Vitamin D3 is converted into the active form 1,25 dihydrocholecalciferol in three
steps, which take place in the skin under the influence of sunlight or ultraviolet light,
the liver and the kidneys. Apart from increasing blood calcium and decreasing blood
phosphorous levels the main function of vitamin D3 is to increase intestinal absorption
of calcium and phosphorous. This increase in intestinal calcium absorption,
commences after sexual maturity and peaks during eggshell. The activity of this
enzyme is oestrogen (female hormone) dependent.