Prospective study of solar exposure, dietary vitamin D intake, and risk of breast cancer among middle-aged women.

The relationship between solar exposure or dietary vitamin D intake and breast cancer risk has not been fully elucidated. These associations were studied within the Women's Lifestyle and Health Cohort Study, a cohort of 49,259 Swedish women ages 30 to 50 years at baseline (1991-1992). Women were asked about solar exposure and completed a food frequency questionnaire and were followed-up through linkages to national registries until December 2004. In the current analyses, 41,889 women were included, 840 of whom were diagnosed with breast cancer during follow-up. Breast cancer risk was not related to solar exposure variables, including sun sensitivity, annual number of sunburns, time spent on sunbathing vacations, or solarium use at any age period of exposure. There was also no association with dietary vitamin D intake or supplementary multivitamin use. These relationships were not modified after stratifying by estrogen or progesterone receptor status.