You may already know that vitamin D can help build strong teeth and bones, but wait until you hear what else it can do for the rest of your body. D can keep you trim, boost your mood, ward off sniffles, drastically cut your risk of cancer, and more.

“We could prevent 150,000 cases of cancer annually if we could just increase vitamin D to optimal levels,” says Cedric Garland, a doctor of public health, a leading vitamin D researcher, and a professor of family and preventive medicine at the University of California, San Diego.

That’s great news, right? Yes, except for one huge problem: A startling report found that more than a third of all women fail to get enough D for healthy bones—and more than 75% of us lack the higher amounts needed for the vitamin to do its disease-fighting best.

Downing a daily glass of milk is a smart way to get more D. But the most significant source is sunlight, and that’s where the trouble lies. Our bodies produce D with exposure to ultraviolet radiation, but as we’ve gotten smarter about dodging skin cancer—staying out of the sun and slathering ourselves with mega-SPF sunblock—our vitamin D levels have plummeted. Fortunately, there are smart and safe ways to boost your intake while you enjoy all the benefits that vitamin D can deliver.

Lower your risk of cancer
Vitamin D may substantially cut the risk of breast, colon, prostate, and ovarian cancers, according to a growing body of research. In fact, Dr. Garland found that women with D blood levels that were more than double the current national average of 25 nanograms per milliliter (ng/ml) had a 50% lower risk of breast cancer than those with the lowest blood levels. Scientists believe that D helps regulate genes in a way that protects healthy cells and stops the growth of cancerous ones.

There are receptors for vitamin D in virtually all of the body’s cells, and to “feed” them you need an adequate blood level of the vitamin. That depends not only on how much time you spend outside and what you eat but also on where you live. People living at higher latitudes, for example, soak up fewer UVB rays from November through March, which means they’re more likely to have low blood levels of vitamin D and a higher risk of cancer.

In fact, studies have shown twice as many colon cancer deaths and 50% more breast cancer deaths in the far North compared with the sunnier South, Dr. Garland says. So how much sun is enough to lower the risk of cancer without upping your risk of skin damage?

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