Adding to the list of illnesses in which vitamin D levels have been found to be decreased, this study found moderately lower levels of vitamin D to be present in Parkinson's and Alzheimer's disease patients. As over 90% of our vitamin D comes from exposure to sunshine, it is possible that the vitamin D deficiency occurs because people with these problems simply cannot get outside as much as healthy people do. Nonetheless, I do recommend good overall nutritional support with a good multivitamin powder for people with these illnesses and for general health maintenance. A good multivitamin powder should contain 2,000 units of vitamin D per dose.
"Prevalence of vitamin d insufficiency in patients with Parkinson disease and Alzheimer disease," Evatt ML, Delong MR, et al, Arch Neurol, 2008; 65(10): 1348-52.