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Breast cancer Patient friendly summary

 Vitamin D has been shown to block breast cancer growth.

Breast cancer is common among women. In the United States, the disease affects about 230,000 females each year compared to 2,000 men. Approximately 20% of those diagnosed with breast cancer die from the disease. Breast cancer rates are much higher in Western countries than in developing countries.

Risk factors

Of the many risk factors associated with breast cancer, the most important include:

  • Lifetime estrogen exposure — This includes estrogen produced in the body and taken orally.
  • High intake of meat and dairy — Eating a diet high in animal products early in life causes the body to produce more estrogen during the course of a lifetime.
  • Alcohol consumption — Studies have shown a link between alcohol and breast cancer.
  • Reproductive factors — Generally, women who have fewer or no children have a higher tumor risk.
  • Night shift work — More electrical light exposure at night reduces melatonin production. This hormone may lower the risk of breast cancer.

Sunlight exposure and breast cancer risk

 Breast cancer is diagnosed less often in summer, when there is more sunlight. It is also diagnosed less often in winter, when there is less sunlight, but the body produces more melatonin.

Sunlight appears to have both direct and indirect effects on breast cancer. The ultraviolet portion of sunlight stimulates the body to produce vitamin D, which protects against breast cancer. Women who spend a moderate amount of time in the sun, especially during midday, may benefit.

Melatonin has been found to be associated with reduced risk of breast cancer. Melatonin is made by the pineal gland, which is connected to the eyes and is sensitive to bright blue light. At night, when there is no bright blue light, melatonin is produced. Melatonin is a hormone that induces sleep and may also reduce the risk of breast cancer. However, during the longer days of summer, the body produces less melatonin. Thus, melatonin seems to help reduce the risk of breast cancer in winter.

The number of breast cancer cases and rate of diagnoses vary with the amount of sunlight and the seasons. According to breast and other cancer studies, there are:

  • Lower rates in the sunny Southwest United States and higher rates in the darker Northeast
  • Higher rates in countries that are further from the equator and receive less sunlight

For breast cancer, there are:

  • Fewer diagnoses in the summer and winter
  • More diagnoses in the spring and fall

Vitamin D and breast cancer

Breast cancer was one of the first cancers identified as having protection from vitamin D. Now there is ample evidence that vitamin D lowers breast cancer risk.

Vitamin D levels

 Vitamin D blood levels greater than 40 ng/mL (100 nmol/L) reduce the risk of breast cancer by approximately 30%.

Based on observational studies of vitamin D levels at the time of breast cancer diagnosis or up to three years later, risk of breast cancer decreases rapidly as vitamin D levels increase from very low levels [less than 10 ng/ml (25 nmol/] out to 20-30 ng/ml, then decreases at a slower rate until levels about 50 ng/ml (150 nmol/l).

The rate of breast cancer appears to decrease by approximately 30% when vitamin D levels in the blood are greater than 40 ng/mL (100 nmol/L) compared to lower levels of 20 ng/mL (50 nmol/L).

How vitamin D works

Vitamin D has been shown to block the growth of breast cancer tumors.  Vitamin D's active form, calcitriol, provides numerous benefits against cancer. This form of vitamin D encourages cells to either adapt to their organ or commit apoptosis. Calcitriol also limits blood supply to the tumor and reduces the spread of cancer.


Most, but not all, studies indicate high levels of vitamin D are associated with a lower risk of breast cancer. The studies that compared incidence rates of breast cancer with vitamin D blood levels or oral intake of vitamin D determined within three years of diagnosis nearly always found lower risk with higher level or intake. Those with longer times between measurement and diagnosis did not. The reason they did not is probably that since breast cancer can grow rapidly, even a drop of vitamin D for a short period can permit breast cancer to grow to where it can be detected.

Vitamin D and calcium

Studies have shown that the combination of vitamin D and calcium provides moderate breast cancer protection for premenopausal women. Calcium intake from diet or supplements of more than 1000 mg/day might be helpful.


A study in Toronto found that women with more than 30 ng/ml (75 nmol/l) at time of breast cancer diagnosis had half the 12-year all-cause mortality rate of those with less than 10 ng/ml (25 nmol/l). Since vitamin D protects against many types of disease, this finding likely relates to deaths from breast cancer and other causes.

A study in Norway found that women diagnosed in summer had a better two-year survival rate than those diagnosed in winter. Vitamin D levels are higher in summer than winter, which may explain the findings.

There have not been any reported studies of treating women with breast cancer with vitamin D. However, some cancer treatment centers are now giving at least 5000 IU (125 mcg)/day vitamin D to patients with many types of cancer.

Need to know more? Read on with our detailed evidence summary for Breast Cancer.

Page last edited: 21 September 2011