The Centers for Disease Control and Prevention (CDC) report that in the United States, around 220,000 women are diagnosed with breast cancer every year. In 2013, 230,815 women were diagnosed with breast cancer, and 40,860 died of the disease.

Researchers have previously suggested that vitamin D might be beneficial in treating cancer.

Some studies have suggested that the active metabolite of vitamin D - known as calcitriol - may be anticancerous. Administering calcitriol in mice has been shown to stop the proliferation and growth of cancer cells, reduce tumor blood vessel formation, and stimulate cell death.

Although there are not yet enough clinical trials in humans to prove this, some clinical and preclinical studies indicate that avoiding vitamin D deficiency and taking vitamin D supplements might be a cost-effective and safe way to prevent cancer and improve prognosis in patients already diagnosed with the illness.

Associating vitamin D with breast cancer survival rates

A new study led by Dr. Song Yao, from the Roswell Park Cancer Institute in Buffalo in New York, analyzed data from 1,666 women diagnosed with breast cancer.

The patients had participated in the Pathways Study, a cohort study of breast cancer survivors established in 2006 at Kaiser Permanente Northern California.

Researchers looked at levels of the vitamin D biomarker 25-hydroxyvitamin D (25OHD) at the time of the diagnosis, and associated them with survival prognosis.

As part of the Pathways Study follow-up, patients were checked for comorbidities and overall health at regular intervals: 12, 24, 48, 72, and 96 months.

The researchers analyzed the data for over a year, from January 5, 2014, to March 15, 2015, and the results were published in the journal JAMA Oncology.

The average age of the patients monitored was 58.7 years. Overall, half of the patients were vitamin D deficient, and over a third had insufficient levels of vitamin D.

Researchers found a lower level of the vitamin D biomarker in women with advanced-stage tumors. The lowest levels were found in premenopausal women with triple-negative cancer.

Triple-negative breast cancer is a type of cancer that tests negative for estrogen receptors, progesterone receptors, and HER2 protein.

Levels of 25OHD were also inversely proportional to disease progression and death rates.

Lower levels of 25OHD were associated with higher tumor stage and grade, and women with the highest levels of 25OHD had higher survival rates overall.

Levels of 25OHD also increased proportionally with vitamin D dietary and supplemental intake.

"Our findings provide compelling observational evidence for inverse associations between vitamin D levels and risk of breast cancer progression and death," the authors conclude.
Further research needed to establish causality

The authors caution against the possibility that vitamin D associations are indicative of a more general aspect of one's health, rather than being specific to breast cancer patients.

They also warn that 25OHD concentrations vary with environmental and physiological changes. However, authors have adjusted their study to account for these changes, as well as for chemotherapy and other forms of treatment that might have affected vitamin D levels.

Dr. Yao and team note that although their study does not indicate causality per se, their findings seem to confirm previous literature that had suggested a causal relationship between 25OHD levels and cancer survival.

However, to establish causality with a higher degree of certainty, researchers would need to implement randomized clinical trials, administering vitamin D supplements and placebo medication to cancer patients.

Such clinical trials may not be entirely feasible, the authors warn, due to the public recognition of vitamin D benefits for cancer patients as well as the vitamin D deficiency or insufficiency found in women with breast cancer. A feasibility study revealed that 84.4 percent of recently diagnosed breast cancer patients had already started taking vitamin D supplements.