A variety of studies have found that low vitamin D levels are associated with increased risk of breast cancer. Early studies did not always find an association, in part because the levels of vitamin D (as measured by 25(OH)D) defined as sufficient were too low. In fact, cancer study results have led researchers to realize that vitamin D levels adequate for bone health might still leave vulnerable individuals at higher than average risk of breast cancer. The optimal 25(OH)D level for breast cancer prevention appears to be at least 45 ng/mL. In fact, 45 to 60 ng/mL might be an appropriate target range.

Vitamin D deficiency associated with more aggressive breast cancer

While not all studies are in agreement, a lack of vitamin D appears to increase the aggressiveness of breast cancer that develops. Variations in vitamin D-related genes also appear to leave some women more susceptible to the cancer-promoting effects of not enough vitamin D. One recent study suggested that genetic variants in the vitamin D pathway may be related to the higher prevalence of estrogen receptor negative (ER-) breast cancer among African-American women.

A recent U.S. case-control study reported that women with breast cancer had lower vitamin D levels (32.7 ng/mL) than cancer-free women controls (37.4 ng/mL). Breast cancer patients with levels under 32 ng/mL had more than three times the risk of having triple negative cancer as those with higher concentrations.

Low vitamin D levels are also associated with reduced survival after a diagnosis of breast cancer. However, chemotherapy reduces vitamin D, which might mean that supplementation with vitamin D during chemotherapy could interfere with treatment. There is also some preliminary evidence that vitamin D supplementation could reduce the effectiveness of aromatase inhibitors in reducing circulating estrogen.

Latest research reports low vitamin D increases breast cancer risk in Chinese

The case-control study referenced at the beginning of this news article was designed to investigate the influence of vitamin D levels on risk of breast cancer. The study included 593 women with breast cancer and 580 cancer-free controls from the Shanghai area. Vitamin D levels were defined either as severely deficient (less than 20ng/mL), deficient (20 to 30ng/mL), or sufficient (> 30ng/mL). Few of the women in either group had sufficient levels of vitamin D.

A total of 80% of the cancer-free women were found to be severely deficient in vitamin D, 15% were deficient, and 5% had sufficient levels. Among the breast cancer patients, 96% were severely deficient, 3% were deficient and only 1% had sufficient levels. In order to conduct additional analysis, the women were divided into five groups based on quintiles of circulating vitamin D. A significant dose-response was found, with women in the highest quintile of vitamin D having the lowest risk of breast cancer and those in the lowest quintile having the highest risk. The authors conclude that the study results provide strong evidence that vitamin D might have a chemopreventive effect against breast cancer.

Please see our article on vitamin D and breast cancer for a detailed discussion of vitamin D and breast cancer risk, treatment and prognosis.