This report at
http://www.news.colostate.edu/Release/4859
states that dementia in PD &AZ is caused by low levels of insulin and insulin growth factors. They say,
"....... these treatments aren’t a permanent solution because of the body’s inability to transfer insulin and insulin-like growth factors from the blood into the brain, called the blood-brain barrier."
They go on to say that a person with Alzheimer's or diabetes may have high levels of insulin in his blood, but the insulin in the brain is abnormally low because of a blockage between the blood and brain.
Around 80% of AZ sufferers have had diabetes.
Ron
A small section of the report is below.

Colorado State University Researchers Discovers What Causes Brain to Shrink; Provides Key to Solving Mysteries of Preventing Dementia
FORT COLLINS - Colorado State University researchers have discovered that low levels of two chemicals in the brain cause the brain to shrink -- a condition that leads to dementia – and the discovery could help scientists prevent the condition. While researchers have suspected that both insulin and insulin-like growth factors may play an important role in dementia, this new research shows for the first time that the two chemicals work together to prevent the brain from shrinking and that low levels are the cause of cell loss and eventual dementia.

“This research provides us with hope for the first time that scientists can find a way to actually prevent the progression of dementia in people with conditions such as Alzheimer’s, diabetes and Parkinson’s disease,” said Douglas Ishii, a professor in the Department of Biomedical Sciences and a leading researcher in the discovery.