Vitamin D is found in only a few foods, and both men and women in the United States have a difficult time meeting their daily needs without supplementation, according to the Office of Dietary Supplements. As a supplement, vitamin D is available in two forms: vitamin D-2 and vitamin D-3. Vitamin D-2, however, may not be as effective as D-3. Consult your doctor if concerned about your vitamin D intake and need for supplementation.
About Vitamin D
The fat-soluble vitamin performs a number of different functions in the body. In addition to helping you absorb calcium, vitamin D helps keep bones healthy and strong by assisting in their growth and maintenance. Vitamin D also helps you fight off infection by supporting immune health, and it helps reduce inflammation.
Even though your body can make its own vitamin D through sun exposure, the Institute of Medicine has established recommended dietary allowances, which for adults range from 600 to 800 international units a day.
Active Vitamin D
Whether from food, the sun or supplements, the vitamin D is initially in an inactive form and must undergo a two-step process to become active and usable by the body. First, the vitamin D is converted to calcidiol in the liver, chemically referred to as 25-hydroxyvitamin D. Then the calcidiol is converted to calcitriol, or 1,25-dihydroxyvitamin D, in the kidneys, the metabolically active form of vitamin D.
Plant-Made Vitamin D-2
Vitamin D-2, also referred to as ergocalciferol, is a form of vitamin D made by plants through photosynthesis when exposed to ultraviolet light. This is the type of vitamin D you get from UV-exposed mushrooms. It's also sometimes used to fortify foods such as milk.
The Office of Dietary Supplements reports that vitamin D-2 is an acceptable supplement because it helps prevent rickets. When given at high doses, vitamin D-2 may not be as effective as vitamin D-3.
Vitamin D-3, also known cholecalciferol, is the form of vitamin D your body makes when exposed to the sun. Most of the vitamin D found in animal foods, including egg yolks, cheese and liver, is in the form of vitamin D-3.
As a supplement, vitamin D-3 is made by irradiating fat from sheep's wool. Most prescription forms of vitamin D are in the form of vitamin D-3.