Genetics versus the environment in Alzheimer’s Disease
Two studies
Japanese-Americans in Hawaii
There is evidence that environmental influences, such as diet and lifestyle, account for at least as much susceptibility to late-onset Alzheimer’s disease as do genetics. A study of Japanese-American men in Hawaii implies that diet and lifestyle play a significant role in the development of Alzheimer’s. the overall prevalence of dementia in Japan is similar to that in the United States and Europe. However, the prevalence of the different type of dementia is reversed. In Japan, vascular dementia is more common than Alzheimer’s dementia. In the United States and Europe, Alzheimer’s disease is more prevalent than vascular dementia. To be specific, vascular dementia is up to three times more prevalent in Japan than Alzheimer’s dementia. In European and American studies, Alzheimer’s dementia is two times more prevalent than vascular dementia. (Vascular dementia is a disease of the arteries that supply blood to the brain. Alzheimer’s is a disease of the nerve cells in the brain.)
In the Honolulu-Asia Aging Study published in the Journal of the American Medical Association in 1996, L. White and other researchers found the prevalence of Alzheimer’s disease in older Japanese-American men in Hawaii to be higher than in Japan and similar to European and American Caucasians. The difference between the Japanese in Japan and the Japanese in Hawaii is not genetics but environment, that is, diet and lifestyle.
African Americans in Indianapolis and Africans in Nigeria
Even more cogent evidence supporting the possibility that environmental influences have more impact on the development of Alzheimer’s disease than do genetics can be found in a study of H. C. Hendrie and others published in the Annals of Psychiatry in October 1995. The title of this article is, “Prevalence of Alzheimer’s disease and dementia in two communities: Nigerian Africans and African Americans.” this was a study on the prevalence of dementia and Alzheimer’s disease among two groups of subjects with the same ethnic background but widely differing environments. The study was conducted among residents aged 65 and older in two communities: Yoruba living in Ibadan, Nigeria, and African Americans living in Indianapolis, Indiana.
The age-adjusted prevalence rate for Alzheimer’s disease in the Ibadan sample was significantly lower than in the Indianapolis sample: among the Yoruba in Ibadan, Nigeria, the prevalence rate for Alzheimer’s disease was 1.41 percent; among the African Americans in Indianapolis, Indiana, the rate was 6.4 percent. The prevalence of Alzheimer’s disease in African Americans was more than four times the prevalence rate of Alzheimer’s disease in Nigerian Africans.
The authors note that this was the first study, using the same research method and the same group of investigators at two different sites, to report significant differences in rates of Alzheimer’s disease in two different communities with similar ethnic origins.
If genetics were the primary risk factor in Alzheimer’s disease, Japanese Americans and African Americans would have a risk of developing Alzheimer’s very similar to that of individuals living in their ancestral homes. The differences between Nigerian subjects and African Americans and between Japanese living in Japan and Japanese living in Hawaii would suggest involvement of environmental factors in the disease process of Alzheimer’s.
Next week’s article will investigate evidence from identical twins in determining whether environment can modify genetics in Alzheimer’s disease and, if so, to what extent.
Mary Lou Williams, M. Ed., is a lecturer and writer in the field of nutrition. She welcomes inquiries. She can be reached at (239) 267-6480.
By Mary Lou Williams, M. Ed.