Most reptiles make their vitamin D₃ from UVB, directly in their skin. Without it, the calcium from their prey is not absorbed. Here is the full path, from light to bone.
UVB creates vitamin D₃
It is stored as a reserve
It becomes active
Calcium from food is absorbed
Bones become strong
Under UVB (300–320 nm), 7-dehydrocholesterol in the skin becomes previtamin D₃, then vitamin D₃. UVA2 (320–350 nm) helps the animal regulate this production, without excess.
Vitamin D₃ reaches the liver, where it is converted into 25-hydroxyvitamin D₃, its storage form.
The kidneys convert it into its active form: 1,25-dihydroxyvitamin D₃, or calcitriol.
The active form lets the gut absorb the calcium provided by food.
Absorbed calcium ensures the mineralisation and strength of the skeleton.
The chain breaks at the very first step: calcium is poorly absorbed, which can lead to bone disorders such as metabolic bone disease.