Women who have higher levels of vitamin D when they are diagnosed with breast cancer appear to have smaller tumors, according to a new study.

Researchers at the University of Leuven (Belgium) conducted a retrospective study involving 1,800 early breast cancer patients treated between 2003 and 2010. Data on serum 25-hydroxy vitamin D3 levels were collected at diagnosis for all patients, who were followed for a mean of four years. The results showed that that lower vitamin D levels were significantly associated with larger tumor size; each 0.4 ng/mL decrease in levels was linked with a 1 cm increase in tumor size.

There were no associations, however, between vitamin D levels and other characteristics such as lymph node invasion, ER or HER2 status, or tumor grade. Patients who were clearly vitamin D deficient--a level below 30 ng/mL at diagnosis--also presented a significant increase in the risk of relapse after three years, while those who were not deficient had a lower risk of relapse at three and six years. The study was presented as a poster session at the San Antonio Breast Cancer Symposium (SABCS), held during December 2011 in San Antonio (TX, USA).

“Lower vitamin D levels were correlated with much bigger tumors. We also saw that higher vitamin D levels were associated with better outcomes, but it wasn't significant,” said lead author and study presenter Barbara Brouwers, PhD. “Anti-inflammatory effects of vitamin D are the likely mediator of the relationship between higher levels and smaller tumor size, though more research is needed to clarify the exact mechanisms.”

Vitamin D is a group of fat-soluble secosteroids; in humans, vitamin D is unique because it functions as a prohormone and because the body can synthesize it (as vitamin D3) when sun exposure is adequate. Vitamin D prevents rickets in children and osteomalacia in adults, and, together with calcium, helps to protect older adults from osteoporosis. Vitamin D also affects neuromuscular function, inflammation, and influences the action of many genes that regulate the proliferation, differentiation, and apoptosis of cells.

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