A new review and meta-analysis of previously published studies has reported that higher circulating vitamin D levels reduce the risk of breast cancer and that a serum 25(OH)D level of 47 ng/ml is associated with a 50% lower risk of breast cancer. Vitamin D3 (cholecalciferol) is the most physiologically active form of vitamin D. The blood test for vitamin D measures the level of 25-hydroxyvitamin D (25(OH)D), which is the primary metabolic product of vitamin D3 in the serum. Low levels of 25-hydroxyvitamin D (25(OH)D) have been reported to be associated with heightened risk of breast cancer.

To conduct the study, the authors performed a search for all case-control studies concerning risk of breast cancer by 25(OH)D concentration. The search identified 11 eligible studies. Data from all of the studies was combined in order to calculate the risk of breast cancer associated with the highest compared to the lowest level of 25(OH)D across all studies. Based on all the studies combined, women with the highest levels of 25(OH)D had a 39% lower risk of breast cancer than women with the lowest levels. The authors conclude that the result supports the hypothesis that higher serum 25(OH)D levels reduce the risk of breast cancer. According to the review of observational studies, a serum 25(OH)D level of 47 ng/ml was associated with a 50% lower risk of breast cancer.

Note that there is some preliminary evidence that vitamin D supplementation could reduce the effectiveness of aromatase inhibitors. Please see our article on vitamin D and breast cancer for more information.