For several decades we have known that alcohol consumption increases a person's risk of developing several types of cancer, but how this actually occurs has never been completely understood. Now a team at Cambridge University has for the first time clearly demonstrated and described how alcohol permanently damages DNA in stem cells which subsequently increases cancer risks.

Scientists have long hypothesized that acetaldehyde, created when our liver breaks down alcohol, is responsible for some of the carcinogenic risk factors of drinking. We know acetaldehyde is toxic, and in cell culture studies it has been shown to cause cancer.

In this new study a team of scientists administered alcohol, or more specifically ethanol, to mice and then observed the effects through chromosome analysis and DNA sequencing. The team found that acetaldehyde clearly, and permanently, damages the DNA in blood stem cells. This discovery potentially explains how alcohol increases the risk of developing several types of cancers.

"Some cancers develop due to DNA damage in stem cells," says Ketan Patel, lead author of the study. "While some damage occurs by chance, our findings suggest that drinking alcohol can increase the risk of this damage."

The scientists further investigated how the body manages this influx of acetaldehyde. It is known that acetaldehyde is broken down in the body by a group of enzymes called aldehyde dehydrogenases (ALDH). Many people around the world carry defective ALDH enzymes, meaning they cannot effectively clear the body of acetaldehyde after drinking alcohol. As well as explaining why some people feel inordinately unwell after drinking alcohol, this build-up of acetaldehyde can result in greater DNA damage.

When mice deprived of ALDH enzymes were administered alcohol in the study, the researchers identified four times the DNA damage in cells compared to the fully-functioning mice. Translating these results to humans means that, for some people, drinking alcohol could be exponentially more dangerous depending on their individual ability to process acetaldehyde.

"Our study highlights that not being able to process alcohol effectively can lead to an even higher risk of alcohol-related DNA damage and therefore certain cancers," explains Patel. "But it's important to remember that alcohol clearance and DNA repair systems are not perfect and alcohol can still cause cancer in different ways, even in people whose defense mechanisms are intact."

This study is the clearest understanding we have attained to date explaining the mechanism behind alcohol and cancer. There are seven common cancers that have been confidently linked to alcohol: mouth, upper throat, laryngeal, oesophageal, breast, liver, and bowel. Further research is planned to help understand why alcohol is more likely to result in these specific cancers and not others.

So is all alcohol created equal? Is any drink potentially safer than others? A study from 2014 suggests that red wine may be the best choice if you must drink alcohol. A chemical called resveratrol, found in red wine, berries and dark chocolate, has been seen to help fight back against the cancer-causing effects of alcohol. Of course this doesn't mean knocking back bottles of red wine is a healthy choice – the safest option ultimately is to simply limit the volume of alcohol one is consuming.

The study was published in the journal Nature.