Vitamin D builds and maintains strong bones; aids with calcium absorption; helps prevent osteoporosis; and helps regulate cells responsible for autoimmune functions. Adequate amounts are linked to improved heart health. In the body, vitamin D also gets converted to a steroid hormone capable of turning genes on or off, signaling them to make enzymes and proteins crucial to maintaining health and fighting disease.

How Much: Recommended dietary allowance (RDA) = 600 international units (IU) daily for adults age 70 and younger; 800 IU daily for adults 71 and older.

Some experts think higher doses are needed. Catherine Peterson, PhD, associate professor of nutrition at the University of Missouri, recommends 1,000 to 2,000 IU daily to maintain healthy levels and at least 4,000 IU to correct deficiencies – common in people with rheumatoid arthritis (RA). One meta-analysis out of Tufts University found taking between 800 IU and 2,000 IU daily helps prevent fractures; taking less than 800 IU does not. When buying a supplement, look for vitamin D3 (cholecalciferol) – the same form the body makes from sunlight. It is better absorbed and more effective than vitamin D2 (ergocalciferol), she says.

Too Much: Tolerable upper limit (UL) = 4,000 IU per day.  

Foods: The most vitamin D comes from vitamin D-fortified milk, yogurt, orange juice and breakfast cereals. It is also produced by the body when skin is exposed to sunlight. Fatty fish such as salmon, tuna and sardines have small amounts. 

Research Note: Many studies suggest vitamin D can prevent or improve autoimmune conditions, including RA. Studies also show that people taking corticosteroids, such as prednisone, are twice as likely as nonusers to have vitamin D deficiency. Researchers at National Jewish Health in Denver say people who have arthritis and are vitamin D deficient “may benefit from vitamin D supplementation.”