What it is
Vitamin D is a fat-soluble vitamin that helps the digestive system absorb and regulate calcium and phosphorous. Naturally present in fatty fish (such as sardines and mackerel), egg yolk, lanolin, and liver, it's also produced by your skin as a result of direct exposure to UVB rays in sunlight. Vitamin D comes in two forms: D2 (ergocalciferol), often derived from yeast, and D3 (cholecalciferol), the type the skin makes as a result of sunlight exposure.
Strengthens bones and prevents osteoporosis. Recently, research has suggested it may prevent high blood pressure, as well as colon, prostate, and breast cancers. It also is highly effective in boosting immunity.
A 2008 study published in the Archives of Internal Medicine found that low levels of vitamin D could lead to a substantially increased risk of death. Researchers analyzed vitamin D levels in 13,331 healthy men and women over the course of seven years. Low levels of vitamin D were linked to an increased risk of mortality from all causes by 26 percent.
How to take it
The current recommended dietary allowance of vitamin D is 200-600 IU. However, the National Osteoporosis Foundation recommends that adults age 50 and older consume 800-1,000 IU per day for bone health. The Institute of Medicine is expected to release new, significantly higher daily requirements for vitamin D next year. Ask your health care practitioner if you should take supplements, especially if you spend little time in the sunshine, live in a cloudy climate, have dark skin, are lactose intolerant, over age 50, overweight, or a strict vegetarian.
Experts disagree on an upper limit of vitamin D, but the Institute of Medicine suggests that anyone over age 14 can safely consume up to 2,000 IU per day.