Vitamin D deficiency is a symptom of the cause. The cause is twofold, the one being gut flora, and the other being industrial (exhaust fumes, chemical waste, etc, primarily metal residues), environmental (mold toxins), or by ill-placed allopathic advice (repeated antibiotic treatments, amalgams in teeth), activities that interfere with the base of the type 1 diabetic's immune system, the gut wall lining and the interplay between it and it's inhabitants.
Seek information on butyric acid (butyrate).
Butyrate is a product of gut flora feeding off of resistant starches present in very fresh fruits and vegetables. It is a short-chain fatty acid, and is used in epithelial tissues, the production of neurotransmitters, reduction of inflammatory responses, raising insulin sensitivity, skin health, and the development of beta islet cells.
Also, Segmented Filamentous Bacteria in the jejunum (2nd part, small intestine) prevent the development of type 1 Diabetes by helping the Th17 immune system cells to develop and mature. These are vital in the immune system's responce to inflammation and microbial attack beyond the gut lining.
The lining of the gut is an epithelial tissue. Progenitor cells for alpha, beta and gamma cells in the islet of lagerhans originate in the gut wall, migrate through the bloodstream to the pancreas, and are matured into their different types in a process also mediated by butyrate (at the mitochondrial level).
Gut flora are also vital for the synthesis of all vitamins, especially the ones lacking to varying degrees in diabetics, A, D, E and K(1 and 2).
The battle begins at the body tissue level for diabetics both type 1 & 2, as systematic biofilm infections live together with whatever toxic agent has allowed their growth to accelerate. (Youtube search biofilms/mycoplasma and type 1 diabetes for confirmation. Don't stop at just knowing the name. Get to know what they are).
'Silver' fillings are the secondary culprit beside stress and secondhand smoke in my case. Biofilm colonies thrive in the presence of metallic compounds, and amalgams are designed to kill colonising bacteria in the mouth - they also do so in the gut. There are /other/ gut flora components that thrive despite the metallic contaminants.
Once the epithelial lining of the gut wall is compromised by a reduction in the production of butyric acid in the colon, food particles and gut flora can spread easily into the surrounding tissues via the bloodstream.
The pancreas is just one of those organs first struck by those ingredients for disaster.
I suggest research on gut diversity, prebiotic and probiotic foods (fermented foods), the gut biome changes in type 1 diabetic children in mexico, techniques for combatting systematic candidiasis, and gentle detoxification therapies.