Overall, more than 100 epidemiological studies have evaluated the association between alcohol consumption and the risk for breast cancer. Taken together, all of the results from these studies suggest that low to moderate alcohol consumption (i.e. in the order of one alcohol drink per day) is associated with an increased risk for breast cancer, and that the risk rises with increasing intake. In a large pooled analysis of more than 50 studies, Hamajima et al (2002)1 found a significantly increased risk (relative risk, 1.13; 95% CI, 1.07–1.20) for an intake of 18 grams (g) alcohol per day.
Recent international systematic reviews have concluded that the evidence is now convincing that alcohol consumption is a cause of pre-menopausal and post-menopausal breast cancer, and that there is a strong dose response pattern without a threshold effect2, 3. Further studies published since these international reviews continue to confirm the association between alcohol consumption and risk of breast cancer; the largest of these studies, conducted by the European Prospective Investigation into Cancer and Nutrition and based on 4300 cases, reported a significant 13% increase in risk for breast cancer for intakes of ≥ 20 g alcohol per day, which corresponds to an increase in risk of 3% per 10 g intake of alcohol per day (95% CI, 1–5%)4.
Despite the overall consistency in the association between alcohol and an increased risk of breast cancer, several important questions remain concerning the nature of the dose response association including whether the association between alcohol intake and breast cancer risk is affected by the timing of alcohol exposure (e.g. current or lifetime), modified by other risk factors and potential confounders or effect modifiers of the relationship such as, reproductive factors, deprivation, folate intake, use of hormone replacement therapy or is more pronounced among women diagnosed with hormone receptor positive tumours or in certain histologic subtypes.
Nevertheless despite these factors and the often small magnitude of excess risk, the association is of great importance because of the apparent lack of a threshold, the large number of Scottish women drinking small amounts of alcohol and the high incidence of the breast cancer in Scotland.
An added confusion to understanding the association between alcohol consumption and an increased risk of breast cancer concerns the term ‘drinks’ used in the epidemiological literature. This term is meant to denote standard drinks in the respective country or area surveyed, yet the use of the ‘standard drink’ concept is complicated by different standards across countries and even within countries. In the UK, there is no ‘standard drink’ measure since the alcohol unit, which varies by drink type and serving size, is the mainstay measure used in the majority of research and for public health messages on safe and excessive drinking levels. The UK standard unit measure is seen as being equivalent to 8 grams, lower than the gram equivalencies (range 12-26 g) of standard drinks in epidemiological studies.
Relying simply therefore on reports from the international literature on the association between alcohol consumption and breast cancer can be confusing; in the current Scottish Government cancer strategy, ‘Better Cancer Care, An Action Plan’, drinking more than three units a day is highlighted as increasing the risk of breast cancer5. The threshold of ‘three units’ (e.g. equivalent to 24 grams in Scotland/UK or 2 medium sized (175ml) glasses of wine) is, however, misleading and, is not consistent with the international evidence which shows an increased risk of breast cancer associated with drinking approximately >15 g/d (= >2 ‘UK’ units). In the end, the best advice in face of the uncertain nature of the alcohol breast cancer association would be not to exceed one medium sized glass of wine a day (i.e. no more than one unit per day).
Dr Ian Grant, Principal Researcher, Scottish Public Health Observatory