I recently watched a webinar by Dr. Perlmutter, renowned integrative neurologist, based on research he has used in his soon to be released book, Grain Brain. In it he showed studies of how blood sugar levels are directly correlated with Alzheimers and Parkinson's disease. Not only are diabetics twice as likely to develop Alzheimer's as non-diabetics, but even people who have modestly elevated blood sugars (above 95) are at an increased risk.
Here's the link if you want to watch it. It's about an hour, but a VERY important hour. (It's not overly complicated or full of doctor-speak, either).