The D3 cycle is a biological process that is dependent on many external factors including access to natural light, heat, cool and rest.
This amazing process begins when a cholesterol called pro vitamin D (7DHC) is produced in the animal’s skin (it is natural in humans too). When this cholesterol is exposed to natural light (including UV-B at 290-315nm), this cholesterol is turned into pre-vitamin D within the skin membrane.
After exposure to warmth, this newly manufactured pre-vitamin D is converted (in the skin membrane) into vitamin D3. Vitamin D3 is then sent out into the blood plasma and is bound with a vitamin D binding protein. This part of vitamin D is then carried to the liver where it converts to a hormone called calcediol (25-hydroxy vitamin D3).
The blood carries this calcediol all around the body into the kidneys. Here, some of the hormone turns into another hormone called calcetriol. This compound plays an essential role in calcium metabolism by controlling blood calcium levels. Calcetriol also plays a huge role in the immune system and the cardiovascular system. It has been shown to lower the risk of cancers in the body and skin.
We can see that exposure to natural sunlight to turn sunlight into life-saving vitamins, is only the start of this amazing ability. If part of the cycle is missed out then it cannot be completed. Changes and reactions would also be hindered. This would result in an under-provision of vitamins and hormones. If supplementation was not used, a calcium crash could happen.
Although vital to the well-being of reptiles, UV exposure plays many more roles in the body than the D3 cycle alone. Impacts on sight, breeding and mental well-being are all documented and undergoing research.