Incident invasive breast cancer, geographic location of residence, and reported average time spent outside

There have been reports of greater breast cancer incidence and mortality at northern compared with southern latitudes postulated to be related to vitamin D exposure. Among 71,662 participants in the Women's Health Initiative Observational Study (WHIOS) free of cancer at baseline (1993-1998), associations were explored between incident invasive postmenopausal breast cancer (n = 2,535), over ̃8.6 years follow-up, and the following: (a) region of residence at birth, age 15 years, age 35 years; (b) region of residence at WHIOS baseline; and (c) clinic center solar irradiance. Hazard ratios and 95% confidence intervals (CI) for breast cancer were estimated after adjustment for individual level confounders. There was no difference in breast cancer risk by region of earlier life, baseline residence, or solar irradiance measured in Langelys (gm-cal) per cm ². There was an observed 15% decreased risk among women residing in areas of low versus high solar irradiance measured in Watts per m ² (95% CI, 2-26%). However, the associated P _trend of 0.20 was not significant. Conversely, women who reported spending on average <30 minutes versus >2 hours outside in daylight year round at WHIOS year 4follow-up (n = 46,926), had a 20% (95% CI, 2-41%; P _trend = 0.001) increased risk of breast cancer. In conclusion, region of residence and geographic solar irradiance are not consistently related to risk of breast cancer and may not be sufficient proxy measures for sunlight/vitamin D exposure. The observed association between time spent outside and breast cancer risk support the hypothesis that vitamin D may protect against breast cancer.