From: http://www.agdnutrition.com/d3-story.html April 2012
The company sells bulk Vitamin D3 for humans and animals (actually made in China)
Vitamin D3 is normally made in animals by the deposition of 7-dehydrocholesterol (a derivative of cholesterol commonly referred to as 7-D) in the skin. Upon irradiation with ultraviolet light from the sun, this precursor undergoes transformation to pre vitamin D (PRE). The pre isomer is converted to cis-vitamin D (CIS) and adsorbed into the blood where it is transported to the liver and metabolized to 25-hydroxyvitamin D (25-OH). The 25-OH analog is circulated in the blood until abnormal calcium levels trigger the need for a change in calcium blood concentration (increase or decrease). The 25-OH analog is transported by the blood to the kidney where it is further metabolized to 1,25-dihydroxyvitamin D3 (1,25-OH2) which acts to facilitate the transport of calcium into or out of bone as well as through the gut wall. The process of the commercial manufacture of vitamin D3 mimics the manufacture of the cholecalciferol in animals.
The process starts with cholesterol, which is obtained from wool grease (15% of wool grease; 30% of wool wax alcohols).
Wool grease and wool wax alcohols are obtained from the cleaning of wool after shearing. The cholesterol is removed from the wool grease and purified.
The cholesterol undergoes a four step process to make 7-dehydrocholesterol. This is the same compound that is stored in the skin of animals.
The 7-dehydrocholesterol is then irradiated with ultra violet light to give pre vitamin D3. Some isomers of vitamin D are formed during irradiation (tachysterol and lumisterol), resulting in a mixture of the pre vitamin D, these isomers and 7-dehydrocholesterol (80%).
The 7-D is removed by crystallization from methanol; the methanol is removed leaving a mixture of pre vitamin D, lumisterol and tachysterol.
The mixture is heated to convert the pre vitamin to cis vitamin D.
This material is in the form of a resin that melts at about room temperature. and usually has a potency of 25,000,000 to 30,000,000 International Units per gram.
This is compared to the pure cis vitamin D which can be isolated from the resin and which has a potency of 40,000,000 International Units per gram in its pure crystalline form.