Calcium, phosphorous, D3 and vitamin A are all important players in bone health...and they play roles in other important body functions too. They need to be in balance.
Exposure to UVB either from sunlight or UVB light (neither of which should pass through glass or plastic) allows the chameleon to produce D3 which allows the chameleon to use the calcium provided for it. If the chameleon is in direct sunlight, then it likely won't need any other D3. As long as the chameleon can move in and out of the sunlight, then its shouldn't end up with too much D3 being produced in the body from exposure to the UVB. D3 from SUPPLEMENTS can build up in the system though.
Most/many of the feeder insects used in captivity have poor ratios of calcium to phosphorous so dusting the insects before they are fed to the chameleon with a phos.-free calcium powder helps to make up for this.
Dusting a couple of times a month with a vitamin powder that contains a beta carotene source (prOformed) of vitamin A. will ensure that it gets all the vitamins it needs without having to worry about overdoing the vitamin A. Beta carotene is only converted as needed. Now, there is controversy as to whether all/any chameleons can convert the beta carotene...so once in a while they can be given a tiny dose of prEformed vitamin A. (However, I have never done this with my veileds and they live long healthy lives.) Excess prEformed vitamin A can prevent the D3 from doing its job and push the chameleon towards MBD.
Since my chameleons get their UVB almost exclusively from an artificial source (UVB light), I dust the insects with a phos.-free calcium/D3 powder twice a month lightly. This ensures that they get enough D3 without overdosing it....something to be careful of since D3 from supplements can build up in the system.
I gutload/feed my insects a nutritious diet. For crickets, roaches, superworms a wide assortment of greens (dandelions, collards, kale, escarole, endive, mustard greens, etc.) and veggies (carrots, parsnips, squash, zucchini, sweet potato, sweet red pepper, celery leaves, etc.).
Now...you said..."gut loader I can find with fresh veggies for moisture. He prefers crickets that also have a bit of that flaky fish food in them. + occasional calcium/vitamin dusting" and "T-Rex Calcium Plus as my main cricket food and a 2:1 Calcium/Phosphorus+Vitamin (also T-Rex) duster"...as I stated above...I dust at most feedings with calcium to make up for the poor ratio of calcium to phos. in many of the feeder insects.
You said..."His stools are regular and when he very occasionally misses passing for more than a 4 or 5 days (usually if the weather is cold) I will give him 2 or 3 crickets dipped in cod-liver oil" and "My gut loader and my duster both have D3 and are cover pretty comprehensively calcium and the rest."...cod liver oil has D3 and preformed vitamin A in it. Check the fish food to see if it also has D3 and preformed vitamin A in it.
Signs of calcium issues include but are not limited to...crooked/deformed/broken bones, failure to exhibit trunkal lifting, tongue dysfunction, tremors, constipation, soft jaw bone and/or casque,
IMHO, it appears that you are not giving it enough calcium and you are giving it too much preformed vitamin A and D3 from supplements...and then there's the phos. in the supplement too. I am not a vet and my opinions come only from 25+ years of experience with chameleons and other reptiles and from what I have read and been told.
You said..."It is a high grade vitamin brand so I do like them to have some of it but will keep an eye out for gout."...gout is better prevented than treated in chameleons...so it would be better IMHO to avoid things that can lead to it rather than try to deal with it when it happens.
Here are some sites that will explain the relationship of calcium, phos., D3 and vitamin A and talk about gout...
http://www.adcham.com/html/veterinary/mbd-fractures-kramer.html
http://chameleonnews.com/?page=article&id=102
http://web.archive.org/web/20060502...rnals.com/vet/index.php?show=5.Vitamin.A.html
http://web.archive.org/web/20060421.../index.php?show=6.Vitamin.D3.and.Calcium.html
http://web.archive.org/web/20060502...als.com/vet/index.php?show=8.Gout.Basics.html
http://www.tracyhicks.com/MBD.htm
There are some nutrition articles on this site...
http://chameleonnews.com/?page=reference
chambabysitter1 said..."MBD usually occurs from improper lighting"...not exactly...its to do with an imbalance in the system and although lack of proper lighting can be a big factor in it, it involves much more than that.