Vitamin D is a fat-soluble vitamin that occurs in two forms in nature. Using cholesterol as a precursor, vitamin D3, or cholecalciferol, is produced in your skin whenever you are exposed to sunlight. Similarly, vitamin D2, or ergocalciferol, is synthesized by plants from ergosterol in response to sun exposure. Cholecalciferol and ergocalciferol are inactive "prohormones," both of which must be converted in your body to an active form of vitamin D. Although vitamin D2 is less potent than D3 in humans, their physiologic effects -- suppression of parathyroid hormone, for example -- are similar.
Controversy
Vitamin D2 has traditionally been used by physicians to treat vitamin D deficiency. However, in 2004, researchers at Creighton University in Omaha, Nebraska, reported equivocal therapeutic responses to high-dose vitamin D2 therapy and demonstrated that D2 has less than one-third the potency of D3 in humans. In contrast, a study published in the March 2008 issue of the "Journal of Clinical Endocrinology and Metabolism" showed that D2 and D3 were equivalent in humans. In 2011, using different analytical methods, researchers at the University of Wisconsin showed that vitamin D3 is "slightly, but significantly, more effective" for increasing levels of active vitamin D in human tissues.
Calcium Balance
About 99 percent of the calcium in your body is sequestered in your bones and teeth, with only 1 percent remaining in your soft tissues and blood. The Linus Pauling Institute at Oregon State University says the concentration of calcium in the fluids around your cells is maintained within a very narrow range to allow normal physiological functioning. Vitamin D and parathyroid hormone play pivotal roles in regulating your calcium balance.
Parathyroid Hormone
Your parathyroid glands are nestled behind your thyroid gland, which is located just beneath your throat's "Adam's apple." Parathyroid hormone, or PTH, is produced by your parathyroids whenever your blood calcium level falls. Under the influence of PTH, calcium is mobilized from your bones, your kidneys retain more calcium and more vitamin D is converted to its active form, thereby increasing intestinal calcium absorption. As blood levels of vitamin D and calcium rise, receptors in your parathyroids are triggered, and secretion of PTH ceases. Once converted to active vitamin D, both D2 and D3 stimulate vitamin D receptors in your parathyroid glands.
Considerations and Recommendations
Although vitamin D2 appears to be less potent in humans than vitamin D3, both have similar effects in your body, and both are capable of suppressing PTH secretion. Dietary recommendations for vitamin D typically refer to vitamin D3. Recommended daily allowances for vitamin D3 vary from 400 IU for infants to 800 IU for adults over the age of 70. A tolerable upper limit for D3 has been set at 2,000 IU daily. Creighton University experts recommend vitamin D2 doses at least three times higher than those recommended for D3. Ask your doctor about the form and dosage of vitamin D that is best for you.
References
- "Journal of Clinical Endocrinology and Metabolism"; "Vitamin D2 is Much Less Effective than Vitamin D3 in Humans"; L.A.G. Armas et al; November 2004
- "Journal of Clinical Endocrinology and Metabolism"; "Vitamin D2 is as Effective as Vitamin D3 in Maintaining Circulating Concentrations of 25-Hydroxyvitamin D"; M.F. Holick et al; March 2008
- "Journal of Clinical Endocrinology and Metabolism"; "Evaluation of Ergocalciferol or Cholecalciferol Dosing, 1,600 IU Daily or 50,000 IU Monthly in Older Adults"; N. Binkley et al; April 2011
- Linus Pauling Institute at Oregon State University: Calcium
- Linus Pauling Institute at Oregon State University: Vitamin D
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