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[NicoleRussell]

(continued)

The UV range is from 180-400 nm, the B range specifically from 280-320 nm.
However, we are most concerned with readings from 290-300 nm. This is the
D-UV range. Why? Because this is the prime range that triggers the miracle
of photo-biosynthesis, creating pre-vitamin D3 from pro vit D3 (= 7-DHC = 7-dehydrocholesterol)).We think that about 80% of this photochemical reaction is triggered by ultraviolet waves in this range. Ingested pro vitD3 (7-dehydrocholesterol or 7-DHC) in the reptile's skin absorbs the UV-B wavelengths, which allows the photochemical reaction that converts the 7-DHC to pre vitD3 which is then thermallyisomerized to cholecalcifero l(=vitD3). The latter is then converted in the liver to 25-hydroxycholecalciferol=25OHvit D3 and the final step towards becoming biologically active vitamin D3 (1,25- dihydroxycholecalciferol = 1,25 (OH)2 vit D3 or 1,25 DHCC) takes place in the kidneys through this process. Evidence indicates that 1,25 (OH)2 Vitamin D3 is NOT stored but is synthesized as "necessary" from the main storage form in the BLOOD, 25 OH vit D3,(therefore, the higher the blood hydroxyl level (OH25), the greater the storage capacity) and its primary function is to regulate calcium metabolism. This gives you some idea of why reptiles suffering from metabolic bone disease (the lack of sufficient D3 to metabolize calcium) may also suffer from forms of liver and kidney disease." (Although this may not be as straight forward as it appears, W.G.)

Having mentioned reptilian skin, let’s quickly consider the amount of exposure of an animal to natural sunlight is necessary for this chemical reaction to take place. The assumption that a few minutes a day is sufficient is based entirely on studies with humans. Keep in mind that the electromagnetic wave lengths (photons) from the UV radiation have to penetrate the skin deep enough to reach the capillaries underneath the skin in order to produce this reaction. Anyone with experience around iguanas can appreciate how tough and thick their hide is. Several hours a day of natural sun exposure is much more appropriate for these large reptiles. (Many different species of reptiles have diverse degrees of UVB sensitivity and require different amounts of UVR in variable lengths of time.)

(continued in Part3)