Higher vitamin D may help prevent breast cancer

(This article was first printed in the June 2006 issue of the Harvard Women’s Health Watch. For more information or to order, please go to http://www.health.harvard.edu/womens.)

We’ve known for a long time that vitamin D is crucial to bone health. Research is now showing that it’s active in many other tissues and may offer some protection against a range of diseases, including certain cancers. To get this benefit, though, we likely need more than the current recommended amounts.

At the American Association for Cancer Research meeting in April 2006, researchers offered compelling evidence that boosting vitamin D intake could help reduce the risk of breast cancer. In a study of 1,760 women, California scientists found that risk fell steadily with increasing blood levels of vitamin D. The highest levels (more than 52 nanograms per milliliter, or ng/mL) correlated with a 50% reduced risk of breast cancer, compared with the lowest amounts (less than 12 ng/mL). To reach a blood level of 52 ng/mL, you would need several times the recommended intake, which is 400 international units (IU) of vitamin D per day for women ages 50–70.

A second study, by Canadian researchers, found that women who spent more time outdoors or got more dietary vitamin D in their teens and early adulthood were 25%–45% less likely to develop breast cancer.

Although not yet peer-reviewed, the data are consistent with mounting evidence that increased vitamin D helps prevent many types of cancer, including breast, ovarian, colon, and prostate cancers.

Tough to get enough

Vitamin D is a hormone whose manufacture begins in the skin with exposure to the sun’s ultraviolet B rays. With enough sun, we wouldn’t need dietary vitamin D. But dependence on the sun is a problem. Skin cancer is one worry. Sunscreens help with that concern, but they also block the rays that spur vitamin D synthesis in the skin. Moreover, people who live above 40 degrees north latitude — in Boston, for example — can’t make enough vitamin D from sunlight in the winter.

Other factors influence the amount of vitamin D you can make from sunlight. The darker your skin, the more sun exposure it needs. And as we age, our skin becomes less capable of triggering vitamin D synthesis.

Natural food sources of vitamin D — primarily fatty fish such as salmon and mackerel — are limited. Fortified foods (milk and some breakfast cereals) supply modest amounts.

What to do

Nutrition experts have already begun to recommend that adults get 800–1,000 IU of vitamin D per day. A standard multivitamin usually supplies 400 IU; you can get an extra 400 IU in a vitamin D supplement. Calcium tablets often contain vitamin D, so include them in your calculations. Don’t overdo it; excessive vitamin D, which usually results from overdosing on supplements, can cause a buildup of calcium in tissues. The tolerable upper limit is still 2,000 IU per day.

The most active form of vitamin D is D3 (cholecalciferol), the type produced in the body. Most supplements contain D2, which is made from plant material. Some experts say that D3 is more effective. (It’s also more expensive and harder to find.)

Don’t take cod liver oil. It contains large amounts of vitamin A as retinol, which at high levels can be harmful to bones.

You can usually get adequate vitamin D from 10 to 15 minutes of sun a couple times a week, without sunscreen, on the face, arms, and hands. Many health experts see little harm in this level of exposure. For longer periods in the sun, use sunscreen.

(This article was first printed in the June 2006 issue of the Harvard Women’s Health Watch. For more information or to order, please go to http://www.health.harvard.edu/womens.)

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