A few years ago, I recall one of my internal medicine teachers talking about an emerging theory that a form of diabetes may be the culprit behind Alzheimer's disease. I had not given much thought to the topic since, but recently stumbled upon an article in the New York Times, called "Is Alzheimer's Type 3 Diabetes?", which has resurrected my interest.
It has been previously shown that those with diabetes have an increased risk of developing Alzheimer's dementia, however since the brain is considered to be an insulin-independent organ, the possible mechanism by which this occurs remained elusive. According to an article posted on Diabetes Forecast, one of the proposed mechanisms by which brain may depend on insulin could include memory and learning processes. Indeed, if that is the case, then this may explain how the insulin-independence of brain in the form of blood sugar regulation has little to do with the decreased insulin sensitivity that may be necessary for other functions.
A small study published in the Archives of Neurology assessed whether intranasal injections of insulin could improve cognitive function in patients with Alzheimer Disease and Amnestic Mild Cognitive Impairment and reported positive preliminary results. Interestingly, they also reported that levels of amyloid and tau proteins were affected in the insulin-treated group compared to controls (these proteins have been strongly associated with Alzheimer's disease). Their findings open the door for larger scale studies to be conducted to validate and further assess this relationship.
A number of previous articles on MemberCentral have focused on Alzheimer's disease and I encourage you to check them out -- I have provided some links below.