Where do you see that?This website states, "Research suggests that Beardies may not utilize much or any of the D3 they ingest (as given in a vitamin supplement), only that made by the UVB-skin interaction."
My question is this: Where can I read this research?
AUC analysis confirmed that the least bioavailable delivery system was micellized vitamin D3
Where do you see that?
To directly answer your question though, I'm not sure there have been any explicit scientific studies on how much D3 actually gets absorbed from supplements in bearded dragons, but we can extrapolate based off of human studies (and studies on other creatures).
For instance, this study on D3 levels in the blood using multiple different types of supplements:
With really the only important part of that for this discussion being:Background and Objectives: The major cause of vitamin D deficiency is inadequate exposure to sunlight. It is difficult to supplement it with food because sufficient concentrations of vitamin D naturally occur only in a handful of food products. Thereby, ...www.ncbi.nlm.nih.gov
The most common form of D3 supplementation for dragons is most likely going to be micellized D3. The most common brand name supplement (repcal) doesn't even list what form of D3 it's using, but its a safe assumption since it's a powder that this is the case.
There are also studies that show D3 synthesis is much more efficient when done through the skin via UV exposure, and that the body has natural pathways to prevent an over production of D3 when D3 is introduced to the body this way. The body does not have that luxury for supplements.
In reality, the amount of D3 in most powdered calcium supplements is so low, that even if a dragon could absorb the D3 extremely efficiently (no waste) it likely still would not be enough D3 for the body alone. UV exposure would still be necessary.
So remember, the supplements for our reptiles most likely uses the least bioavailable form of D3 possible, and it's already in such a small quantity, you aren't going to be able to fulfil the bodies D3 needs from supplementation.
So while the statement in your original post hasn't been verified specifically with bearded dragons, it's a safe assumption given what we know about bioavailability of D3 and tests on humans/animals. It also lines up with decades of first hand experience from bearded dragon owners around the world (that being that MBD sets in eventually when housed without proper lighting).
Apologies to all for my original post's description of the location/source of my quoted question, which is actually from Beautiful Dragon's diet page. However, that page is strongly recommended by your website's page: Bearded Dragons - Care Sheet | General Care | Bearded Dragon .org
Thank you, Brandon, for your very understandable description and a URL to a research paper on this matter, which I will read assiduously.