Vitamin D is well known for its important role in calcium and phosphor homeostasis. Recent research suggests that vitamin D also prevent some type of cancers. We studied solar vitamin D effective UV radiation (VD dose), dietary vitamin D, sun-seeking holidays, use of solarium, frequency of sunburn and breast cancer risk in a large population-based cohort study. A total of 41,811 women from the prospective Norwegian Women and Cancer Study, aged 40-70 years at baseline, were followed from 1997/1998 to 2007. Dietary vitamin D intake was calculated at baseline. Information on historical VD dose was used as a proxy for cutaneously obtained vitamin D status. Cox proportional hazards model was used. We adjusted for age, height, BMI, baseline menopausal status, use of hormone replacement therapy, use of oral contraception, alcohol, mother's history of breast cancer, mammography and parity. During 8.5 years of follow-up, 948 new cases of breast cancer were registered using data from the Norwegian Cancer Registry. We found no significant associations between VD dose, or vitamin D intake, or sun-seeking holidays, or use of solarium, or frequency of sunburn, and breast cancer risk. Relative risks (95% confidence intervals) for highest versus lowest category were 1.17 (0.95-1.44), 0.95 (0.75-1.21), 1.07 (0.87-1.32), 0.93 (0.76-1.14) and 1.10 (0.89-1.36), respectively. Our results do not support an association between vitamin D status, and breast cancer risk.
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