My biology teacher at school told us: "You don't buy a beer, you rent it." And the added cost of renting it, he used to say, was the hangover.

But how does alcohol really affect our body and brains, and how much do we know about hangovers? Well, scientists aren't exactly sure what's going on, but everyone who's had a drink knows how miserable they can be.

This holiday season, many of us will be celebrating with a drink or three. But is it possible to come out the other side without any aftereffects?

The chemistry of hangovers

Alcohol is metabolized in the liver by the enzyme alcohol dehydrogenase. As ethanol breaks down, it forms acetaldehyde, which is a toxic chemical the body needs to clear away before it starts causing serious damage.

Studies have shown that hangover symptoms reach their peak around the point when all the alcohol has been converted to acetaldehyde. It's here when the blood alcohol content is returning to zero.

But it's not only alcohol that contributes to hangovers — other compounds in alcoholic drinks like congeners and sulfites, which are particularly common in darker drinks like red wines and whiskeys, also play their part.

Hangover symptoms and their causes

Alcohol affects multiple systems in the body, which explains the many symptoms of hangovers. Here's what we know:

Dehydration: alcohol is a diuretic, meaning it increases urination. Alcohol supresses the release of vasopressin, a hormone that signals the kidneys to retain fluid. This loss of fluid leads to mild dehydration, contributing to headaches and fatigue.

Headache: headaches are a staple of hangovers. One reason is the mild dehydration, which is caused by a minor shrinkage in your brain due to loss of water. This shrinkage pulls the brain away from the skull, dragging on nerves outside the brain that cause pain. Secondly, alcohol is a vasodilator, meaning it can trigger migraine attacks in people who are prone to them.

Nausea: Alcohol irritates the stomach lining and increases stomach acid release, leading to nausea and stomach upset.

Fatigue: Along with the late night, alcohol consumption causes fragmented and disrupted sleep, making you tired and irritable the following day. Second to this is the fact that alcohol increases general inflammation in the body, which is caused by a general immune response to clear out damaging chemicals. This contributes the general malaise you might feel when hungover, like when you're ill.

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Worse hangovers could have genetic origins

It's obvious that drinking more alcohol causes worse hangovers, but it's never quite that simple. There's a huge range in people's experiences with hangovers: some get them worse than others, and from less alcohol too.

One reason could be your genes. Studies have shown that many people are less effective at metabolizing alcohol in the liver due to genetic variances.

The culprits are two enzymes important for breaking down ethanol: alcohol dehydrogenase and aldehyde dehydrogenase.

In fact, about 45% of hangover severity is due to inherited variations in the genes that encode these enzymes. Genetic variations causing sensitivity to alcohol and hangovers are particularly common in people of Asian descent.

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On the other side, about 10-20% of fortunate drinkers report not having a hangover, even after consuming large amounts of alcohol. Either they're just better at pretending to be healthy, or their livers may be more effective at breaking down alcohol.

Taking a poo, and other hangover cures

Are there any scientifically proven hangover cures? Bad news: no, not really.

Sure, there are plenty of folk cures involving raw eggs, coffee, sex or isotonic mixtures, but none of these are definitive cures for hangovers. All they do is replenish nutrients, fluids and endorphins lost during the hangover. Sure, they're all restorative, but they aren't going to make the hangover magically disappear.

But one study has found that an effective way to speed up recovery is to have a poo.

The reason is that ethanol stays around in the stomach and intestines for a long time after consumption, from where it continues to be absorbed into the bloodstream. The authors of the study call this "intestinal drinking."

The intestine absorbs ethanol faster than the liver can metabolize it, meaning that taking a poo is an effective way of evacuating the ethanol in your intestines that has yet to be absorbed into the blood.

The study suggests having a poo or two is an effective way to alleviate hangover symptoms and reduce the risk of liver damage.

But the best way to avoid a hangover is simply to have a healthy day, an early night — and lay off the booze.

Edited by: Andreas Illmer

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