And the Vitamin D Council, partly sponsored by supplement makers, recommends healthy adults with little sun exposure take 5,000 IUs of D daily — 1,000 IUs more than what the Institute of Medicine (IOM) deemed the safe upper limit of 4,000 IUs.

The IOM cautionary message that more vitamin D is not necessarily better has largely fallen on deaf ears, says JoAnn Manson, M.D., a professor of medicine at Harvard Medical School who helped draft the guidelines. "Very few people have been affected by that because there's so much marketing and promotion of vitamin D," she says.

She doesn't expect the promotional din to die down before the 2016 completion of a National Institutes of Health study of the vitamin. Led by Manson, it follows 20,000 people to test whether taking 2,000 units of D daily lowers the risk of cancer and cardiovascular disease. Until the results are in, she says, the public can be misled. "I think the enthusiasm for vitamin D supplements has outpaced the evidence."

Should you supplement?

So in the meantime, should you or your family members take a supplement? Be sure to speak with your doctor before buying any supplements. But one key factor to consider is skin color. Most African Americans should at least consider a supplement, says Susan Harris, a vitamin D expert at Tufts University. According to the CDC report, a majority of African Americans had insufficient blood levels of vitamin D, and nearly one-third had a frank deficiency, compared with 12 percent of Mexican Americans and 3 percent of whites. (The information the CDC collected for this report included only Mexican Americans, not American Hispanics in general.) Dark skin pigmentation serves as a natural sunscreen, she explains, blocking the ultraviolet rays that trigger vitamin D synthesis.

Some older people also should consider supplementing. In May the U.S. Preventive Service Task Force endorsed vitamin D supplements of 800 IUs a day for people who are at risk of falls.

You also can assume you're low on vitamin D if you live in the northern part of the country or spend most of your time indoors. Says Harris, "In Boston, I tell pretty much everyone that they should be taking a supplement during the winter."

Also of interest: Supplements can interact with prescription meds.