SAN ANTONIO – A new meta-analysis challenges the notion that breast cancer risk is inversely related to serum vitamin D level.

The meta-analysis included 16 published studies deemed by investigators to have made the grade in terms of scientific rigor. In 10 studies, vitamin D was measured before diagnosis of breast cancer, while in the other 6 studies, blood samples were gathered for measurement of vitamin D only after the diagnosis.

When data from all 16 studies were pooled, lower vitamin D levels were associated with a significant 1.5-fold increased rate of breast cancer (P less than .001). So far so good for the theory that low vitamin D might be causally related to breast cancer, and by extension for the corollary that vitamin D supplementation might be an effective option for breast cancer prevention.

However, a major difficulty with this line of thinking arose when the two groups of studies were analyzed separately, Dr. Eitan Amir explained at the San Antonio Breast Cancer Symposium.

Indeed, only 1 of the 10 studies in which vitamin D was measured before diagnosis of breast cancer showed a significant relationship between low levels of vitamin D and subsequent increased likelihood of the malignancy.

In contrast, all six studies in which serum vitamin D was measured following diagnosis of breast cancer showed a significant inverse relationship. In the pooled analysis of these six studies, lower serum vitamin D was associated with a highly significant, 2.63-fold increased likelihood of breast cancer (P less than .001), according to Dr. Amir of the University of Toronto.

Breast cancer cells have been shown to express vitamin D catalytic enzymes that may interfere with accurate measurement of serum levels of the vitamin, added Dr. Amir.

Dr. Amir declared having no relevant financial interests.