Cigarette smoking and postmenopausal breast cancer risk in a prospective cohort.

BACKGROUND: The relationship between cigarette smoking and breast cancer risk has been inconsistent, potentially due to modification by other factors or confounding. METHODS: We examined smoking and breast cancer risk in a prospective cohort of 186 150 female AARP (formerly American Association of Retired Persons) members, ages 50-71 years, who joined the study in 1995-96 by responding to a questionnaire. Through 2006, 7481 breast cancers were diagnosed. Multivariable-adjusted hazard ratios (HRs) were estimated, overall and stratified by breast cancer risk factors, using Cox proportional hazards regression. Multiplicative interactions were evaluated using the likelihood ratio test. RESULTS: Increased breast cancer risk was associated with current (HR 1.19, 95% confidence interval (CI) 1.10-1.28) and former (HR 1.07, CI 1.01-1.13) smoking. The current smoking association was stronger among women without (HR 1.24, CI 1.15-1.35) as compared to those with a family history of breast cancer (HR 0.94, CI 0.78-1.13) (P-interaction=0.03). The current smoking association was also stronger among those with later (≥ 15 years: HR 1.52, CI 1.20-1.94) as compared with earlier (≥12 years: HR 1.14, CI 1.03-1.27; 13-14 years: HR 1.18, CI 1.05-1.32) ages at menarche (P-interaction=0.03). CONCLUSIONS: Risk was elevated in smokers, particularly in those without a family history or late menarche. Research into smoking's effects on the genome and breast development may clarify these relationships.

Physical activity, sedentary behaviours, and the prevention of endometrial cancer.

Physical activity has been hypothesised to reduce endometrial cancer risk, but this relationship has been difficult to confirm because of a limited number of prospective studies. However, recent publications from five cohort studies, which together comprise 2663 out of 3463 cases in the published literature for analyses of recreational physical activity, may help resolve this question. To synthesise these new data, we conducted a meta-analysis of prospective studies published through to December 2009. We found that physical activity was clearly associated with reduced risk of endometrial cancer, with active women having an approximately 30% lower risk than inactive women. Owing to recent interest in sedentary behaviour, we further investigated sitting time in relation to endometrial cancer risk using data from the NIH-AARP Diet and Health Study. We found that, independent of the level of moderate-vigorous physical activity, greater sitting time was associated with increased endometrial cancer risk. Thus, limiting time in sedentary behaviours may complement increasing level of moderate-vigorous physical activity as a means of reducing endometrial cancer risk. Taken together with the established biological plausibility of this relation, the totality of evidence now convincingly indicates that physical activity prevents or reduces risk of endometrial cancer.