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The Sunshine Vitamin

If you’ve seen the many recent headlines claiming that "sunlight may prevent cancer," you probably wonder if the world isn’t going crazy. Sun exposure causes skin cancer, so how can this be? There is indeed some research showing that a small amount of sun exposure may reduce the risk of certain cancers, as well as help keep bones strong. But you don’t need to spend lots of time in the sun—and expose yourself to all the known dangers of the sun—to get the potential benefits.

Vitamin D: not just for strong bones

Researchers have found that mortality rates for some cancers—notably breast, colon, ovarian, and prostate—tend to be lower in sunnier regions. And some studies have found that people who get little or no sun exposure tend to have higher rates of breast and colon cancer. Why? Since sunlight’s ultraviolet-B radiation is responsible for producing vitamin D in the body, researchers have wondered if this could be the connection. There is some laboratory evidence that vitamin D helps inhibit the growth of some types of cancer cells and their spread. One study found that women who have the lowest blood levels of this vitamin have a higher risk of breast cancer than women with the highest levels.

Unique among vitamins, vitamin D is a hormone. It helps the body utilize calcium and builds bones and teeth. You don’t actually need to consume vitamin D, provided you get a minimal amount of sunlight. All it takes is about 10 to 15 minutes of sunlight on your hands, face, and/or arms several times a week, depending on your location, the time of year, the darkness of your skin, and your age (see below). Most of us get that much sun without even trying. And because D is a fat-soluble vitamin, most people can store enough to supply them in the days, or even months, when they don’t get any sun.

People who live in the northern third of the U.S. (for instance, Boston, Detroit, or Chicago) can manufacture sufficient vitamin D from a minimal amount of sun exposure from April through October. Those in the center band (Washington, D.C., Kansas City, or San Francisco) can manufacture the vitamin from March through November. Farther south (Los Angeles, Dallas, or Atlanta), vitamin D can be produced year round. Canadians have six months or more of shorter, darker days when there isn’t enough sunlight to manufacture D.

If you have dark skin, especially if you are African-American, you may need longer exposure to sunlight—perhaps up to twice as much as a light-skinned person—to produce the same amount of vitamin D, since skin pigmentation screens sunlight and reduces vitamin D production.

The best D-fense for those over 60

As you grow older, your ability to manufacture vitamin D declines, and just increasing your sun exposure may not do the trick. By the time you are 70, your vitamin D production is only 30% of what it was when you were 25. That’s why the Recommended Dietary Allowance for D is higher for older people: while those under 50 need only 200 IU (international units) daily, those 50 to 70 should get 400 IU, and those over 70 need at least 600 IU.

It’s a good idea to drink nonfat or low-fat milk for many reasons, among them that milk is fortified with vitamin D. Each cup contains 100 IU. Other foods containing vitamin D are fatty fish such as salmon and sardines, egg yolks, and fortified breakfast cereals. (Yogurt and cheese are not made from fortified milk.)

Everyone over 70 should take supplemental vitamin D. So should people in their sixties who don’t get adequate amounts of vitamin D from milk and other foods and also seldom get sun. Those at highest risk are the homebound or institutionalized, as well as those living in the northern third of the U.S. and in Canada. Vegans and others who don’t drink milk may also need a supplement if they live up north.

Supplements are tricky, however, because overdoses of D can be toxic, leading to kidney stones, kidney failure, muscle and bone weakness, and other problems. Danger starts at 2,000 IU a day. It’s nearly impossible to get too much D from food. A daily multivitamin with 400 IU of vitamin D is usually the best solution for those over 60. Some calcium supplements also contain D.

Sunscreen note: Sunscreen can reduce or even shut down the synthesis of vitamin D if you coat all exposed skin. This is a problem chiefly for older people, who produce less D.

UC Berkeley Wellness Letter, January 2003