Strictly speaking, vitamin D is not a real vitamin. According to the definition, vitamins are vital organic compounds that the body has to take in regularly with food because it cannot produce them or cannot produce them in sufficient quantities. However, this does not apply to vitamin D: If there is sufficient sunlight, the body can produce most of the vitamins it needs (80 to 90 percent) itself. At around 10 to 20 percent, diet only accounts for a relatively small proportion of the supply of vitamin D.
Hormone precursor (prohormone) would actually be the more appropriate name for vitamin D. The body converts it into a hormone called calcitriol. It is the biologically active form of vitamin D.
What is Vitamin D3?
The collective term vitamin D includes several fat-soluble compounds. One of them is vitamin D3, also called cholecalciferol or colecalciferol. It is converted into the active hormone calcitriol in the liver and kidneys. In addition, the body can convert vitamin D3 into a storage form, the so-called calcifediol (also 25-hydroxy-vitamin D or 25-OH-vitamin D).
Vitamin D2, also known as ergocalciferol, also belongs to the vitamin D group. It is converted in the body into the more effective form vitamin D3.