EPI LIFE COACH articles

Vitamin D is a steroid hormone that functions to regulate specific gene expression following interaction with its intracellular receptor.
YEAR 2017
DATE Friday, March 03
TOPIC Vitamins
AUTHOR Dr. Carlos Orozco (BSc, MSc, ND, MD, PhD, FPAMS)

Biological Function
Vitamin D is a steroid hormone that functions to regulate specific gene expression following interaction with its intracellular receptor. The biologically active form of the hormone is 1,25-dihydroxy vitamin D3 (1,25-(OH)2D3, also termed calcitriol). Calcitriol functions primarily to regulate calcium and phosphorous homeostasis.

Active calcitriol is derived from ergosterol (produced in plants) and from 7-dehydrocholesterol (produced in the skin). Ergocalciferol (vitamin D2) is formed by uv irradiation of ergosterol. In the skin 7-dehydrocholesterol is converted to cholecalciferol (vitamin D3) following uvirradiation [1;2].
Vitamin D2 and D3 are processed to D2-calcitriol and D3-calcitriol, respectively, by the same enzymatic pathways in the body. Cholecalciferol (or ergocalciferol) are absorbed from the intestine and transported to the liver bound to a specific vitamin D-binding protein. In the liver cholecalciferol is hydroxylated at the 25 position by a specific D3-25-hydroxylase generating 25-hydroxy-D3 [25-(OH)D3] which is the major circulating form of vitamin D. Conversion of 25-(OH)D3 to its biologically active form, calcitriol, occurs through the activity of a specific D3-1-hydroxylase present in the proximal convoluted tubules of the kidneys, and in bone and placenta. 25-(OH)D3 can also be hydroxylated at the 24 position by a specific D3-24-hydroxylase in the kidneys, intestine, placenta and cartilage [3;4].

Activation of Vitamin D
Vitamin D itself is biologically inactive, and it must be metabolized to its biologically active forms. After it is consumed in the diet or synthesized in the skin, vitamin D enters the circulation and is transported to the liver. In the liver, vitamin D is hydroxylated to form 25-hydroxyvitamin D [25(OH)D], the major circulating form of vitamin D. Increased exposure to sunlight or increased intake of vitamin D increases serum levels of 25(OH)D, making the serum 25(OH)D concentration a useful indicator of vitamin D nutritional status. In the kidney and other tissues, the 25(OH)D3-1-hydroxylase enzyme catalyzes a second hydroxylation of 25(OH)D, resulting in the formation of 1alpha,25-dihydroxyvitamin D [1,25(OH)2D]—the most potent form of vitamin D. Most of the physiological effects of vitamin D in the body are related to the activity of 1,25(OH)2D (2).

References:

  1. DeLuca HF and Zierold C. Mechanisms and functions of vitamin D. Nutr Rev 1998;56:S4-10.
  2. Reichel H, Koeffler H, Norman AW. The role of vitamin D endocrine system in health and disease. N Engl J Med 1989;320:980-91.
  3. Van den Berg H. Bioavailability of vitamin D. Eur J ClinNutr 1997;51 Suppl 1:S76-9.
  4. Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes: Calcium, Phosphorus, Magnesium, Vitamin D and Fluoride. National Academy Press, Washington, DC, 1999.
  5. Goldring SR, Krane S, Avioli LV. Disorders of calcification: Osteomalacia and rickets. In: LJ D, ed. Endocrinology. 3rd ed. Philadelphia: WB Saunders, 1995:1204-27.

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