Get the 25-hydroxyvitamin D, aka 20[OH]D, test done. Find out if you are deficient. Type 2 diabetes is thought to be multifactorial in nature. IIRC, over two dozen genes have been implicated in type 2 diabetes. This study was limited to: "The new study involved 81 South Asian women with insulin resistance living in New Zealand." People of color living in temperate climates are routinely found to be deficient in vitamin D.
A quart of fortified milk is only supposed to have 400 international units of vitamin D, IIRC. That level is still the recommended daily allowance, but many now suspect that level to be far below what is needed. (Search the keyword vitamind. I posted at least a two articles about that.)
Vitamin D in foods: development of the US Department of Agriculture database.
abstract excerpt
The Nutrient Data Laboratory has prioritized foods for analysis and has identified the following as important contributors of vitamin D: finfish and shellfish, naturally occurring sources, and fortified foods such as milk, calcium-fortified orange juice, breakfast cereals, American cheese, margarines, and yogurt.
Adequate levels of vitamin D may not be sufficient to avoid type 2 diabetes. Insulin resistance is also associated with obesity.
Correction: Get the 25-hydroxyvitamin D, aka 25[OH]D, test done.
Some typos are unreal.