I have previously posted regarding the effect of vitamin D on cognitive disability, autism and other brain disorders. With all of these disorders, there is a clear correlation between sunlight and/or vitamin D deficiency and decreased brain function. Considering that most elderly people are severely vitamin D deficient and that there are vitamin D receptors throughout the brain, it would not be surprising to also find the same correlation with Alzheimer’s, which is a brain disease. An excellent paper by Dr Frederick Dyer makes a case that Alzheimer’s is, at least in part, a vitamin D-deficiency disease.
Dr Dyer makes the point that many diseases or disorders—those that are well-established as correlating to vitamin D deficiency—are themselves risk factors for Alzheimer’s. These include depression, osteoporosis, diabetes, poor cognitive abilities, periodontal disease, dental caries, inflammation, tooth loss, low cognitive performance, poor strength, depression, congestive heart failure, peripheral artery disease, hypertension and arterial plaque. In other words, Alzheimer’s shows a "co-morbidity” with these disorders, meaning that they may have the same underlying causes. Lack of sunlight/vitamin D is likely one of those causes.
Until we know for sure, it is certainly a great idea to maintain high levels of vitamin D in ourselves and in our aging parents; there is no downside, and it may prevent us from losing our minds.
Another excellent paper on vitamin D and Alzheimer’s will be published soon; I will keep you posted.
 Dyer, F. Deficient Vitamin D in the Pathogenesis of Alzheimer’s Disease. Unpublished manuscript furnished to author December 2008. Used by permission.