Ok, awhile ago I had a discussion with someone regarding sunlight exposure to gecksos as it applies to Vitamin D3. There's this big hullabaloo about whether D3 is needed in reptiles and how much is too much. What I'm going to post is what I know about D3 and my opinion. Please feel free to post your thoughts on the subject, as well.
(and my apologies, in advance, if it's not terribly organized or sounds confusing - I'm pulling it from my logs; you should catch my basic points)
UV radiation helps to trigger Vitamin D production in the skin. Humans don't really need it since most foods have vitamins and [bleep] added. Some people wonder if keeping their geckos inside prevents them from triggering D3 synthesis, since windows naturally block UVA radiation and some windows are treated to block UVB. Therefore, supplementation is needed to add D3 to their diet, to ensure it does it's thing (if you don't know what it does, I'll cover that in a bit).
So goes the theory, anyway.
Keeping geckos in a sunlight room or a dark room makes no difference with the D3 issue. I have to question what the big concern is about, actually, since they are nocturnal. Nocturnal animals don't require UV exposure to trigger the Vitamin D synthesis in their skin. Here's how the D3 process works in animals...
* the liver produces something called....well, I forget the name, but it's precursor-D, for short (not the chemical name)
* it moves to the skin and when exposed to mid-level UV it turns into something else. I don't know the name offhand (lol), but it's essentially a pre-vitamin-D
* the liver and kidneys make more changes to it, turning it into proper vitamin D (D3); part of the process is body temperature..the normal temp of an animal is enough to trigger it.
Other points to conisder...
* UV exposure isn't needed if calcium is introduced into the body (well...a form of calcium)
* provided you get the calcium -- other nutrients are important, but calciferol is the trigger for synthesis -- you don't need UV exposure, period.
* nocturnal animals have no D-deficiency problems, normally - they pick it up in their diet; same goes for any animal with fur (no direct skin exposure to UV); same goes for burrowing animals who see no sun; marine life, too
What does D3 do?
* regulates blood levels of calcium and phosphorous (increases or decreases absorbtion in the small intestine, as needed)...helps the bones maintain proper levels (stimulates bone cell uptake of those minerals).
(actually...D vitamin is unique in the way it stimulates absorbtion.....usually that's a function of hormones. *shrugs* that's why they always push women and kids to drink milk....lack of calcium and phosphorous lead to osteoporosis)
Also, D is also the most toxic of the vitamins....so too much is bad; but since no one can agree on the proper levels for geckos, you'll have to experiment to see what kills your gecko and what doesn't (it's a joke, relax). With people, though, it's like 10-15 times the recommended level before toxic effects manifest.
My opinion: geckos don't need D3 supplementation, but it doesn't hurt. In fact, it might be a good idea with juveniles and pregnant geckos to toss a little in their diet, since they have special nutritional needs. How much is too much? I think it depends on the D3 content in the supplements, but I suspect toxicity isn't a huge issue unless you are forcing your gecko to take in inordinate amounts of D3 (animals have natural processes to control things like that). D3 is not needed if you provide them plenty of Calcium...there should be no problem - let them regulate Calcium intake themselves (just make sure it's available).
Your turn.
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-Marcus The Great
Owner, 1.1 Leopard Geckos (T^2) 
Master, sotik.com
Photographer, imesho.com
