Vitamin D Supplementation: An Update

Christine Gonzalez, PharmD, CHHC

US Pharmacist

Pharmacology

Vitamin D is a fat-soluble vitamin that acts as a steroid hormone. The body makes vitamin D from cholesterol through a process triggered by the action of the sun's ultraviolet B rays on the skin (FIGURE 1). Factors such as skin color, age, amount and time of sun exposure, and geographic location affect how much vitamin D the body makes. Vitamin D influences the bones, intestines, immune and cardiovascular systems, pancreas, muscles, brain, and the control of cell cycles.^[6] Its primary functions are to maintain normal blood concentrations of calcium and phosphorus and to support bone health.

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Figure 1.

Vitamin D synthesis.
UVB: ultraviolet B.
Source: Reference 32.

Vitamin D undergoes two hydroxylations in the body for activation. There are several metabolic products or modified versions of vitamin D (TABLE 1). Calcitriol (1,25-dihydroxyvitamin D₃), the active form of vitamin D, has a half-life of about 15 hours, while calcidiol (25-hydroxyvitamin D₃) has a half-life of about 15 days.^[6] Vitamin D binds to receptors located throughout the body.

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Cite this: Vitamin D Supplementation: An Update - Medscape - Oct 01, 2010.

Abstract and Introduction
Pharmacology
Deficiency, Blood Concentrations, and Toxicity
Disease Prevention
Dosing
Drug Interactions
Future Research
Conclusion
References

Table 1. Forms of Vitamin D
Cholecalciferol (vitamin D₃) Naturally occurring form of vitamin D; made by the skin upon sun exposure; found in foods and most supplements
Calcidiol (25-hydroxyvitamin D₃) Prehormone made directly from cholecalciferol in the liver; low biologic activity, but major form circulating in the bloodstream; concentrations measured by typical blood test for vitamin D deficiency
Calcitriol (1,25-dihydroxyvitamin D₃) Activated form made from calcidiol in the kidneys and tissues; the most potent steroid hormone in the body
Ergocalciferol (vitamin D₂) Made in the laboratory by radiating fungus; not naturally occurring in the body; used in prescription vitamin D (Drisdol), available in 50,000-IU capsules

Table 2. Dosing and Blood Concentrations
Dosage	Expected Increase in Blood Concentration
100 IU	1 ng/mL
200 IU	2 ng/mL
400 IU	4 ng/mL
800 IU	8 ng/mL
1,000 IU	10 ng/mL
2,000 IU	20 ng/mL

Source: Reference 30.

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Vitamin D Supplementation: An Update

Pharmacology

Tables

Tables

References

Authors and Disclosures

Authors and Disclosures

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Figure 1.

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