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Distinction to Plain Vitamin D, Clinical Evidence and Practical Recommendations.
The term vitamin D designates a group of closely related seco-steroids with antirachitic activity. The two most important members of these are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). About 80% of the vitamin D supply in man is produced in the skin and only 20% is provided by nutrition.
The first analogue of vitamin D, dihydrotachysterol, was developed in the 1930s and used in the treatment of primary or secondary hypoparathyroidism. It then took 40 years to learn that vitamin D was not the biologically active principle for healing bone disease. It has to be hydroxylated in the liver at position 25 to build 25-hydroxycholecalciferol and then finally in the kidney at position 1 to become 1,25-dihydroxycholecalciferol [= 1,25-(OH)2-D3].
Further generations of vitamin D analogues have since been developed. The most important of these are the so-called D hormone 1,25-(OH)2-D3 (= calcitriol) and its analogue alfacalcidol (= 1-OH-D3) which are highly effective drugs for the treatment of renal bone disease. Further successful indications for active vitamin D analogues include hypoparathyroidism and osteomalacia.
Concerning osteoporosis, the role of an insufficient vitamin D supply or impairment in activation was underestimated for a long time. A breakthrough was achieved with studies showing that the prevalence of mild to moderate vitamin D deficiency was very high and increasing with age.
