A direct association between moderate drinking and breast cancer incidence has been found in most epidemiological studies albeit the association is less clear for premenopausal than for postrnenopausal women. The underlying mechanisms are not firmly established but may include an influence on circulating oestrogen, immune function, enhanced permeability of chemical carcinogens, decreased absorption of nutrients or the established carcinogen acetaldehyde. The aim of this study was to examine, in a large prospective population-based follow-up study, the Norwegian Women and Cancer Study (NOWAC), how the use of oral contraceptives (OC) interacts with alcohol on breast cancer risk.
Between January 1991 and January 1997, a total of 179,388 women aged 30-70 from the general population of Norway drawn at random from the central person register were invited to participate in the NOWAC cohort. The study questionnaire contained 28 dietary questions including four questions about alcohol use (alcohol use per se and, for drinkers, average frequency of use of beer, wine and spirits). On the basis of these answers, women were classified as non- drinkers, light drinkers (<2 drinks/week), moderate drinkers (2-4 drinks/week) and heavy drinkers (>4 drinks/week).
A total of 1,130 cases of invasive breast cancer were diagnosed during 618,638 person-years of follow-up. Use of >10 g alcohol per day was associated with a PR of 1.69 (95% Cl 1.32-2.1 5), consistent with a linear relationship (P<0.0001 for trend). Among drinkers, an excess risk of breast cancer was found for total duration of OC use only for those drinking less than 5 g/day (P = 0.0009 for trend). Duration of OC showed a negative interaction with alcohol consumption effects (P = 0.01 for interaction). The association between heavy drinking and breast cancer risk was more prominent for postrnenopausal than for premenopausal women (P = 0.01 for heterogeneity). There was no interaction between alcohol use and duration of OC use after stratification on menopausal status
These findings, in conjunction with biological data, imply that alcohol use and OC use have antagonistic effects on breast cancer risk through a common pathway. Whether the interactive effect differs by menopausal status remains to be clarified.
Source: Dumeaux V, Lund E, Hjartiker A. Use of oral contraceptives, alcohol, and risk for invasive breast cancer. Cancer Epidemiol Biomarkers Prev 13 (2004) 1302-1307.