Thanks to Hilde:
Here goes Hilde's post on D3:
Hilde
Super Moderator Join Date: Mar 2006
Location: S. Ontario Canada
"The gecko needs both calcium and Vit D3, so you need both to some extent.
Vit D3 helps absorb calcium (Ca). Without enough D3, the gecko won't be able to absorb enough Ca, resulting in MBD. Vit D also plays a role in cell growth, calcium/phosphate regulation, neuromuscular and immune functions.
With a properly balanced diet, the gecko can make D3 if exposed to UVB in the correct wavelength range (direct sunlight exposure or UVB bulbs which produce it in the correct range). Adding D3 to the diet forces the body to absorb it, possibly causing an overdose of D3 - which produces a form of MBD. Too much D3 or not enough can both result in MBD, so it's important to use the proper dosage. Vit D3 is fat soluble, meaning it is stored in the body, unlike some vitamins which are water soluble - excess is eliminated from the body making it almost impossible to overdose in healthy geckos. Once enough D3 has been absorbed, excess can start causing new problems. If the gecko gets the D3 via exposure to UVB, then there's no overdosing - the body only makes what it needs, making more as necessary.
If your gecko is healthy, has a balanced diet, then don't use D3 with every dusting. Depending on your feeding routine, every 2nd or 3rd feeding is probably enough. It also depends on how much D3 is added to the dusting - lots of D3 would mean including it less often, while skimping on D3 might mean including it at every feeding. It's hard to say how much and how often you should add D3 without knowing how much is in the vitamin or calcium supplement. Follow some of the suggested supplementing routines, watch the gecko carefully (it might take a few months to see if it's getting enough or too much), then adjust accordingly."