Types of Vitamin D and How Vitamin D is Metabolized
There are two major types of vitamin D1,2:
- Vitamin D2 (ergocalciferol) – which is synthesized by plants and is not produced by the human body
- Vitamin D3 (cholecalciferol) – which is made in large quantities in the skin when sunlight strikes bare skin. It can also be ingested from animal sources
Factors that impact the ability of the body to synthesize vitamin D through the skin are geographic latitude, time of year, time of day, presence of clouds and/or smog, skin melanin content, and whether or not sunscreen has been applied. For example, residents at 42° N latitude or higher are unable to synthesize vitamin D via the skin during the winter months (from November through February).
In supplements and fortified foods, vitamin D can be either D2 or D3. The two forms have traditionally been regarded as equivalent based on their ability to cure rickets, but evidence suggests that vitamin D3 is approximately three times more effective at maintaining serum concentrations because the binding protein has a higher affinity to vitamin D3 than vitamin D2. This allows vitamin D3 to reside in the circulatory system longer and increase the concentration to sufficient levels more quickly. The major preparations of vitamin D for prescription use in North America are in the form of vitamin D2, while more over-the-counter vitamin / multivitamin preparations use vitamin D3.
Whether it is absorbed through unprotected skin or ingested then absorbed by the intestines, vitamin D is bound to the binding protein (both albumin and vitamin D binding protein) and carried to the liver via the bloodstream. From there it begins two hydroxylation processes. Beginning in the liver it is transformed into 25(OH)vitamin D (calcidiol), which is the primary circulating form of Vitamin D and the most commonly measured form in serum. Then in the kidneys it is transformed into 1,25 dihydroxy-vitamin D (calcitriol), which is the biologically active form of vitamin D.
1,25 dihydroxy-vitamin D is the primary steroid hormone involved in mineral homeostasis. When serum calcium dips to below 8.8 mg/dL it prompts a proportional increase in the secretion of parathyroid hormone (PTH). PTH signals to the kidneys to increase the production of 1,25 dihydroxy-vitamin D by increasing the production of 25(OH)vitamin D-1α-hydroxylase. Subsequently, the increase in 1,25 dihydroxy-vitamin D stimulates the increased absorption of calcium in the intestines to stimulate bone remodeling. When phosphorous and bone genes levels signal a normal state of bone remodeling, the kidney reduces the production of 1,25 dihydroxy-vitamin D to a normal level.
1: Dietary Supplemental Fact Sheet: Vitamin D. Office of Dietary Supplements. National Institutes of Health. Updated 11/13/2009. Accessed 08/17/2010.
2: Bringhurst FR, et.al., “Bone and Mineral Metabolism in Health and Disease”. Chapter 23 of “Harrison’s Endocrinology”, J. Larry Jameson, editor, McGraw-Hill Medical Publishing Division, copyright 2006