Alcohol and breast cancer – some absolute numbers
This was a meta-analysis of data on light alcohol drinking and breast cancer. The authors reported that they looked at 113 papers which reported breast cancer risk estimates for light drinkers. Only 36% of the reported estimates were adjusted for the main risk factors (age, family history, parity, menopausal status, oral contraceptive/hormonal replacement therapy use). The findings reported in the abstract were:
A significant increase of the order of 4% in the risk of breast cancer is already present at intakes of up to one alcoholic drink/day.
This fits with the random-effect summary relative risk (RR) of 1.04 (95% CI, 1.02 to 1.07) for the overall analysis. However, when pooling for the 36% of studies that had adjusted for the main risk factors the RR was 1.03 (95% CI, 1.00 to 1.07). Hmm, a 95% confidence interval that includes 1.0 – I’m starting to feel a little anxious about the numbers here.
These epidemiological studies can only show an association. However, alcohol as a causative agent in breast cancer doesn’t seem that unlikely. There is secure evidence that higher consumption of alcohol is associated with an increased risk of breast cancer. There is a plausible mechanism with alcohol affecting oestrogen levels and given that breasts are a highly oestrogen-sensitive tissue. The studies also seem to suggest a dose-response relationship – a further piece of evidence suggesting alcohol is causative.
So, seeing a small effect (and it is small) with light drinking is hardly surprising. On population terms the epidemiologists may be impressed but your average person may wonder at their real risk. It is notable that at no point in this paper, or the press release, or the Daily Telegraph article does anyone attempt to put it in absolute risk terms.
Telling people who drink heavily that they need to cut down isn’t controversial but how much can a woman who cuts down on her light drinking expect to benefit? Well, firstly, there is no guarantee that stopping or reducing drinking will reduce risk anyway- no one has ever done that study and they’re not likely too either. But we can do a very rough and ready calculation with the absolute numbers.
How about if we do something as simple as reduce the current age-related risks by 4%?
That’s an absolute risk reduction of 0.08%.
In women under 39 the absolute risk is estimated as 1 in 215 (0.465%). A reduction of 4% would put the risk at 0.447%.
That’s an absolute risk reduction of 0.018%.
(Age-related risk data from Cancer Research UK.)
Of course, these numbers wouldn’t make quite such impressive headlines. But it’s a pity that the authors, the journal, or the newspaper haven’t made any referral to absolute risk at all.
Seitz, H., Pelucchi, C., Bagnardi, V., & Vecchia, C. (2012). Epidemiology and Pathophysiology of Alcohol and Breast Cancer: Update 2012 Alcohol and Alcoholism DOI: 10.1093/alcalc/ags011