tough to evaluate our own experiences objectively – especially in the face of long-held assumptions. So to drive the point home, let’s take a minute to look at the real science and what it has to say about this idea.
The science has been clear since 1942
The common dehydration=hangover idea is probably based on the observation that alcohol makes you pee more. Amazingly, (and this is something I didn’t expect when starting my research) even this observation is pretty much wrong! Digging through the scientific literature, it turns out that published experiments dating back as far as 1942 completely debunk the myth that alcohol has a dehydrating effect (citation).
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.
A picture is worth a thousand words; 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 understanding of alcohol’s effect on the body.
OK, so what if we give someone a bunch of drinks in succession? Given our popular assumptions about drinking and dehydration, I think we’d all assume that we’d see a repeat of this chart 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.
So 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.
To summarize, since at least 1942 (and probably even longer, as Eggleton actually cites another study from the 1930s), we’ve 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 summarize the point well:
“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.” (citation)
So at most you’re maybe 200-300mL in the hole on water loss (i.e. one glass of water), and you have normal electrolyte levels. The science is unequivocal and clear – and has been so for decades – that dehydration, electrolytes, and vitamins make no sense when it comes to hangovers.
OK, so then what is a hangover, really?
The truth is that despite over a century of careful research, science still doesn’t completely know. It turns out that a hangover is quite complicated, elusive, and variably dependent on several factors. There’s been a lot of good characterization of symptoms and general bodily responses, but science has yet to truly understand the complete picture.
A good example of this is that researchers have observed a “hangover resistant” subset of people who never experience hangovers, regardless of what they drink. But even though we know these lucky people exist, researchers have no idea why they don’t experience hangovers, nor can they find any appreciable difference between these people and the general population.
However, 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. These two represent big and important swaths of the hangover, but it’s important to note that even this is not comprehensive.
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 that can contribute to your hangover:
- 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).
- 4. 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 that can contribute to a hangover:
- 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
Ethanol and acetaldehyde both cause a lot of issues your body has to deal with when you drink. As I said before, this is not a comprehensive list, but it definitely paints a pretty good picture.
To massively condense (and over-simplify): the biggies you’re dealing with are a lack of quality sleep, GI irritation, and a bunch of cellular damage resulting in inflammation. Then there’s a whole host of other smaller factors such as glucose-insulin pendulum swings, psychological and neurological effects of alcohol and acetaldehyde tinkering with your brain, and vasopressin and endocrine system disruption, all of which can help create that very visceral pain you might deal with the day after a big night out.
The takeaway is that a hangover is complex. It’s not dehydration, and it’s not just one thing. To truly understand hangovers, we need to continue investigating the full spectrum of issues our bodies are actually dealing with, and not default to long-held but, unfortunately, mistaken assumptions.
A final note
I’ll leave you with a final thought and piece of advice.
Even though alcohol-induced dehydration is not the hangover cause you grew up believing it to be, that doesn’t mean you should stop “hydrating” while drinking alcohol. You absolutely should drink plenty of water while drinking. This is not to combat dehydration, though. Instead, it’s to help you pace yourself (spread your alcohol consumption over time) and support your organs (such as your liver and kidneys), which are working overtime to deal with the alcohol you’re ingesting. Just leave the rehydration aids and electrolyte powders at the door; they’re not doing anything for you.