Not exactly rocket science, is it? Part 2

If there's a deficiency in "X", taking supplement "X" will correct the deficiency in "X".
If problem "Y" is caused by a deficiency in "X", then taking supplement "X" will fix problem "Y".

If there's no deficiency in "X", taking supplement "X" won't do anything.
If problem "Y" isn't caused by a deficiency in "X", then taking supplement "X" won't fix problem "Y".

If a person spends a lot of time outdoors in skimpy clothing in sun that's higher than 45deg in the sky, it's highly probable that they will not be deficient in Vitamin D3. Therefore, supplementing with 5,000iu/day of Vitamin D3 will highly probably do nothing.

If the above person has type 2 diabetes, supplementing with 5,000iu/day of Vitamin D3 won't fix their type 2 diabetes.

Not exactly...

Rocket Science!

There will be some people for whom all of the supplements & exercise that I recommend don't help their type 2 diabetes. Sorry about that. A low-carb (but not very-low-carb) diet will minimise your serum glucose level fluctuations without increasing your serum NEFA level excessively. See The problem with Diabetes.