Vitamin D is a fat-soluble vitamin and pro-hormone that exists in several different forms.

Ergocalciferol (Vitamin D2) is not produced in the body and comes from plant sources whereas cholecalciferol (Vitamin D3) is found in cold-water fish such as salmon, mackerel, and sardines, is fortified in milk and is manufactured in the skin with adequate exposure to the suns UVB rays.

The New England Journal of Medicine estimated that 30-50% of children and adults in the US are at risk for vitamin D deficiency and 32% of healthy adults age 18-29 were measured as Vitamin D deficient at the end of winter in Boston. This is likely a consequence of inadequate dietary intake that may be exacerbated by fat malabsorption coupled with insufficient exposure to UVB sunlight.

Note: Synthesis from sunlight requires that the sun be greater than 45˚ above the horizon and most sunscreens block UVB rays. People with increased melanin (darker skin) require longer exposures to sunlight to produce the same amount of Vitamin D.


Bioactive vitamin D binds to specific receptors in the cell (known as VDRs or Vitamin D receptors) and induces the transcription of more than 50 genes that have far-reaching effects in the body including:

  • Osteoporosis / Calcium Balance – Vitamin D increases intestinal absorption of dietary calcium, increases reabsorption of calcium filtered by kidneys and mobilizes calcium from the bone when there is insufficient dietary calcium. We have long known the influence vitamin D had in the formation of bone, as evidenced by the incidence of rickets with low Vitamin D levels.
  • Anti-Cancer – Vitamin D inhibits proliferation and stimulates the differentiation of cells when bound to VDR receptors. Vitamin D has also been shown to induce apoptosis (programmed cell death).
  • Immunity – There are a significant number of VDR receptor on T cells and macrophages and there is evidence that when bound to these receptors vitamin D acts as a selective immunosuppressant and can either prevent or significantly affect many autoimmune diseases including rheumatoid arthritis, SLE, type 1 diabetes and IBD. Vitamin D can also enhance innate immunity and has been shown to be beneficial against tuberculosis and through this mechanism may be useful against additional infectious agents such as influenza.
  • Diabetes – The bioactive form of Vitamin D has been shown to stimulate insulin production in the pancreas in type 1 diabetics and may play a role in the pathogenesis of type 2 diabetes through its ability to impair insulin synthesis and secretion.
  • Blood Pressure Regulation – Vitamin D can decrease the expression of the gene that codes for rennin, and therefore plays a role in controlling hypertension.


Vitamin D is a fat-soluble vitamin that is stored in the liver and fatty tissue and is eliminated more slowly than water-soluble vitamins. Excess levels of Vitamin D can cause hypercalcemia, hypercalciuria, hypertension, constipation, fatigue and more. While adequate levels of Vitamin D are very important, you can get too much of a good thing. Monitoring therapy is important to ensure adequate, but not excessive dosage.

Vitamin D: What to Test

Whether supplemented or manufactured in the skin, cholecalciferol (D3) is hydroxylated in the liver to form 25-hydroxycholcalciferol (25(OH)D3) and this is the major circulating form of the vitamin. Though it goes through an additional hydroxylation (primarily in the kidney) to form 1,25-dihydroxycholecalciferol before it is biologically active, the 25(OH)D3 form is considered the most accurate measure of the amount of vitamin D in the body.

The Vitamin D blood spot test is an inexpensive, quick and painless test that will enable you to test and monitor Vitamin D supplementation without the inconvenience of venipuncture.


1. Vitamin D Deficiency. Holick MF. N Engl J Med. 2007 Jul 19; 357(3):266-81.
2. Vitamin D: Importance in the Prevention of Cancers, Type 1 Diabetes, Heart Disease, and Osteoporosis. Holick MF. Am J Clin Nutr 2004;79:362-71.
3. Overview of General Physiologic Features and Functions of Vitamin D. DeLuca HF. Am J Clin Nutr 2004;80:169S-96S
4. Vitamin D: It’s Role and Uses in Immunology. Deluca HF, Cantorna MT. FASEB J. 2001 Dec; 15(14):2579-85.
5. Vitamin D and the Immune System: Role in Protection Against Bacterial Infection. Bikle DD. Curr Opin Nephrol Hypertens. 2008 Jul;17(4):348-52.

Health Disclaimer:

All information given about health conditions, treatments, products, and dosages are not intended to be a substitute for professional medical advice, diagnosis or treatment. This is provided only as a suggested guideline.