Vitamin D is a critical fat-soluble vitamin required for healthy bones and immune function. People with an increased risk of vitamin D deficiency, such as the elderly, may need to take vitamin D supplements to maintain proper health. Because vitamin D sometimes comes in the form of vitamin D3 or vitamin D2, confusion can arise concerning the best form to take. In short, all forms have similar effects on the body.
Vitamin D3
One form of vitamin D is vitamin D3, which is also called cholecalciferol. Vitamin D3 is produced in the skin of humand and animals after exposure to ultraviolet-B, or UVB, radiation from the sun, explains the Mayo Clinic. Many health supplements contain vitamin D3. Whether your own skin produces vitamin D3 or you consume it in a supplement, vitamin D3 is converted to vitamin D in your body.
Vitamin D2
Another form of vitamin D that is commercially available is vitamin D2, also known as ergocalciferol. Vitamin D2 is produced almost exclusively in plants, reports the Mayo Clinic, and is the other major type of vitamin D found in health supplements. Like vitamin D3, vitamin D2 is converted into vitamin D inside your body. However, D2 demonstrated only 1/3 the potency of a similar quantity of D3 in a study released in 2004 in "The Journal of Clinical Endocrinology & Metabolism."
Activation of Vitamin D
After vitamin D2 or vitamin D3 is converted into vitamin D, it undergoes two more activation steps before it becomes biologically active in the body. In the liver, vitamin D undergoes a process called hydroxylation and becomes 25-hydroxyvitamin D, or calcidiol. Then in the kidneys, 25-hydroxyvitamin D is hydroxylated again to form 1,25-dihydroxyvitamin D, or calcitriol, which is the most active form of vitamin D and responsible for most of the biological activity of vitamin D.
Vitamin D Sources
Almost all vitamin D supplements contain either vitamin D2 or vitamin D3. Because human skin produces vitamin D3 after exposure to sunlight, it is possible for most people to meet all of their vitamin D needs by spending five to ten minutes outside in the sun two or three times a week, according to the Linus Pauling Institute at Oregon State University. However, people with very dark skin, people who live at very high latitudes and elderly people may require additional vitamin D from food or supplements. Foods that are rich in vitamin D include fatty fishes, such as salmon and sardines, and egg yolks. Many foods, including milk, orange juice and cereal, are fortified with extra vitamin D.
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