How Vitamin D May Fight Breast Cancer

As early as the 1940s, scientists observed that populations with greater sun exposure had higher rates of skin cancer, but lower rates of other cancers, including breast cancer. Since then, it has become clear that vitamin D (produced by the body in response to sunlight) is largely responsible for these variations, and there is strong evidence that maintaining sufficient vitamin D levels is crucial for cancer prevention.

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The Link Between Vitamin D and Cancer

Among women with breast cancer, about 75 percent are deficient in vitamin D. A 2018 review found that women who have adequate blood vitamin D levels are less likely to be diagnosed with breast cancer; 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.

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 early onset of menstruation and excess body fat, is therefore associated with a greater risk of breast cancer.

Research has indicated that vitamin D supplements may counteract the cancer-causing effects of estrogen.

A 2015 study of over 57,000 postmenopausal women found that women who took vitamin D supplements had a 26 percent reduction in breast cancer risk. What made this finding particularly notable was that it occurred specifically in women who had also used hormone replacement therapy, which contains estrogen. (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.)

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.

How to Get Enough Vitamin D

The current recommended daily allowance of vitamin D for people ages 1 to 70 is 600 international units (IU). This includes pregnant women. Adults over the age of 70 should get 800 IU.

Oily fish such as salmon, sardines, and oysters are good sources of vitamin D. Supplements are another way to boost your intake.

Vitamin D is also produced in the body and stored in its inactive form, predominately in the skin. Exposure to the UVB light in the sun's rays transforms the vitamin to its active form.

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. Using sunscreen typically doesn't prevent the sun's effect on vitamin D synthesis, but it's best to limit time in the sun under any circumstances due to the risk of skin cancer.

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

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