Vitamin D May Fight Breast Cancer by Blocking Estrogen

Women in the sun

As early as the 1940s, scientists had observed that populations with greater sun exposure had higher rates of skin cancer, but lower rates of other cancers. Over the years, studies found geographical variations in the rates of 24 different cancers, with lower rates of cancers in areas with greater typical sun exposure.

Today, it is clear that vitamin D is largely responsible for these variations, and we have strong evidence that maintaining sufficient vitamin D levels helps to prevent many different cancers—breast cancer included, plus cardiovascular disease, autoimmune conditions, and of course osteoporosis.

The Link Between Vitamin D and Breast Cancer

Among women with breast cancer, about 75 percent are deficient in vitamin D. Women who have adequate blood vitamin D levels are less likely to be diagnosed with breast cancer, and those who already have breast cancer are less likely to experience a recurrence of the disease if their vitamin D levels are sufficient. In 2014, a meta-analysis of 56 trials concluded that vitamin D3 supplementation was associated with a 12 percent reduction in the risk of death from any cancer.

Vitamin D may have a particularly protective effect against breast cancer. This possibility was raised when a study of over 57,000 postmenopausal women found that women who took vitamin D supplements had a 26 percent reduction in the risk of breast cancer. What made this finding particularly notable was that it occurred specifically in women who had also used hormone replacement therapy. The use of hormone replacement therapy in postmenopausal women dropped sharply after 2002, when a large study, called the Women’s Health Initiative, was stopped early because of increases in the risk of heart disease, stroke, and breast cancer in the group receiving hormone therapy.

The Link Between Vitamin D and Estrogen

Estrogen is known to stimulate growth and proliferation of breast cancer cells. Cumulative exposure to estrogen, due to factors such as earlier onset of menstruation and excess body fat, is associated with a greater risk of breast cancer. Vitamin D supplements may have counteracted the cancer-promoting effects of the hormones.

Other dietary factors that oppose the actions or production of estrogens are associated with reduced risk of breast cancer. A few examples: mushrooms contain aromatase inhibitors (aromatase is an enzyme that produces estrogen); soybeans and flax and chia seeds contain phytoestrogens which have anti-estrogenic effects; a high-fiber diet facilitates the excretion of estrogen. Vitamin D appears to have similar effects.

According to in vitro studies on breast cancer cells, the active form of vitamin D suppresses aromatase expression, resulting in lower production of estrogen. Vitamin D has also been shown to reduce the number of estrogen receptors on cultured breast cancer cells, making the cells less responsive to the hormone’s cancer-promoting signals.

Is Sunshine Enough?

Vitamin D synthesis in the skin varies between individuals. There is no specific duration of daily sun exposure that will work for everyone, and for many of us, a reasonable amount of sun exposure will not be enough.

In a study of Hawaii residents with abundant sunlight exposure—an average of 29 hours per week— around 50 percent still had vitamin D levels below 30 ng/ml. Plus, some of us live in higher latitudes that make Vitamin D adequacy difficult.

The best way to know for sure is to have a 25(OH)D blood test. We recommend using supplements to reach the sweet spot of 30-45 ng/ml. For many people, a moderate daily dose of supplemental vitamin D3 (approximately 1000-2000 IU/day) is appropriate to reach that 30-45 ng/ml window. 

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