High levels are linked to...

Greater resistance to viruses
During a recent study, researchers at the Yale University School of Medicine discovered that people with high levels of vitamin D got sick about half as often as people with low levels. And when they did fall ill, they recovered in fewer days. The reason: Vitamin D instructs your white blood cells to manufacture a protein that kills infections.

Less cancer
Specifically, a 30 to 50 percent lower chance of breast cancer, and a 50 percent lower chance of colon cancer. D regulates some of the genes responsible for cellular growth and survival, says Holick, and it does its job cleverly: "It helps shut down any out-of-control growth to prevent malignancy. If that doesn't work, it will help kill the cell. And if a tumor grows anyway, it will work to cut off blood supply."

Higher cancer survival rate
At the Dana-Farber Cancer Institute, researchers found that colon cancer patients with high levels of D had a 39 percent lower chance of dying from the disease. And this might actually apply to all cancers, says Edward Giovannucci, MD, ScD, professor of nutrition and epidemiology at the Harvard School of Public Health.

Reduced risk of Parkinson's
Researchers believe the correlation, which Archives of Neurology reported in July, may have to do with D's protective effect on the brain: It regulates calcium levels, enhances the conduction of electricity through neurons, and detoxifies cells, among other handy functions.

Low levels are linked to...