Women who develop breast cancer often have low vitamin D levels. Vitamin D appears to influence the development and progression of breast cancer in important ways, but the relationship is not well understood. Vitamin D inhibits the expression of breast cancer-related gene activity, including cell differentiation, cell growth, and angiogenesis (new blood vessel formation). Normal breast cells lose their ability to respond to Vitamin D's favorable effects during their transformation to a malignant state while increasing their ability to degrade vitamin D. Now a new Belgian study has reported that relatively low vitamin D levels at diagnosis are associated with larger tumor size and poorer prognosis than above-average levels.

Study included 1,800 women with early-stage breast cancer

The study was designed to investigate the associations between circulating vitamin D and tumor characteristics and breast cancer outcomes. The authors also examined the influence of genetic determinants of vitamin D insufficiency. The blood test for vitamin D measures the level of 25-hydroxyvitamin D (abbreviated 25OHD, 25(OH)D, 25-OH-D or 25-OHD), which is the primary metabolic product of vitamin D3 in the blood. The study included 1,800 women with early-stage breast cancer, from whom blood was collected at diagnosis.

Lower vitamin D levels linked to larger tumor size but not positive lymph nodes

Lower circulating vitamin D levels were found to be correlated with larger tumor size at diagnosis, but not with positive lymph nodes, estrogen or progesterone receptor status, or tumor grade. Genetic variants in how the women processed vitamin D influenced their vitamin D levels but not the association between vitamin D and tumor size. Women with 25(OH)D levels over 30 ng/mL at diagnosis had better overall survival and breast cancer-free survival than women with lower levels. In addition, levels over 30 ng/mL at diagnosis were found to have a modest favorable effect on disease-free interval (the time between diagnosis and the first recurrence, if any), which became apparent only after the first three years of follow up.

Link to vitamin D levels established only in postmenopausal women

When the analysis was conducted according to menopausal status at diagnosis, vitamin D levels were found to have a statistically significant influence on breast cancer-free survival and disease-free interval only in postmenopausal women. The authors conclude that vitamin D levels over 30 ng/mL at diagnosis correlate with smaller tumor size, better overall survival, and improved breast cancer-specific outcomes, especially in postmenopausal breast cancer patients.

Other studies have reported associations in premenopausal women

Other studies have reported a link between low levels of vitamin D and increased risk of breast cancer-specific death among premenopausal women. In fact, there is some evidence that low vitamin D is associated with more aggressive tumor profiles (ER- and triple negative disease) in younger women.

Chemotherapy and vitamin D

One study reported that the association between vitamin D and breast cancer survival was statistically significant only for levels measured before the start of chemotherapy. This is at least in part because levels of vitamin D normally drop during chemotherapy. Vitamin D supplementation during chemotherapy can raise circulating vitamin D levels, but it is not known whether this enhances or interferes with chemotherapy's treatment effects. Therefore, based on available evidence to date, it is not advisable to take vitamin D supplements during chemotherapy.

Please see our article on vitamin D before and after breast cancer diagnosis for more information vitamin D and breast cancer.