Alzheimer's Disease Declining in African Americans

Fran Lowry

August 14, 2015

Rates of Alzheimer's disease (AD) and other dementias in African Americans have been significantly declining during the past 2 decades, new research shows

However, rates of AD have remained unchanged in the Yoruba, a cohort of native West Africans.

The findings are published in Alzheimer's and Dementia.

Surprising Results

They come from the Indianapolis-Ibadan Dementia Project, a longitudinal study comparing dementia prevalence, incidence, and other health outcomes in two community-based cohorts: African Americans aged 70 years and older living in Indianapolis, and the Yoruba, aged 70 years and older, living in Ibadan, Nigeria.

The cohorts were recruited at both sites in 1992 and again in 2001, with a view to comparing incidence rates of AD in the two populations.

The current analysis, which was led by Sujuan Gao, PhD, from Indiana University School of Medicine, in Indianapolis, compared age-specific incidence rates for dementia and AD between the cohorts assembled in 1992 to those enrolled in 2001.

The 1992 cohort consisted of 1440 African Americans and 1774 Yoruba, and the 2001 cohort consisted of 1835 African Americans and 1895 Yoruba. None of the individuals had any evidence of dementia at the time of enrollment. The participants were evaluated every 2 to 3 years until 2009.

In the 1992 cohort, 191 of the 1440 African Americans (13.2%) had developed incident dementia. In the 2001 cohort, 94 of the 1835 African Americans (5.1%) were diagnosed with incident dementia.

The estimated annual incidence rate of dementia among African Americans dropped by more than half, from 3.6% in the 1992 cohort to 1.4% in the 2001 cohort.

Among the Yoruba, rates of incident dementia remained low. Of the 1992 cohort of 1174 Yoruba, 108 (09.2%) were diagnosed with dementia. Of the 2001 cohort of 1895 Yoruba, 82 (04.3%) were diagnosed with dementia.

The estimated annual incidence rate of dementia among Yoruba went from 1.7% in the 1992 cohort to 1.4% in the 2001 cohort.

"The rates in the Yoruba were lower to start with, but now they are about the same as those for the African Americans," senior author Hugh Hendrie, MBChB, DSc, from the Regenstrief Institute and the Indiana University Center for Aging Research, Indianapolis, told Medscape Medical News.

"When we looked at the comorbidities in the Indianapolis African American group, the 2001 cohort had higher rates of hypertension, diabetes, and stroke ― all the things that are considered risk factors for dementia ― and yet their incidence rates for dementia and Alzheimer's disease declined, whereas they stayed about the same in the Yoruba," Dr Hendrie said.

"We can only speculate as to why," he said.

"The major difference was that, even though the comorbidities had increased, so also were treatment rates, so many more African Americans with hypertension were being treated with antihypertensive medication, and similarly for those with diabetes, they were being controlled at increasing rates in the 2001 cohort than in the 1992 cohort. From that, we presume that perhaps it was a better treatment of these risk factors that was causing the decline in dementia and Alzheimer's disease rates, which is very reassuring," Dr Hendrie said.

"This is just one study. We'd like to see our findings replicated in more places across the country, although one or two are reporting lower rates now," he added.

Provocative Findings

Commenting on the findings for Medscape Medical News, Dillip V. Jeste, MD, Estelle and Edgar Levi Chair in Aging and Distinguished Professor of Psychiatry and Neurosciences at University of California, San Diego, said the new finding is "surprising."

Dr Dillip Jeste

"The authors postulate that it may reflect a greater use of cardiovascular and other medications to treat diabetes, hypertension, and stroke in the US. This suggestion is based on the fact that both in 1992 and 2001, cohorts of African Americans had similarly high rates of these disorders at baseline. However, the 2001 cohort in Indianapolis had significantly higher treatment rates with antihypertensive, antidiabetic, and lipid-lowering medications than the 1992 cohort. If this explanation is correct, it may indicate the importance of preventive measures, such as treatment of diseases predisposing to dementia, in seeking to lower its incidence," he said.

"Of course, it would be premature to make a definitive conclusion about the cause of reduced incidence of dementia, since various other medical and social factors might have contributed to the result," Dr Jeste added.

"Also, the generalizability of the findings remains to be tested in other populations of Africans and African Americans. Additionally, the participant refusal rate among African American individuals was higher in 2001 than in 1992. This might have introduced a selection bias. Notwithstanding these limitations, the findings of this study are provocative and deserve further research in attempts to replicate and interpret the results," he said.

Dr Hendrie and Dr Jeste report no relevant financial relationships.

Alzheimers Dement. Published online July 25, 2015. Abstract

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