

While the vitamin is best known for helping build strong bones and absorb calcium, a vitamin D deficiency can raise the risk of everything from immune disorders to colds and flu, according to recent research.

But testing for a vitamin D deficiency may raise more questions than it answers. The tests aren't standardized, so the results can differ from one lab to another. Earlier this year, Quest Diagnostics recalled several thousand test results because of concerns about their accuracy.

Meanwhile, even if the tests are accurate, there's debate about how much vitamin D a person needs and whether supplements are worthwhile.

The body makes vitamin D when the sun's ultraviolet rays strike the skin, something that doesn't happen often in Chicago and other Northern cities in the winter. Lesser amounts are found in some foods -- mainly fortified beverages -- and dietary supplements.

Knowing your vitamin D level can be useful, said Gregory Plotnikoff, medical director of the Institute for Health and Healing in Minneapolis. He recommends a baseline measurement for those with recurrent or chronic illness, or for people with mysterious medical symptoms.

"Vitamin D is the single most cost-effective medical treatment in the U.S. today," said Plotnikoff, who tells his patients to get 1,000 IUs of vitamin D a day, well above the government guidelines of 200 to 600 IUs per day. "My recommendation is to check levels, replenish and maintain."