Understanding rural–urban differences in risk factors for breast cancer in an Indian population

Purpose: Although cancer registry data indicate that there are large differences in breast cancer (BC) rates between rural and urban regions of India, the reasons for these differences are not well understood. Methods: We conducted a hospital based case–control study (1,637 breast cancer cases; 1,515 visitor controls) in Mumbai, India, during the years 2009–2013. Extensive questionnaire data, anthropometry measurement and blood samples were collected on all participants. Using logistic regression models, we estimated risk based on odds ratio (OR) and 95 % confidence intervals (CI) for various reproductive and anthropometric measures, stratified by rural–urban status depending upon residence in first 20 years of life. Results: Waist-to-hip ratio of ≥0.95 compared to ratio ≤0.84 was strongly associated with risk of BC in both rural and urban populations (OR_urban = 4.10, 95 % CI 3.03–5.56; OR_rural = 3.01, 95 % CI 1.85–4.90). First full-term pregnancy after the age of 25 compared to first full-term pregnancy below 20 years of age was associated with risk of BC in both urban and rural women (OR_urban = 1.78, 95 % CI 1.32–2.41; OR_rural = 2.24, 95 % CI 1.13–4.43). The prevalence of age at first full-term pregnancy was significantly lower in rural (mean age at first full-term pregnancy = 19.39 years) versus urban women (mean age at first full-term pregnancy = 22.62 years), whereas mean waist circumference was much higher in urban women (82.13 cm) compared to rural women (79.26 cm). We did not observe any association between breast feeding and risk of BC. Conclusions: Differences in the prevalence of central adiposity and age at first full-term pregnancy between rural and urban women from India may explain some differences in breast cancer rates between these two populations.