Vitamin D refers to a group of fat-soluble secosteroids. Three molecules are of prime importance in this group. The first is cholecalciferol, or “native” vitamin D, the second is 25-hydroxy-vitamin D [25(OH)D], and the third is 1,25-dihydroxy-vitamin D [1,25(OH)2D] also known as calcitriol.
In the 1970 and 80s, circulating calcitriol was thought to be the most important vitamin D molecule. However, numerous epidemiological studies examining serum levels of calcitriol and various diseases showed no significant associations.
Around 1990, scientists began to discover that serum levels of the circulating form of the molecule, 25(OH)D, was associated with many diseases. The accepted concept was that cholecalciferol was made in the skin, transported to the liver where it was turned into 25(OH)D, which in turn was circulated around to numerous tissues, where it was transported across cell membranes and turned into calcitriol, which regulated numerous genes.