I have endured over 10 years of useless, expensive and painful medical tests, only to be told over and over that it is "idiopathic" with no discernible cause. Finally, a doctor I spoke to asked about gluten sensitivity; I had never been asked before. My brother was born with celiac disease, and it does run in families; not hard to connect the dots.
I did a great deal of reading on the subject; the most helpful is Wiliam Davis' book, "Wheat Belly." He states absolutely that the explosion of diabetes is caused by wheat; and he has the research to prove it. According to Davis, if you eat a lot of gluten, by around age 50, you will likely start to experience nerve problems.
Here's what's wrong with wheat:
1. The gluten tends to burn out the receptors in your gut that process B vitamins. That's why so many of us have deficiencies. I have just finished my 6th month of going gluten-free, and have hopefully grown back most of my receptors.
2. Gliadin, the protein in wheat, can breach the blood/brain barrier, causing a great deal of damage, as well as being able to penetrate the intestines, causing toxins that you would want to eliminate to enter the blood stream directly. Given that, you name a condiltion, wheat either caused it, or is making it worse. Diabetes and arthritis are almost a slam dunk.

I have not bothered with more testing. I'm fanatically gluten free (it was even in my shampoo), and I'm working on building up my system with large doses of B-12, as well as other supplements that the wonderful folks on this site have recommended. I have lost 45 pounds of excess weight, which has made walking, etc. much easier.
There is diabetes on both sides of my family, and while I was never diagnosed with it, it always felt inevitable. Now I don't even consider it possible.
I still have PN, but for the first time, I'm optimistic about improvement. And happy not to be making the medical establishment wealthy at my expense.

Good luck to you in your journey toward feeling better.
Steven