Vitamin D metabolites increase communication between cells by switching on a protein that blocks aggressive cell division,” said Garland. “As long as vitamin D receptors are present tumor growth is prevented and kept from expanding its blood supply. Vitamin D receptors are not lost until a tumor is very advanced. This is the reason for better survival in patients whose vitamin D blood levels are high.

Women in the high serum group had an average level of 30 nanograms per milliliter (ng/ml) of 25-hydroxyvitamin D in their blood. The low group averaged 17 ng/ml. The average level in patients with breast cancer in the United States is 17 ng/ml.

The study has implications for including vitamin D as an adjuvant to conventional breast cancer therapy,” said co-author Heather Hofflich, DO, UC San Diego associate professor in the Department of Medicine.

Garland recommended randomized controlled clinical trials to confirm the findings but suggested physicians consider adding vitamin D into a breast cancer patient’s standard care now and then closely monitor the patient.

There is no compelling reason to wait for further studies to incorporate vitamin D supplements into standard care regimens since a safe dose of vitamin D needed to achieve high serum levels above 30 nanograms per milliliter has already been established.