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Vitamin D Status and Risk of Breast Cancer in Iranian Women: A Case–Control Study

Objective: Considering the rising incidence of breast cancer and high prevalence of vitamin D deficiency in Iran, this case–control study aimed to investigate the relationship between serum concentration and intake of vitamin D and risk of breast cancer. Methods: A total of 135 incident breast cancer cases at the Cancer Research Center of Shahid Beheshti University of Medical Sciences were matched with 135 controls by age and menopausal status. A validated and reliable 168-item food frequency questionnaire was completed by participant interviews. To determine the vitamin D content of foods we used the U.S. Department of Agriculture (USDA) nutrient database. To analyze the food frequency questionnaires we used the data collected in the Iranian Household Food Pattern Study, conducted by the National Nutrition and Food Technology Research Institute and the Iranian Ministry of Agriculture. Five-milliliter blood samples were collected to measure serum 25-hydroxyvitamin D (25(OH)D) using an enzyme-linked immunosorbent assay method. Results: Women in the fourth quartile of serum 25(OH)D level had 3 times lower risk of developing breast cancer compared to those in the first quartile. In the adjusted model the inverse relationship remained significant (odds ratio [OR] = 0.269; 95% confidence interval [CI], 0.122–0.593). In the stratified model by menopausal status the inverse association was only seen in premenopausal women (OR = 0.25; 95% CI, 0.094–0.687). Dietary intake of vitamin D was inversely associated with risk of breast cancer (OR fourth quartile [Q4] vs first quartile [Q1] = 0.39; 95% CI, 0.196–0.784; p = 0.008). After adjusting for the confounding factors, this inverse association remained significant. Conclusion: Results from this case–control study support the protective effect of higher serum concentration of 25(OH)D against breast cancer. Moreover, dietary but not total intake of vitamin D was associated with decreased risk of breast cancer.