Vitamin D is
a steroid hormone made up of a group of fat-soluble secosteroids responsible
for enhancing intestinal absorption of calcium, iron, phosphate, magnesium, and
zinc and involved in the regulation of homeostasis. Vitamin D exists in two
forms, specifically ergocalciferol (vitamin D2), and cholecalciferol (vitamin

D3 is formed in the skin after exposure to sunlight or ultraviolet light, and vitamin
D2 is acquired by irradiation of plants or plant materials or foods. Both D2
and D3 forms are not yet biologically active
and are bound by a protein called VDBP (vitamin D binding protein)
in the bloodstream. Subsequently, the bound form is metabolized in the liver
where a conversion occurs into storage forms (with limited biological activity)
of the vitamin: 25-OH vitamin D2 (calcidiol) and 25-OH vitamin D3 (calcitriol).
This metabolite has been found to be the predominant circulating from of
Vitamin D and had been recognized as an accurate indicator of an individual’s
vitamin D status.

Figure 1:
D2 and D3 Structures (Norman, A. et. al)

Reasons to Measure Vitamin D

D is not only important for bone health but can also play a role in the
development of several non-bone related diseases. There has much debate over the past 6 decades
with regards to of which form of vitamin D to measure and which best reflects
vitamin D status. Since both D2 and D3
are used for therapy, it is necessary to measure both circulating forms. The
only way to assess a person’s vitamin D status (vitamin D deficient, sufficient,
or intoxicated) is to measure their circulating level of 25-OH Vitamin D.

D deficiency has been associated with many diseases such as:

  • Rickets
  • Osteomalacia
  • Osteoporosis
  • Hyperparathyroidism
  • Multiple
  • Cancers
  • Cardiovascular
  • Depression


1. Lips, P. et al. “Vitamin D deficiency and secondary hyperparathyroidism in the elderly: consequences for bone loss and fractures and therapeutic implications” Endocrine Rev. 22 2001; 477–501.

2. Lips P. Vitamin D physiology. Prog Biophys Mol Biol 2006; 92: 4-8.

3. Mavroeidi, A et al. “Seasonal 25-hydroxyvitamin D changes in British postmenopausal women at 57 degrees N and 51 degrees N: a longitudinal study. J Steroid.” Biochem Mol Biol 2010; 121:459-461.

4. Macdonald, HM et al. “Sunlight and dietary contributions to the seasonal vitamin D status of cohorts of healthy postmenopausal women living at northerly latitudes: a major cause for concern?” Osteoporos Int 2010.

5Norman, et. al. “From vitamin D to hormone D: fundamentals of the vitamin D endocrine system essential for good health1,2,3,4.” Am J Clin Nutr 2008; 88: 2 491S-499S.

6. Tsur, A. et al. “Effect of different dress style on vitamin D level in healthy young Orthodox and ultra-Orthodox students in Israel”. Osteoporos Int 2010; 26: 6.

7. Holick MF, Chen TC. “Vitamin D deficiency: a worldwide problem with health consequences”. Am J Clin Nutr 2008; 87: 1080-1086.

8. BW J Hollis Editiorial: “The Determination of circulating 25-Hydroxyvitamin D: No Easy Task” Clin Endocrinol Metab 2004; 89: (7):3149–3151.

9. Snellman, G. et al. “Determining Vitamin D Status: A Comparison between Commercially Available Assays.” PLoS One 2010; 13:5(7):e11555.

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