Add brain health to the list of benefits of vitamin D. A new study found that low vitamin D levels can cause oxidation of the brain's proteins, which can lead to cognitive decline and may factor into Alzheimer's disease. "The bottom line is the brain can become damaged if it's exposed for a fair time to a low-vitamin-D diet," says study author D. Allan Butterfield, a professor of biological chemistry.

Given the vitamin's role in bone and muscle development, immunity, heart disease, and brain health, getting a regular blood test for D levels is important. Talk to your doctors about taking a supplement (especially if you have to contend with northern winters), eat D-rich foods like fatty fish, and get out in the sun daily.