Reproductive factors, obesity and risk of colorectal cancer in a cohort of Asian women

•Obesity is positively associated with colonic cancer but not rectal cancer.•Abdominal obesity exerts an independent effect on colonic cancer.•Reproductive factors have at best a weak effect on colorectal cancer. This study evaluated reproductive factors and obesity in relation to colorectal cancer (CRC) in Asian women. The study cohort comprised 28191 women who were recruited between 1994 and 1997. During 18 years of prospective follow-up, 404 and 212 women developed colon cancer (CC) and rectal cancer (RC) respectively. Cox proportional hazards regression was used. Menstrual factors were not related to the risk of CRC, CC and RC. Gravidity and parity were not associated with CRC or RC, but women who were ever pregnant had a HR of 1.87 (95%CI 1.12–3.14) compared to those never pregnant, and parous women had a HR of 1.79 (95% CI 1.10–2.92) compared to nulliparous women for CC. Use of oral contraceptives and hormone replacement therapy were not associated with CRC, CC or RC. Compared to women with normal BMI, women who were obese had HRs of 1.39 (95%CI 1.12–1.74) and 1.64 (95%CI 1.24–2.16) for CRC and CC respectively. No increased risk was seen for RC. Adjusted for BMI, for colonic cancer, women in the highest quartile for Waist Circumference had a HR of 2.14 (95%CI 1.42–3.25) compared to the lowest quartile, for Waist Hip Ratio, a HR of 1.74 (95%CI 1.30–2.34), and for Waist-Height ratio, a HR of 1.80 (1.26–2.57). None of these measures were significantly associated with RC. Obesity is positively associated with CC but not RC, and abdominal obesity exerts an independent effect. Reproductive factors had at best a weak effect on CC and RC.