A long-running study comparing African-Americans and Nigerians has found the incidence of dementia has fallen significantly over two decades among the African-Americans, but remained the same for the Nigerians (for whom it was lower anyway).
The study enrolled two cohorts, one in 1992 and one in 2001, who were evaluated every 2–3 years until 2009. The 1992 cohort included 1440 older African-Americans (70+) and 1774 Nigerian Yoruba; the 2001 cohort included 1835 African-Americans and 1895 Yoruba. None of the participants had dementia at study beginning.
The overall standardized annual incidence rate was 3.6% for the 1992 African-American cohort, and 1.4% for the 2001 cohort. For the Yoruba, it was 1.7% and 1.4%, respectively.
It's suggested that one reason for the improvement among African-Americans may be medications for cardiovascular conditions. Although both groups had similar rates of high blood pressure, this was recognized and treated in the American group but not in the Nigerian.
As you can see, African-Americans in the earlier cohort were more than twice as likely as Africans to develop dementia. Their decrease has brought them into line with the African rate.
Although the rate of new cases of dementia decreased, the African-Americans enrolling in 2001 had significantly higher rates of diabetes, hypertension and stroke, but also higher treatment rates, than the African-Americans who enrolled in 1992.
The finding offers hope that treatment can offset the expected increase in dementia resulting from the rise in lifestyle diseases.
 . Dementia incidence declined in African-Americans but not in Yoruba. Alzheimer's & Dementia [Internet]. Submitted . Available from: http://www.sciencedirect.com/science/article/pii/S1552526015021494