• (calcitriol 1,25(OH)2D3) – fat soluble modulator of calcium homeostasis
  • 7-dehydrocholesterol ? vitamin D in the skin under the influence of UV light
  • Transportation of all vitamin-D metabolites by an alpha globulin synthesized in the liver (by vitamin D-25-hydrolase and Mg++)
  • Production of less active metabolites 24,25- and 25,26-OH vitamin D also in the kidney
  • Actions: matures intestinal lining cells producing calcium-binding proteins (? GI Ca++ absorption), and affects osteocalcin production, monocytic maturation, myelocytic differentiation, skin growth, insulin secretion
  • Production increased with reduced [Ca++] and [PO4=] and increased PTH
  • 24,25-diOH vitamin D serves as alternative breakdown pathway for vitamin D
  • Absorption reduced with excessive GI fat or failure of emulsifying action of bile salts (biliary cirrhosis, alcoholism) or with small bowel wall injury (celia disease, Crohn disease, short loop-shunts or fistulae, rapid transit-gluten-sensitive enteropathy and other sprue-like syndromes) or prolonged TPN
  • Augments PTH in recruiting osteoclasts to resorb bone as a macrophage stem cell maturation hormone (resorp-tion of Ca++ from bone)
  • Alfacalcido (1?(OH)D3), synthetic vitamin D, is hydroxylated in the liver, bypassing renal 1? hydroxylation
  • Recommended intake: 400-800 IU
  • Overdosage on a regular basis ? kidney stones, nausea, hypercalcemia