Alcohol can strengthen the heart because it teaches cells how to cope with stress, a study has found.
The organisms the body produces to clear toxic chemicals from alcohol are the same ones it needs to protect the heart from damage, researchers say.
The body produces organisms called enzymes which break down the toxic parts of alcohol when people drink it, and they are the same enzymes which are released to protect the heart when it suffers major damage, such as during a heart attack.
So exposing the heart to low levels of alcohol effectively trains the body how to create the enzymes needed to cope with a heart attack.
However, drinking excessively has the opposite effect and makes the heart more vulnerable than not drinking at all.
Researchers at the University of São Paulo in Brazil say the findings could lead to a drug which does the same thing but does not carry the risks of drinking alcohol.
In their research the scientists said it is already known that drinking alcohol in moderation can improve heart health, but the reasons are not well understood.
Now they say it could be because being exposed to the poisonous ethanol in alcohol builds up the same protective reaction that is needed to reduce damage during a heart attack.
The researchers do not explain how much alcohol is optimal and say it varies depending on the individual.
They used the equivalent of two cans of beer for an average man, but drinking that every day would be 28 units per week – double the NHS's recommended amount.
Although high levels of the chemicals in alcohol are toxic to heart cells, lower levels – like those produced by an average sized man drinking two cans of beer or two glasses of wine – could make it stronger.
And the enzymes needed to break down this deadly chemical are the same as those used to get rid of alcohol from the body, the researchers say.
Alcohol builds up the heart's ability to deal with damage
A drinker's body builds up a chemical memory of how to create these enzymes to protect itself, so it is better prepared for heart damage than the heart of someone who does not drink alcohol.
In tests on mice's hearts researchers interrupted the flow of oxygen and nutrients to the heart for 30 minutes to simulate myocardial infarction – a heart attack.
This was less damaging for hearts which had been exposed to alcohol, they found.
Some 50 per cent of cells died during the heart attack in the hearts which was not exposed to alcohol.
'Our data suggest moderate exposure to ethanol causes minor stress in heart cells but not enough to kill them,' says Professor Julio Cesar Batista Ferreira.
'Heart cells eventually create a biochemical memory to protect against stress, also known as preconditioning.
'When the cells are submitted to a higher level of stress, they know how to deal with it.'
Millions have genetic intolerance to alcohol
However, the same process does not happen in everybody. Some people have a genetic mutation which means they are 80 per cent less able to make the enzyme.
This condition is thought to affect around 500 million people worldwide – it is particularly common in East Asian people – and is easy to identify because they have a poor tolerance of alcohol.
Professor Ferreira said: 'It all depends on people's DNA
'The [chemical] that results from digesting ethanol may protect most people if a small amount is produced, but it can also maximize the damage done by a heart attack in an individual with the ALDH2 gene mutation.
'It's easy to identify these people. After one glass of beer, they get flushed and complain of a headache. Their resistance to alcohol doesn't improve over time.'
Drinking too much makes heart damage worse
Heart damage may also get worse among people who drink too much because – like people with the gene mutation – the body cannot produce enough enzymes to cope with the alcohol.
In the study hearts which mimicked this condition, 70 per cent of heart cells died during a heart attack after alcohol consumption, so the scientists' theory did not work.
In the same group using a drug which simulated the effects of alcohol, however, only 35 per cent of cells died during a heart attack.
This could pave the way for a drug to be developed which is safer than drinking alcohol, the researchers say.
The findings were published in the journal Cardiovascular Research.