Most observational epidemiologic studies have shown a slight increase in the risk of breast cancer for women who consume alcohol; the degree of increase is usually small for light-to-moderate drinkers (between 5% and 15% increase for consumers of no more than one drink/day), but the risk may be higher for women consuming greater amounts of alcohol. However, there are a number of factors that affect this relationship, including the type of study (cohort or case-control), the pattern of drinking (regular versus binge), the type of beverage consumed, folate intake, use of hormone replacement therapy, as well as genetic factors. It has also been shown that the risk of breast cancer among obese women tends to be higher than among non-obese women, but there are limited data on how obesity interacts with alcohol in affecting the risk of breast cancer. It is clear that alcohol consumption cannot be evaluated in isolation, without considering other factors that relate to the development of breast cancer.
The present study is based on a large cohort of Swedish women who were examined as part of the Women’s Lifestyle and Health Study in 1991 – 1992, then followed through 2009 for the development of breast cancer: 1,385 cases were ascertained among the more than 45,000 women in the study. Alcohol was assessed at baseline and again at the follow-up examination, and the diagnosis of breast cancer was gleaned through linkage to the nationwide health registries in Sweden. Almost all of the women were pre-menopausal at baseline.
Forum members considered this to be a very well-done study, with almost complete ascertainment of cancer cases. There were some concerns, however, including the fact that the height and weight of subjects was not measured but self-reported, perhaps resulting in less accurate estimates of BMI. Further there may have been residual confounding from other factors related to breast cancer that were not assessed. Finally, the reported levels of alcohol consumption among the women in this study were very low (72% of the cohort drank either zero alcohol or below 5 g per day), but it cannot be determined if these are indeed the levels in this population or whether there may have been under-reporting of consumption.
The key findings as reported by the authors were that, overall, there was “no statistically significant association between alcohol intake and breast cancer risk after adjustment for confounders.” While this is indeed supported by their data, Forum members warned that readers should also note that there was an increase in risk found in less-obese women (BMI ≤ 25 kg/m2), among whom there was a step-wise increase in the risk ratio for cancer from 1.0, to 1.05, to 1.19, to 1.32 with increasing category of alcohol intake. This association was not seen in the more obese women for total cancer or for any of the sub-categories of cancer.
While the overall results did not show an increase in risk from alcohol intake, the results among non-obese women are similar to those seen in many other studies, and suggest a slight increase in the risk of breast cancer associated with alcohol for at least some women. The differences in effect according to BMI reported by the investigators are interesting and add to our understanding of the association of alcohol with breast cancer which, however, remains unclear.
Reference: Shin A, Sandin S, Lof M, Margolis KL, Kim K, Couto E, Adami HO, Weiderpass E. Alcohol consumption, body mass index and breast cancer risk by hormone receptor status: Women’ Lifestyle and Health Study. BMC Cancer 2015;15:881 (DOI 10.1186/s12885-015-1896-3) For the full critique of this paper by members of the International Scientific Forum on Alcohol Research, please click here.