Diabetes effects on nerves begin before the actual diabetes starts. And this can be many years for some people.

There is a paper on this in fact....

http://neurotalk.psychcentral.com/post810705-1.html

A long glucose tolerance test may show more for you. But you can test yourself if you have a glucometer. Do 1.5-2 hr after eating to see if you have spikes. Do testing before a meal to see if you get really low.

I just spent two days testing every 2 hours myself, because the amitriptyline was raising my sugars and right in there a tooth crown came off, due to a gum injury from the edge of the crown which had slipped to the side minutely...and I had swelling and a minor infection there. Those 2 things threw my morning fasting up 40 pts! Of course just being phobic about going to the dentist didn't help. My dentist was really nice... he fixed it all and showed me how it happened.(it is a gold crown, and metal and if they shift even a tiny bit, can cut the gum).

So if you have a highly reactive cortisol system, like I do, you can get high over various medical or emotional stressors.

This morning I am back to normal! 3 days of highs however!

Getting HbA1C done twice or 3 times a year, will show you if
you are spiking after eating. Large meals will spike a prediabetic more than smaller more conservative meals. So avoiding huge meals with long intervals, is important, to normalize the metabolsm.

Prediabetes can also be insulin resistance, and this shows up early as low or normal morning fasting for several years before the diabetes begins. Insulin resistance will show a reactive hypoglycemia on the long glucose tolerance test, and low blood sugars can impact nerves and starve them.