The generic term Vitamin D – actually refers to a small family of two sub vitamins – Vitamin D2 and Vitamin D3.
Vitamin D is fat soluble and is produced in the skin when it is exposed to sunlight and also occurs naturally in fish. Staple foods are fortified with vitamin D – very useful for people who live in countries in the arctic circle that have 24hrs of darkness during the winter months.
Image kindly reproduced according to the licence at:http://en.wikipedia.org/wiki/File:Cholecalciferol-3d.png
Vitamin D is carried to the liver (calcidiol) where it is processed and transported to the kidneys where it is synthesised into its biologically active form.
In its active form (calcitriol) it functions as a hormone, regulating the amount of phosphate and calcium in the blood – this promotes bone growth.
A defficiency of vitamin D can lead to brittle bones.