This Vikki Peterson video explains why Celiacs may not be absorbing vitamin D or calcium from food.
Dr Vikki suggests 2000iu for each 10ng/ml you need to raise status and thinks most people aren't taking sufficient.
To maximise the absorption of calcium you first need to raise your 25(OH)D to above 30ng/ml and for some people more than 40ng/ml is required. (Dr Vikki suggests 50ng/ml should be your aim. Dr Davis prefers 60ng/ml. The point is at 40ng/ml you've no safety zone whereas above 50gn/ml your body will have a reserve store of D3.
Since around 1960s when new shorter faster growing varieties of wheat were produced we've had an increase in celiac incidence because these new wheat varieties contain more gluten than the older traditional wheat varieties. This has combined with a general decline year on year in vitamin D status as populations become more urbanized and local ozone pollution levels prevent UVB reaching ground level. So eliminating wheat from your diet and reducing gluten intake while improving vitamin D status may well help the situation.