Vitamin D3 in lighting

[Paleosuchus]

I have recently read and asked a few exotic vets about full spectrum lighting for iguanas. The material i read aswell as the vets where telling me that it would take a few lights to do anything for producing vitamin D3. Simply they do not need it, any bulb will work for heat purposes, in captivity they can get D3 simply from there diet. The only thing the full spectrum lighting does is cost money. I was wandering if this is true. I have found it to be true from asking 2 local iguana shelters. Was wandering what you all thought and if this actually is correct?