About 12 percent of women in the United States will develop invasive breast cancer at some point in their life. Breast cancer is the second leading cause of death by cancer among women. Although breast cancer death rates have dropped in previous years, aggressive forms of this disease are still proving hard to treat.

Researchers are always looking for new ways to prevent breast cancer. In recent years they’ve focused on vitamin D’s ability to both prevent breast cancer and improve survival in women who get this disease. Here’s a look at the research on vitamin D and breast cancer prevention.

What Is Vitamin D?

Vitamin D is a vitamin found in foods like:

  • fatty fish, such as tuna, salmon, and sardines
  • eggs
  • fortified orange juice
  • milk
  • cereal

Your body also produces vitamin D when your skin is exposed to ultraviolet rays from the sun.

Vitamin D has a number of roles:

  • It helps the body absorb calcium and phosphorous
  • It works with calcium and phosphorous to promote strong bones and teeth.
  • It’s involved in cell growth, immunity, and reducing inflammation throughout the body.

In recent years, researchers have been investigating whether vitamin D might also help prevent or treat chronic diseases such as diabetes, high blood pressure, and cancer.

How Vitamin D Might Lower Breast Cancer Risk

Vitamin D entered the cancer spotlight when researchers noticed that people who live in southern regions, or areas with greater sun exposure, have a lower incidence of certain cancers. People living in these regions have also been found to have a lower death rate from these cancers. Breast cancer is included among these.

Researchers have theorized that the extra vitamin D these people absorbed from sunlight might have played a role in preventing breast cancer and other cancers.

Vitamin D has several actions in the body that could lower cancer risk. These include:

  • slowing the growth of cancer cells
  • stimulating cancer cell death, or apoptosis
  • reducing the formation of blood vessels that feed cancerous tumors

What the Research Says About Breast Cancer and Vitamin D

Whether eating foods rich in vitamin D or taking supplements will lower your risk of getting breast cancer is still unclear. Research on the subject is mixed. A 2013 study in PLoS One found no real evidence that postmenopausal women who take vitamin D supplements have a lower risk of breast cancer.

Another study published the same year in Nutrition and Cancer similarly found no link between vitamin D in the diet and breast cancer risk. A 2011 study in PLoS One found that women with higher vitamin D levels in their blood were 6 percent less likely to get breast cancer than women with lower blood levels of this nutrient.

There’s some evidence that vitamin D might improve survival in women who already have breast cancer. In research from the University of Rochester, women with lower vitamin D levels had more aggressive breast cancer and faced a poorer outcome as a result. A study in the journal Anticancer Research found women with breast cancer who had higher levels of vitamin D in their blood had a lower death rate than women with lower levels of this nutrient.

Can Vitamin D Lower Your Cancer Risk?

The research done so far doesn’t prove that eating foods high in vitamin D or taking supplements will lower your risk for breast cancer. Doctors can’t say for sure how much vitamin D you need to take, and for how long, to lower your risk. Future research should help clarify this nutrient’s role in cancer prevention and identify what dose is most effective.

While it’s too early to recommend extra vitamin D for preventing breast cancer, you should get the daily recommended allowance of at least 400 to 800 international units through diet or supplements. It’s estimated that 42 percent of Americans don’t get enough of this vitamin.

What Else Can You Do to Lower Your Breast Cancer Risk?

Taking vitamin D daily may or may not lower your breast cancer risk. Other lifestyle strategies have been shown to reduce your odds:

Keep Your Weight at a Healthy Number

Fat tissue produces estrogen, a hormone that fuels breast cancer growth. Where the fat is centered on your body is also important. Excess fat around your middle may increase your breast cancer risk more than fat in your legs and other parts of your body.

Stay Active

Not only does exercise keep your weight in check, but it also lowers breast cancer risk on its own. In the Women’s Health Initiative Study, women who walked for 1.25 to 2.5 hours a week decreased their risk of breast cancer by 18 percent compared to inactive women.

Limit or Avoid Alcohol

Two or more glasses of wine or beer daily can make you more likely to develop breast cancer. This risk increases the more you drink.

Avoid Hormone Therapy

Combined hormone therapy with estrogen and progesterone to treat menopause symptoms might make women more likely to get breast cancer and to die from it. Birth control pills also slightly increase the risk. This risk diminishes once you go off the pill.

Speaking with Your Doctor

If you’d like to take vitamin D supplements to lower your breast cancer risk or for any other reason, talk to your doctor first. Find out how much of this supplement is safe for you to take each day, especially if you’re currently being treated for breast cancer.