Background. Results of research on the effects of caffeine consumption and invasive breast cancer remains inconsistent. The aim of this study was to determine the association between caffeine intake from coffee and tea on incident invasive breast cancer in a large cohort of postmenopausal women.

Methods. This analysis included 79,871 women without previous invasive breast cancer in the Women's Health Initiative Observational Study prospective cohort. Coffee and tea intake were self-reported in a baseline questionnaire. Cox proportional hazard models were used to estimate crude and multivariate-adjusted hazards ratios (HR) and 95% confidence intervals (CIs) for invasive breast cancer incidence. Subgroup analyses by beverage type, age, race/ethnicity, smoking status, BMI, sleep duration, history of hormone therapy use, and invasive breast cancer receptor subtypes were also assessed.

Results. A total of 4,719 incident cases of invasive breast cancer were identified with a mean follow-up time of 13.5 years. No significant association was found between overall reported caffeine intake from coffee and tea and invasive breast cancer incidence. In the beverage-specific analysis, participants consuming 2-3 cups/day of caffeinated or decaffeinated coffee, compared to non-drinkers, were at increased risk of invasive breast cancer in a multivariate model adjusting for age, race/ethnicity, education, and other caffeinated beverages (caffeinated coffee HR = 1.12, 95% CI = 1.02-1.24; decaffeinated coffee HR = 1.24, 95% CI = 1.09-1.41). Invasive breast cancer risk was not associated with tea consumption. In analysis of invasive breast cancer receptor subtypes, there was decreased risk of ER+/PR- breast cancer among those in the highest quartile (279 -- 736 mg/d) of caffeine consumption compared to the lowest (0 -- 93 mg/d) (HR = 0.74, 95% CI = 0.59, 0.93).

Conclusion. Our findings suggest that total caffeine intake from coffee and tea is not associated with invasive breast cancer incidence. Consumption of 2-3 cups/day of coffee is associated with increased incidence of invasive breast cancer. There was no association of risk with tea consumption.