Vitamin D appears to influence the development and progression of breast cancer in important ways. Women with the highest levels of vitamin D intake have been reported to have reduced risks of developing breast cancer in numerous population studies. On the other hand, women with deficient vitamin D levels have been shown to have significantly increased risks of distant recurrence and death compared with those with sufficient levels. Now a new study has reported that women with low levels of vitamin D at diagnosis tend to have more aggressive breast cancer than women with adequate levels. Such women are also have a poorer prognosis even after taking into account breast cancer type and other tumor-specific prognostic indicators.

Vitamin D and breast cancer

Vitamin D inhibits the expression of breast cancer-related gene activity, including cell differentiation, cell growth, and angiogenesis (new blood vessel formation). Vitamin D deficiency has been shown to influence genes in triple negative (ER-/PR-/HER2-) breast cancer cells in ways that promote invasion and metastasis. Vitamin D deficiency has been shown to increase the growth of bone metastases. Normal breast cells lose their ability to respond to Vitamin D's favorable effects during their transformation to a malignant state.

The blood test for vitamin D measures the level of 25-hydroxyvitamin D (25(OH)D), which is the primary metabolic product of vitamin D3 found in the blood. Early studies did not always find an association between vitamin D levels and breast cancer risk, in part because the levels of vitamin D defined as sufficient were too low. Vitamin D levels are typically defined either as severely deficient (< 20 ng/mL), deficient (20 to 30 ng/mL), or sufficient (> 30 ng/mL). However, breast 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 vitamin D level for breast cancer prevention appears to be at least 40 ng/mL (40 to 50 ng/mL may be an appropriate target range). However, taking mega doses of vitamin D to achieve extra-high levels of vitamin D carries its own risks of adverse effects and is not recommended.

Best sources of vitamin D

Vitamin D3 is made in the skin when skin is exposed to sunlight (when the UV index is at least 3). In addition to exposing the skin to sunlight, the following foods and supplements are sources of vitamin D, while also having been found to protect against breast cancer:

Arctic char, wild
Lake trout

Salmon, wild
Vitamin D3 supplements

Both smoked fish and pan or deep frying of fish should be avoided since these preparation methods introduce significant levels of carcinogenic polycyclic aromatic hydrocarbons (PAHs) or heterocyclic amines (HCAs). Although some vitamin D supplements contain Vitamin D2 (ergocalciferol), it makes more sense to take Vitamin D3 since Vitamin D2 is not produced by vertebrates.

Latest research finds low vitamin D levels are associated with poor prognosis

The study referenced at the beginning of this news story was designed to investigate the association between vitamin D (25(OH)D)) level at diagnosis and breast cancer prognosis. The study included participants in the Pathways Study, a prospective study of breast cancer survivors which was established by Kaiser Permanente Northern California and enrolled breast cancer survivors between 2006 and 2013. The enrollment rate was 46% (4,505 of 9,820). Participants generally were enrolled within two months of diagnosis and continue to be followed. To conduct the study, the authors selected 1,666 Pathways Study participants to ensure representation of various ethnic backgrounds and breast cancer subtypes. This group of women, whose average age was 58.7, was followed for breast cancer recurrence, second primary cancer, and death.

Vitamin D levels at diagnosis were found to be lowest in premenopausal women with triple negative breast cancer. Vitamin D levels were also low in women with advanced-stage tumors at diagnosis. Generally speaking, the lower the vitamin D Level at diagnosis, the higher the likelihood of breast cancer progression and death. Compared with women in the lowest third of circulating vitamin D levels, women in the highest third experienced superior overall survival. This finding held after breast cancer type and other prognostic factors were taken into account. The association with overall survival was strongest among premenopausal women. The authors conclude that vitamin D (serum 25(OH)D) levels at diagnosis are independently associated with breast cancer characteristics and patient prognosis, especially among premenopausal women.

Please see our article on vitamin D for more information.