Benefits: Vitamin D enables your body to absorb calcium and maintain blood levels of calcium and phosphates for a strong skeleton. D may also boost immune function and help control inflammation.

Delivery: Your skin converts sunlight into D, but many Americans, especially those who have dark skin or live up North, don't get enough rays for D production. Oily fish such as salmon, sardines, catfish and tuna contain D, as do dairy foods and juices labeled “fortified with vitamin D.”

Dose: The recommended daily dose has been 400 IU, but some doctors think that's too low. Many people—especially those with conditions associated with D deficiency, including HIV—take 1,000 or more units a day.

Type: Pick a supplement with vitamin D3 (more effective than D2). To help your bones, choose one that includes calcium too.