Journal of Neurological Science  Jan 20. [Epub ahead of print] (Bosco D, Plastino M, Cristiano D, Colica C, Ermio C, De Bartolo M, Mungari P, Fonte G, Consoli D, Consoli A, Fava A.) Dementia has been found to be associated with insulin resistance in Parkinson's Disease. Dementia commonly occurs in Parkinson's Disease when Parkinson's Disease progresses. This is not inevitable because the biochemistry of Parkinson's Disease and Dementia are completely distinct. There is therefore no reason why they should coincide. Their common association has never been fully explained apart from the fact that Dementia and Parkinson's Disease are both far more common with age.
People who have Parkinson's Disease and dementia were assessed for their resistance to insulin. When insulin is produced in order to prevent high blood glucose levels, insulin sometimes does not have affect. This can be due to insulin resistance, which is the inability of insulin to stimulate the insulin receptors. Brain function largely requires glucose in order to function. Nearly two thirds (62%) of people with Parkinson's Disease who had dementia were found to have insulin resistance. 30% of them also had impaired glucose tolerance. These percentages were significantly higher when the disease duration was longer and when the movement disability was worse. So dementia in Parkinson's Disease appears to be affected by the inability to make use of glucose rather than be a direct result of Parkinson's Disease.