Is there evidence that vitamin D can help reduce breast cancer risk?
Epidemiologic studies of the association between vitamin D and breast cancer risk have had conflicting results. Although several studies have suggested an inverse association between vitamin D intake and the risk of breast cancer, others have shown no association or even a positive association (that is, individuals with higher intakes had higher risks). A meta-analysis of six studies that investigated the relationship between vitamin D intake and breast cancer risk found no association (19). However, most women in these studies had relatively low vitamin D intakes, and, when the analysis was restricted to women with the highest vitamin D intakes (>10 μg, or 400 IU, per day), their breast cancer risks were lower than those of women with the lowest intakes (typically <1.25 μg, or 50 IU, per day) (19). In the Women’s Health Initiative, calcium plus vitamin D supplementation for an average of 7 years did not reduce the incidence of invasive breast cancer compared with placebo (20). The ass