How much vitamin D do I need?

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.

Optimal vitamin D levels depend on many factors, including age, body weight, skin color, air quality and latitude.

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.

The American Academy of Pediatrics recently doubled the amount of recommended daily vitamin D for children to 400 IUs per day, but some say the new guideline is still too low. The Vitamin D Council, for example, suggests giving children 2 years old and younger 1,000 IUs per day if they have little exposure to the sun's ultraviolet B rays.

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."

The most common way to have your vitamin D level tested is to see a doctor. But a new $65 at-home test is available through the Vitamin D Council, which has partnered with ZRT Laboratory. The test measures 25-hydroxyvitamin D or 25 (OH) D. Stick a finger or heel to get a few drops of blood and mail the kit back. The results will be mailed to you.

Plotnikoff urges going through a qualified doctor. "I've always thought that self-diagnosis and self-treatment put one at risk for self-malpractice," he said.

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