Worrying news: alcohol is a carcinogen. But what does this actually mean? Should you be concerned about that couple of drinks you enjoy with your friends every so often?
What is a carcinogen?
A carcinogen is anything that has the potential to cause cancer, a disease caused by DNA mutations that lead to uncontrolled cell division. Different carcinogens can act in different ways – while some may directly act on DNA, others may cause cells to divide at a faster rate than normal, which could increase the chances of mutation.
You should note that carcinogens don’t cause cancer in every single case of exposure – being exposed doesn’t automatically mean a person will definitely develop cancer. Different carcinogens may have different levels of cancer-causing potential, with some that may only cause cancer after prolonged, high levels of exposure have occurred. In addition, there are a range of factors that affect whether someone exposed to a carcinogen will actually develop cancer – these may include how they are exposed, the length and intensity of the exposure, other environmental factors and the person’s genetic makeup.
There are different groups used to classify carcinogens:
Group 1: Carcinogenic to humans
Group 2A: Probably carcinogenic to humans
Group 2B: Possibly carcinogenic to humans
Group 3: Unclassifiable as to carcinogenicity in humans
Group 4: Probably not carcinogenic to humans
Sadly, alcohol is in group 1. That means alcohol definitely can cause cancer.
What types of cancer does alcohol cause?
It is estimated that 5.5% of new cancer cases worldwide can be traced back to drinking.
Consumption of alcohol has been conclusively shown to be the direct cause of seven types of cancer: oropharynx, larynx, oesophagus, liver, colon, rectum, and breast. In addition, there is growing evidence to implicate alcohol consumption in the development of skin, prostate, and pancreatic cancer.
How much alcohol increases the risk?
Sadly, research suggests no amount of alcohol is safe, as even modest alcohol consumption may increase risk of some cancers. A meta-analysis found that drinking one drink or fewer per day is associated with some increased risk for squamous cell carcinoma of the oesophagus, oropharyngeal cancer, and breast cancer.
However, the greatest risks occur with heavy, long-term use. The most common type of excessive drinking is binge drinking (consuming four or more drinks during a single occasion for women, or five or more for men), whereas heavy drinking is defined as eight or more drinks per week/three or more drinks per day for women fifteen or more drinks per week/four or more drinks per day men. Moderate drinking is defined as up to one drink per day for women and up to two drinks per day for men.
The more a person drinks and the longer period of time they drink for, the greater their risk of developing cancer – especially head and neck cancers.
Evidence overall supports a positive dose-response relationship for alcohol and liver cancer.
One review found that people consuming four or more drinks (one drink defined as around 1.5 units of alcohol) per day have about five times the risk of mouth and pharynx cancers, compared with people who never drank or only drank occasionally. Even light drinkers (no more than one drink a day) had a 20% higher risk.
The specific type of alcohol consumed does not affect risk.
However, there can be difficulties in investigating dose-response relationships due to differences in what is defined as a ‘drink’ in different countries and self-reports being used to measure alcohol consumption. Many people are not aware of how many units they are consuming, and people may lie about their alcohol consumption – this could lead to ‘light’ or ‘moderate’ drinking being wrongly associated with increased. cancer risk
How does alcohol cause cancer?
There are a few different mechanisms by which alcohol could cause cancer.
When you drink alcohol (C2H6O), this needs to be processed. This is carried out sequentially, catalysed by two enzymes – alcohol dehydrogenase converts alcohol into acetaldehyde (C2H4O), and then aldehyde dehydrogenase converts that into acetate (C2H3O2). While ethanol itself is not mutagenic, acetaldehyde is – acetaldehyde binds to DNA and proteins, which can lead to them being damaged. DNA damage can lead to mutations, which could cause cancer. In research carried out on mice and rats, the animals were a lot more likely to develop tumours if they drank water with ethanol or acetaldehyde.
Another way alcohol may cause cancer is by generating what are called reactive oxygen species (ROS). ROS are chemically reactive, toxic molecules containing oxygen, which can damage DNA, proteins and fats in the body. This is done by the reaction of oxidation.
Also, alcohol can impair the ability of the body to absorb and use some nutrients associated with cancer risk, including, vitamins A, the B complex vitamins such as folate, vitamin C, vitamin D, vitamin E, and carotenoids.
Alcohol raises oestrogen levels. Oestrogen is a sex hormone that is linked to a higher risk of breast cancer.
There are some ways in which alcohol may interact with other carcinogens to produce a larger combined risk. Alcohol may change the activity of detoxifying enzymes, which could interfere with the body’s ability to deal with carcinogens. Also, alcohol is a solvent, which can lead to enhanced penetration for carcinogens.
A key factor for a type of liver cancer, hepatocellular carcinoma, a type of liver cancer, is cirrhosis – 90-95% of sufferers have underlying cirrhosis. Cirrhosis is scarring of the liver as a result of previous damage, which can often by caused by chronic alcohol consumption. Therefore, alcohol consumption increases the risk of liver cirrhosis, which in turn predisposes someone to liver cancer.
How do genetics affect this?
The enzymes responsible for processing alcohol are coded for by two different genes. Genes can have multiple forms, called alleles. Certain alleles may increase the mutagenic effects of consuming alcohol.
For the gene encoding aldehyde dehydrogenase 2, there is an allele encoding a catalytically inactive protein. That means, when alcohol is consumed, there is an excessive accumulation of acetaldehyde. As acetaldehyde is a carcinogen, people with this defective gene are expected to be more susceptible to alcohol-induced cancer. Studies carried out in East Asian populations, who have the highest prevalence of the defective allele, show that drinking alcohol is more strongly associated with cancers of the upper aerodigestive tract in people with the defective allele.
In addition, some people have an allele of the gene coding for alcohol dehydrogenase, which leads to enhanced production of acetaldehyde in the liver. Studies of high alcohol intake found that this allele is significantly associated with increased risk for liver cancer and some other cancers.