After yesterday’s new year celebrations, you might well be nursing a sore head this morning after a few drinks too many. This is a chemical consequence of consuming alcoholic beverages, but it’s one that, surprisingly, we still don’t fully understand. There are, however, a number of chemical suspects that have been identified as contributing to the symptoms of a hangover; here, we take a look at each in turn, and the evidence for their contribution.

It’s worth clarifying, before discussing the factors causing a hangover, that there’s a significant number of things that can affect a hangover’s severity. We all know that one person who enviably claims never to experience hangovers; studies have shown that genetic factors could go some way towards explaining this, and health, age, and sleep can all also have effects on how bad your hangover is. In general, most studies have shown that reaching a blood alcohol concentration of 0.1% is required for most people to experience the symptoms of a hangover the following day, though obviously this will still be variable from person to person. It’s also generally accepted that the higher your peak blood alcohol concentration (simple translation: the more alcohol you drink), the worse your hangover symptoms are likely to be.

It’s commonly assumed that these symptoms are largely down to dehydration after a night of drinking. It’s widely thought that alcohol does have a diuretic effect on the body, causing an increase in urination, and therefore water loss (though there’s some debate as to the magnitude of this effect). It does this because it lowers the level of the antidiuretic hormone (ADH) vasopressin, which usually acts to increase the retention of water in the kidneys. It therefore leads to increased urination and water loss.

Although dehydration is a commonly assumed cause of a hangover, it’s actually the case that there’s little research to support this. Whilst it’s certainly possible that it contributes in part to some of the symptoms, and drinking a few glasses of water after a night of drinking might help alleviate symptoms such as a dry mouth and thirst, there’s little evidence that this also helps reduce the presence or severity of hangover symptoms.

Another hangover suspect is a compound produced by the metabolism of alcohol. Alcohol (more specifically, ethanol) is broken down by enzymes in the liver into acetaldehyde, which is subsequently broken down by another enzyme into acetate. Acetate can be broken down into carbon dioxide and water. Your body is capable of breaking down alcohol at a rate of around one unit (8 grams or 10 millilitres of pure alcohol) per hour, though of course this rate will vary marginally from person to person.

Acetaldehyde is the particular compound that’s been implicated in hangovers. It’s a toxic compound, which is usually broken down very quickly into acetate. However, the enzyme that converts ethanol to acetaldehyde works faster than that which converts acetaldehyde to acetate, leading to a build-up of acetaldehyde if you have several drinks in a row.

It’s been suggested that acetaldehyde’s toxic effects on cells may play a part in the development of hangover symptoms, particularly nausea, sweating, increased heart rate and headache. There’s still no definite answer one way of the other as to the extent of acetaldehyde’s involvement, though studies have found that the concentration of acetaldehyde in the blood of test subjects didn’t show significant correlation with hangover severity. It may well play a part, but it seems likely that it isn’t the major player.

Other studies have suggested that the problem may lie with compounds other than alcohol in the drinks you were drinking the night before. Most alcoholic drinks will contain a whole range of other chemical compounds as well as ethanol, and these compounds are generally referred to as congeners. Different drinks have different levels of congeners; for example, brandy, red wine and whiskey have much higher congener levels that drinks such as beer, vodka and gin. It’s suggested that higher congener levels could increase the severity of hangover symptoms the next day.

Potential major players when it comes to congeners are alcohols other than ethanol. These are present in much lower quantities, but can include methanol. Whether you’re a chemist or not, you’ve likely heard of methanol, and you might be surprised to learn that it’s actually found in small quantities in most alcoholic drinks, as a byproduct of distillation or brewing processes. Though methanol is dangerous to ingest in large amounts, it’s not problematic in the amounts found in these alcoholic drinks – though it may make a contribution to hangover severity.

Methanol is actually broken down in the body by the exact same enzymes that help to break down ethanol. However, these enzymes are more specific for ethanol; that is, if there’s any ethanol around, they prefer to break it down over methanol. As such, the methanol hangs around until your body’s finished breaking down ethanol. It’s been suggested that its breakdown could account for some hangover effects, as it’s metabolised into toxic formaldehyde and formic acid. The delay in its breakdown could help explain the delayed action of hangovers, and also why ‘hair of the dog’ might help: providing your body with ethanol to break down instead essentially ‘distracts’ the enzymes from breaking down methanol.

It’s generally thought that congeners can have a significant effect on the severity of a hangover, though they still don’t explain all of the symptoms. More recent research is now suggesting that our immune systems may also have a part to play in a hangover’s genesis. Studies have shown that alcohol can have an effect of cytokines, small proteins produced by cells in the body that help control the immune system and fight disease. It can increase the concentrations of certain cytokines in the body, causing ‘imbalance’ in the immune system that could result in symptoms such as headache, fatigue, and memory loss. Changes in hangover severity have been significantly correlated with the increased levels of some of these cytokines.

To draw things to a close, it’s clear that there’s still a lot more work to be done on our knowledge of hangovers; a phenomenon the majority of us experience, but one that we still don’t fully understand. Because of this, it’s hard to suggest an effective hangover cure, and consequently there’s no cure that’s been shown to be particularly efficacious in studies. In the meantime, it looks like there’s little more to do on New Year’s Day than recline on the sofa for most of the day and wait for the symptoms to pass!

There’s more on hangover chemistry, including whether mixing drinks really worsens your hangover, in the Compound Interest book on the chemistry of food and drink, available now!

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