D Is for Defense: Why Vitamin D Is the Ultimate Super Vitamin

How to Get More Vitamin D

To find out if you're low in this super supplement, ask your doctor for a simple vitamin D blood test, called 25-Hydroxyvitamin D. (It costs $50 to $200 and is typically covered by insurance.) Current guidelines say you're deficient if your level is below 20 nanograms per milliliter, the amount needed to prevent rickets, a bone disorder. "But if you bring blood levels above 30, your body absorbs more than twice as much calcium," Dr. Holick explains. This is the point at which vitamin D, finished with its bone-strengthening duties, is free to work its magic on the rest of the body. James Dowd, MD, an associate clinical professor of medicine at Michigan State University and author of The Vitamin D Cure, goes even further: "I consider 45 to 65 to be the sweet spot to maintain health and prevent disease," he says.

Because most of us can't get enough D from the sun, experts recommend taking a supplement daily. But don't think your multivitamin or a calcium-vitamin D combo has you covered. These typically supply the RDA of 200 to 400 international units (IU), which the latest research indicates is far too low. So how much is enough? Studies indicate that 1,700 IU of D a day nudges blood levels to 32. A growing number of experts even recommend at least 2,000 IU a day -- which, according to the Institute of Medicine, is the highest amount that's deemed safe to take.

Billie Jo Coomer used sunshine and daily supplements to boost her level of vitamin D from 11 to 52 nanograms per milliliter, and the results have been dramatic. "My life has totally turned around," Billie Jo says. "I have the energy to get together with friends again, and I can climb the three flights of stairs to my apartment without feeling out of breath." She adds, "The difference vitamin D has made is astonishing. I recommend it to everybody."