Does light drinking increase breast cancer risk?

By Sophie Ramsey

BMJ Group News

What do we know already?

Several factors can raise a woman's risk of breast cancer, including being older, having children later in life (or not at all), and taking hormone replacement therapy (HRT) during the menopause.

Drinking alcohol can also play a role, with many studies showing that heavier drinking can increase a woman's breast cancer risk. However, the effect of lower-level drinking - for example, less than a glass of wine a day - hasn't been as well-studied. Also, few studies have looked at women's alcohol consumption over many years, to account for changes in drinking habits over time.

To fill these gaps, researchers gathered data from a large US study that tracked the health, diets, and lifestyles of more than 100,000 female nurses over nearly 30 years.

What does the new study say?

Women who drank three to six alcoholic drinks a week had a slightly higher risk of breast cancer than women who didn't drink at all. A US standard alcoholic drink is just over twice a UK unit of alcohol, or about the same as a 175 millilitre glass of wine.

The researchers estimated that, over a year, 333 in 100,000 light-to-moderate drinkers would develop breast cancer, compared with 281 in 100,000 women who did not drink. This represented a 15 percent increase in risk.

Women who drank two or more alcoholic drinks per day had a much steeper increase. Among these heavier drinkers, the researchers estimated that 431 in 100,000 would develop breast cancer over a year - a 51 percent increase in risk compared with women who didn't drink.

The researchers also found that women who tended to 'binge drink' (drink heavily in a short period) had a raised risk of breast cancer compared with those who didn't drink in this manner.

However, the study didn't find any difference in risk based on how old the women were when they drank. Drinking levels at ages 18 to 40 and after age 40 were equally linked to breast cancer risk, suggesting that the effects of drinking in early adulthood persist long term. The researchers also didn't find that what types of alcohol women drank made a difference to their risk.

How reliable is the research?

This was a very large study that followed up with participants numerous times over nearly three decades. The researchers also took account of several other factors that might have affected the women's breast cancer risk, such as their age, their family history of cancer, and whether they smoked. All these things make the findings more reliable.

However, this type of study can't prove cause and effect. So we can't be 100 percent certain that women had a higher risk of breast cancer because of their drinking habits. It's possible that other unknown factors might also have played a role.

What does this mean for me?

Having a glass of wine with dinner, or a drink after work, is a pleasure enjoyed by many of us. These findings suggest that even such moderate drinking may slightly increase your breast cancer risk. But does this mean you should give up alcohol? Not necessarily. The researchers point out that other studies have found that moderate drinking may also lower the risk of heart and circulation problems. These are all factors to take into account when deciding what's right for you.