Providing UVB for your dragon allows it to produce D3 which allows it to use the calcium in its diet. To ensure that it gets enough D3 I dust twice a month with a phosphorous-free calcium/D3 powder. D3 from supplements can build up in the system so you need to be careful with it.
Since most insects we use as feeders have a poor ratio of calcium to phos. I dust with a phosphorous-free calcium powder at most feedings.
I also dust with a vitamin powder that has a beta carotene source of vitamin A. Beta carotene won't build up in the system like preformed vitamin A can. Excess preformed vitamin A can prevent the D3 from doing its job and lead to MBD.
Phos., calcium, D3 and vitamin A are major players in bone health and they need to be in balance. When trying to attain the balance you need to look at the supplements, what you feed to the dragon and what you feed to the insects for the content of these 4 things.