Everyone knows that hangovers happen after consuming too much alcohol, but surprisingly little is known about the science behind them. Despite all of the advances in scientific knowledge and its applications, science has not been able to agree on a common understanding of the hangover, but let's take a look at we do know.

How is alcohol processed by the body?

After taking a swig of your favourite tipple, a quarter of the alcohol is absorbed directly into your bloodstream. The remaining three quarters is stored in your stomach where it is gradually absorbed into the bloodstream.

The rate at which alcohol is absorbed into the bloodstream depends on the it's strength, whether it is carbonated and how full your stomach is. So if you drink a glass of strong champagne on an empty stomach it will be absorbed quicker than a shandy had after a big meal.

Once alcohol has been absorbed into the bloodstream, it stays in the body until it’s processed by the liver and a small amount is expelled by breathing, sweating and urinating.

How does the liver break down alcohol?

Alcohol is mainly processed by the largest organ in the body; the liver. Sitting in the upper right-hand side of the abdomen, the liver is a crucial organ that performs over 500 functions including energy storage, bile production and nutrient intake.

The liver uses the enzyme alcohol dehydrogenase (ADH) to break alcohol down into the enzyme acetaldehyde (ALDH). Then, a further enzyme named acetaldehyde dehydrogenase continues to process the acetaldehyde further into acetate. From there, the acetate is further broken down and exits the body as water and carbon dioxide.

However, a co-enzyme called NAD+ is required to partner both of the other enzymes (ADH and ALDH) that is used up in the process of breaking down alcohol in the liver.

Traditional hangover theories

The issue the body has is that NAD+ is used in a number of different critical bodily functions, such as regulating electrolyte and blood glucose levels. So if the body's NAD+ supply is used up on dealing with large amounts of alcohol, it would make logical sense that there would be repercussions of the type that are experienced during a hangover.

The above process describes what we can refer to as the traditional theory of the cause of hangovers and, barring a few long words, is a relatively straightforward explanation of the hangover.

The traditional theory for hangovers held up to scientific scrutiny for a number of years, but has since been discredited after researchers found that people experiencing hangovers do not have lower levels of electrolytes and glucose as would be expected. Furthermore, hangovers are not solely the cause of dehydration as many people believe.

Modern hangover theories

More modern theories seeking to explain hangovers have observed high levels of cytokines, which are the body’s signalling chemicals. Increased cytokine levels are otherwise seen when the body is battling a viral infection, like the common cold or flu. This would suggest that the body responds to excessive alcohol intake as it would a microbial attack firing up the immune system.

Under this new explanation hangover symptoms such as fatigue, achiness, and impaired cognitive capabilities could be an incorrect reaction to a boozy night out rather than depleted levels of NAD+.

Let’s take a look at closer look into science’s current understanding of hangovers.

The four key hangover contributors

There are four key contributors which are central to science’s current understanding of the cause of hangovers: nutrient depletion, build up of acetaldehyde, immune system disruption and increased glutamine production.

1. Nutrient depletion

Alcohol is a well known diuretic, which is why you have to take frequent toilet breaks while drinking. A side effect of increased urination is that vitamins and minerals stored by the body are lost much faster when drinking alcohol. Vitamins B and C are water soluble and are particularly vulnerable to being expelled at a faster rate.

One study found that depletion of vitamin C levels suffered by 47% when alcohol had been consumed compared to a water-drinking control group. Alcohol was determined to be the causal factor in vitamin C depletion and would suggest that a lack of this essential vitamin is a contributing cause of a hangover.

Evidence supporting the theory that nutrient depletion is a contributing cause of hangovers, is also seen in the cures that have been put forward to treat them.

Tomatoes are known to be excellent sources of vitamin B6 and C and science has found evidence to suggest that they can assist in hangover prevention. A 2016 study found that alcohol was metabolised at a faster rate in subjects who ate tomatoes directly after drinking alcohol (check out our hangover remedy page on tomatoes for more information).

It would seem that lowered levels of key vitamins are a contributing factor to hangovers and that foods high in vitamins (particularly B and C) are key to curing them.

2. Acetaldehyde build up

Acetaldehyde is a toxic substance which, under normal circumstances, is oxidised by an enzyme called acetaldehyde dehydrogenase into acetate. However after a night of heavy drinking acetaldehyde isn’t able to be oxidised quickly enough causing a build up. It is thought the body’s inability to break down acetaldehyde quickly enough is a contributing factor to a hangover and responsible for nausea and headaches symptoms.

One of the reasons why women often experience more severe hangovers than their male counterparts is because women have less acetaldehyde dehydrogenase, increasing the time it takes for the liver to break down alcohol.

A study by Yokoyama et al. (2005) concluded that acetaldehyde build was central to the development of a hangover. The experiment tested 251 Japanese participants and found individuals who had lower levels of alcohol dehydrogenase and, therefore were less equipped to metabolise acetaldehyde, suffered significantly worse hangovers.

To further support the claim that acetaldehyde build up is a contributing factor, there is a well-established genetic component to hangovers. A genetic mutation called ALDH2, primarily observed in people from East-Asia, has been identified as being responsible for metabolising acetaldehyde more slowly than people without ALDH2.

3. Disruption of the immune system

Key to current theories on the causes of hangover symptoms is the involvement of the immune system. When excessive amounts of alcohol are consumed there is an alteration to the parameters of the immune system resulting in an inflammatory response.

The impact of alcohol on the immune system is seen through increased levels of cytokines; the name for a category of small proteins involved in cell signalling, particularly during immune responses. Production of inflammatory cytokines increases when the body comes under oxidative stress; a biological imbalance known to be caused following alcohol consumption.

The body’s response to alcohol, through increased cytokine production would suggest that alcohol is treated as a threat because of the immunological response that is triggered. This theory makes intuitive sense as it is consistent with the cold and flu-like symptoms experienced during a hangover.

4. Increased glutamine production

Glutamine is a neurotransmitter and a natural stimulant used by nerve cells in the brain to send signals to other cells. When alcohol enters your system glutamine signalling is inhibited for a short time, so when you stop drinking the brain goes into overdrive and overcompensates by producing higher levels of glutamine than it needs. This process is known as the glutamine rebound.

The result of increased glutamine levels after drinking is that it disrupts your ability to sleep deeply resulting in commonly experienced symptoms of restlessness and fatigue.
The increased production of glutamine is also central to our best understanding of the biology of alcoholism. Alcoholics have been found to have increased levels of glutamate (an amino acid closely related to glutamine) during acute alcohol withdrawal. It is thought that the spike in glutamate levels following alcohol abstinence results in the fatigue symptoms previously described and is a primary driver behind relapse.

Now that you have an insight into the causes of a hangover, check out Hangover Hacks' library of Hangover remedies to find out what treatments science recommends for curing hangovers.

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