Smoking, diet, pregnancy and oral contraceptive use as risk factors for cervical intra-epithelial neoplasia in relation to human papillomavirus infection.

Smoking, nutrition, parity and oral contraceptive use have been reported as major environmental risk factors for cervical cancer. After the discovery of the very strong link between human papillomavirus (HPV) infection and cervical cancer, it is unclear whether the association of these environmental factors with cervical cancer reflect secondary associations attributable to confounding by HPV, if they are independent risk factors or whether they may act as cofactors to HPV infection in cervical carcinogenesis. To investigate this issue, we performed a population-based case-control study in the Vasterbotten county of Northern Sweden of 137 women with high-grade cervical intra-epithelial neoplasia (CIN 2-3) and 253 healthy age-matched women. The women answered a 94-item questionnaire on diet, smoking, oral contraceptive use and sexual history and donated specimens for diagnosis of present HPV infection (nested polymerase chain reaction on cervical brush samples) and for past or present HPV infections (HPV seropositivity). The previously described protective effects of dietary micronutrients were not detected. Pregnancy appeared to be a risk factor in the multivariate analysis (P < 0.0001). Prolonged oral contraceptive use and sexual history were associated with CIN 2-3 in univariate analysis, but these associations lost significance after taking HPV into account. Smoking was associated with CIN 2-3 (odds ratio (OR) 2.6, 95% confidence interval (CI) 1.7-4.0), the effect was dose-dependent (P = 0.002) and the smoking-associated risk was not affected by adjusting for HPV, neither when adjusting for HPV DNA (OR 2.5, CI 1.3-4.9) nor when adjusting for HPV seropositivity (OR 3.0, CI 1.9-4.7). In conclusion, after taking HPV into account, smoking appeared to be the most significant environmental risk factor for cervical neoplasia.