New research provides fresh insight into the socioeconomic factors that could explain why African Americans are more likely to develop Alzheimer's disease.
In total, the illness is estimated to affect more than 5 million U.S. adults, but African Americans are thought to be twice as likely to develop the condition than their white counterparts.
Until now, it was not known whether these disparities continue over the age of 90. The reasons for these discrepancies in incidence rates have also not been sufficiently explored.
Now, four new studies - presented at the 2017 Alzheimer's Association International Conference, held in London, United Kingdom - confirm that these racial disparities do persist into old age. They also shed light on some previously under-investigated risk factors for the illness.
African Americans at highest risk at age 90
Researchers jointly led by Rachel Whitmer, Ph.D., from the Kaiser Permanente of Northern California (KPNC) health center, and Dr. Maria M. Corrada, of the University of California, Irvine, set out to examine whether there are any racial differences in the incidence and risk of Alzheimer's disease among the so-called oldest-old.
They examined 2,351 seniors aged 90 and older, all of whom were registered with the KPNC health plan and who did not have dementia in 2010. The researchers examined the data available on the participants' electronic records between 2010 and 2015.
The racial distribution in the cohort was as follows: 72 percent of the participants were white Americans, 16 percent were African Americans, 4 percent were Latino, and 7 percent of the participants were Asian American.
In epidemiology, "cumulative incidence" refers to the total number of new disease cases in a given period of time divided by the number of people who were considered "at risk" at the beginning of the study period.
Overall, the study found that Asian Americans have the lowest incidence of Alzheimer's disease, while African Americans have the highest. Specifically, the African American oldest-old had a cumulative incidence of 39 percent, compared with that of Asian Americans, who had a cumulative incidence of 21 percent.
After adjusting for several factors - including education, sex, and other diseases in later life - African Americans over the age of 90 were still 28 percent more likely to develop Alzheimer's disease than their white American counterparts.
"These are the first estimates of dementia incidence in a diverse cohort of individuals age 90 and older. Given the increasing ethnic diversity in the coming decades, it is imperative to identify what factors contribute to the differences in rates, whether genetic, social, or lifestyle, as some of these factors may be modifiable."
Senior author Rachel Whitmer, Ph.D.
Stress puts African Americans at risk
A team of researchers led by Megan Zuelsdorff, Ph.D., of the University of Wisconsin School of Medicine and Public Health in Madison, decided to investigate the influence of lifetime stressors on the risk of cognitive impairment.
Dr. Zuelsdorff and team analyzed 1,320 patients from the Wisconsin Registry for Alzheimer's Prevention Study. The cohort consisted of 1,232 non-Hispanic whites and 82 African Americans.
The researchers examined the impact of stressful situations on the cognition of these two racial groups. Dr. Zuelsdorff and colleagues considered stressful experiences including losing one's job and facing financial insecurity, being raised by a parent with substance abuse issues, surviving the death of a child, or engaging in military combat.
The participants were asked to complete a questionnaire that probed their experiences and any potential psychosocial trauma that they might have faced. The participants were also asked to undertake a series of tests that measured their cognitive abilities - namely, their memory and problem-solving skills.
Overall, the study found that the more stressful events a person was subjected to in early life, the more impaired their cognition became in later life.
Specifically, in the case of African Americans, the study revealed that each stressful experience amounted to the equivalent of 4 years of cognitive decline.
Additionally, the study found that in a small population sample of highly educated individuals, African Americans were exposed to 60 percent more stressful situations over the course of their lifetime compared with their non-Hispanic white counterparts.
The authors note that according to their research, life adversity is a stronger predictor of cognitive performance than "established risk factors," such as age, education, and even the APOE e4 gene, which is known to significantly raise a person's risk of Alzheimer's disease.
Other risk factors for African Americans
Further findings presented at the conference include the link between a high infant mortality rate and the risk of developing Alzheimer's disease. Infant mortality rate is generally considered to be an indicator of the health and well-being of a population, and a high rate is seen as a marker of social adversity.
One of the new studies determined that African Americans who were born in states with a high infant mortality rate were almost 80 percent more likely to develop dementia than white Americans living in areas with low infant mortality rates. In the case of white Americans, the infant mortality rate seemed to have no impact on their chances of developing Alzheimer's.
Another study showed that living in a disadvantaged neighborhood significantly increased the odds of having Alzheimer's-related biomarkers and impaired cognition.
Neighborhoods were considered to be disadvantaged if they were "challenged by poverty, low education, unemployment, and/or substandard housing."
"For a racially diverse nation like the United States, and to address Alzheimer's and dementia on a global scale, these findings support the need for targeted interventions, whether preventive or service-driven, to help address the gaps we know exist - and for more research."
Maria C. Carrillo, Ph.D., Alzheimer's Association
"In addition to research on Alzheimer's risk factors and biology, the Alzheimer's Association is particularly interested in increasing understanding of stigma and concern related to Alzheimer's and other dementias in diverse communities," Dr. Carrillo adds.