People with diabetes are known to be at higher risk for developing Alzheimer’s disease.  However, the interaction between the two diseases is not well understood.  A French study published 10/27/2009 just found that Alzheimer’s progresses more slowly in people who have diabetes.   An article on DiabetesHealth.com says:

“Diabetes is thought to increase the risk of developing Alzheimer’s, possibly because of vascular damage in the brain that mimics the dementia seen with Alzheimer’s.  But once patients display symptoms of the disease, the current study suggests that the progression is slower than in people without diabetes. … One complication may be that the medications used to help control blood sugar have a protective effect on the brain….”

http://www.diabeteshealth.com/read/2009/10/29/6427/diabetes-and-alzheimers-disease/