Scientists have found that prostate cancer cells don't have uncontrolled growth when treated with vitamin D, Vieth says. And there's a good reason for that. Vitamin D acts like a control switch, telling cells how they are to function. Skin cells, which constantly replace themselves, normally spread out in a smooth, flat sheet. "What keeps them from cauliflowering out?" Heaney asks. "Turns out it's vitamin D. Colon, breast and prostate cancer may be lying at the feet of vitamin D insufficiency." That's not to say that if you're short on vitamin D, you will develop one of those cancers. "Lots of things have to go 'right' for cancer," Heaney says. "But a low level of vitamin D could mean that you've lost one of your lines of defence."

Researchers also have looked for a relationship between vitamin I) and autoimmune disorders, particularly multiple sclerosis and rheumatoid arthritis. Autoimmune disorders happen when the body's immune system turns against an organ or a system of the body, rather than on a foreign invader like a virus. Vitamin D is known to depress the body's immune system. It's well-established that MS rarely occurs in people living near the equator and becomes increasingly common in people at higher latitudes with less sunshine. MS also tends to occur less frequently at higher altitudes, where the sun shines more potently. A recently published study found that women getting at least 400 IUs of vitamin D daily from food and supplements were 40 per cent less likely than others to develop MS. A team tracking rheumatoid arthritis and vitamin D intake among a group of older women found that women who got less than 200 IDs a day were 33 per cent more likely to develop rheumatoid arthritis than women who consumed more. All of which raises the question: how much vitamin D is enough? The Women's Health Initiative, a large clinical test of the role of hormone therapy and other factors in women's health, expects to add information to the picture next year. A group of women has been receiving 1,000 milligrams of calcium and 400 IUs of vitamin D daily. They're being monitored for bone fractures and colon cancer.

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