Article In Brief
Black individuals who lived in racially segregated neighborhoods throughout young adulthood performed worse on certain cognitive tests as early as midlife.
Black Americans who live in segregated neighborhoods throughout young adulthood may have signs of possible cognitive decline by the time they reach midlife, according to a new study that tracked where participants lived over a span of 25 years.
While the study indicates an association, not a causal effect, between living in racially segregated neighborhoods and worse performance on cognitive testing in middle age, its findings point to yet another possible reason why there is a well-documented disparity in the risk of dementia for older black Americans compared with their white counterparts.
“The way we think conceptually about this is that residential segregation is really a factor upstream for a lot of other factors. When neighborhoods are segregated by race, that causes downstream effects such as low income and environmental exposures,” said lead author Michelle Caunca, PhD, who is a third-year medical student in the MD/PhD program at University of Miami Miller School of Medicine.
“The study gives us insight into how social and other contextual factors can play into cognitive pathology down the road,” Dr. Caunca said.
The investigators of the study, published online May 4 in JAMA Neurology, noted that previous studies have found “disparities in cognitive performance, risk of dementia and markers of brain aging between non-Hispanic black and white older adults in the United States, largely attributed to structural racism.”
The researchers—including senior author Adina Zeki Al Hazzouri, PhD, assistant professor of epidemiology at Columbia University Mailman School of Public Health—noted that racial residential segregation persists in the US, even though more than 50 years have passed since the enactment of the Fair Housing Act of 1968, which prohibited discrimination in the sale, rental and financing of housing based on race, religion and other factors.
“Residential segregation has been associated with worse built environments, psychosocial factors, educational quality, health behaviors, environmental exposures, and cardio-metabolic disease, all of which may contribute to cognitive and brain aging,” the researchers said. However, research examining a possible connection between racial residential segregation per se and cognitive function is sparse, they said.
Analysis Drew from Larger Study
To examine the issue, the researchers analyzed data collected on a cohort of black individuals participating in the Coronary Artery Risk Development in Young Adults (CARDIA) study. The CARDIA study is an ongoing, multicenter prospective cohort study of both black and white individuals that was designed to explore risk factors for the development of subclinical and clinical cardiovascular disease. It has generated hundreds of publications on risk factors such as diet, physical activity, stress, and healthcare access.
Participants were between the ages of 18 and 30 years at the start in 1985 and they have been followed up eight times since. At the 25-year examination in 2010, cognitive testing was added to the evaluation.
The new study on racial residential segregation was conducted using the 2010 cognitive testing results collected on 1,568 black CARDIA participants.
Racial residential segregation was determined by a statistical method that used census tracts to determine the racial composition of the neighborhood where the study participant lived compared with the surrounding community.
Using residential information collected in six follow-up visits over 25 years, the researchers calculated each participant's cumulative exposure to racial residential segregation, categorizing it as high, medium, or low. Those living in highly segregated neighborhoods were more likely to have fewer years of education, lower income, smoke and be less active.
The researchers considered cumulative racial residential segregation exposure in relation to the participants' performance on three cognitive tests, the Digit Symbol Substitution Test (DSST), Stroop color test (reverse coded) and the Rey Auditory Verbal Learning Test.
The DSST, which measures processing speed, stood out as being significant, while the other two tests did not reach statistical significance in the analysis.
The study found that “relative to living in low segregation neighborhoods, black participants who were exposed to highly segregated neighborhoods in young adulthood exhibited worse performance in processing speed,” the study said.
Greater cumulative exposure to segregated neighborhoods was associated with a worse DSST z score (for high segregation, β=−0.37); for medium segregation, β=−0.25) relative to exposure to low segregation, Dr. Caunca said.
The association between high racial residential segregation and poorer performance on the processing speed test accounted for income as it related to probability of segregation status, and it was also a stronger determinant than one year of added age.
The researchers concluded that racial residential segregation “may potentially explain black-white disparities in dementia risk at older age,” though they also said it was impossible to say precisely how that would be the case.
The study had limitations, including the fact that no black-white comparisons could be made since the cohort used for the analysis was all black. Also, cognitive performance was only considered at one point in time, so there was no way to see how midlife results compared with baseline or other earlier points.
Dr. Caunca said the “findings are generalizable in theory to similar communities across the US, but more work needs to be done to replicate the findings in other cohorts.”
A 2018 report from the Centers for Disease Control and Prevention said that among Americans age 65 and older in 2014, 13.8 percent of African-Americans had Alzheimer's disease or related dementia, compared with 1.0 percent of Hispanics and 10 percent of non-Hispanic whites. Such racial disparities exist for many health conditions and diseases, including the COVID-19 disease, which is disproportionately impacting black communities in this country.
The new study using the CARDIA data noted that “racial residential segregation has been called the fundamental cause of black-white health disparities because it influences the distribution of resources and opportunities that can protect health and/or increase risk of disease.”
William T. Hu, MD, PhD, FAAN, associate professor of neurology at Emory University School of Medicine, said he would have preferred it if the paper on racial residential segregation and cognitive performance would have weighed the strength of that association against other risk factors, such as diet and income volatility, that have been analyzed in the CARDIA study. He also thought it would have been helpful for the participants to take a full battery of neuropsychological tests, not just the three that were used in the analysis.
“Neighborhood segregation is certainly a serious issue that needs attention, Dr. Hu said. But suggesting there is “a connection between that and brain health needs more work, more validation.”
“Racial residential segregation could be a surrogate for other things that may be more amenable to intervention though public policy or clinical care,” Dr. Hu said. “Changing residential disparities is a lot more challenging than improving the residents' diet.”
Rachel Whitmer, PhD, professor of public health sciences at University of California Davis School of Medicine, said the study on racial residential segregation and midlife cognition is “really innovative and work like this is highly needed.”
While she said there are multiple factors that influence dementia risk—such as poorer quality of schools, more stress and often limited access to healthcare and parks in racially segregated communities with large inequities in distribution of wealth have worse health outcomes for all residents.
Dr. Whitmer, who uses large population-based cohorts to study racial and ethnic differences in risks for cognitive decline and dementia, said it was interesting that the new study looked at cognitive performance in midlife because that is a critical time.
“The fact we are seeing a signal in midlife is very compelling,” she said. “We know these changes in the brain happen over many years...and from young adulthood to midlife is really important.” But she stressed that dementia risk “is not set in stone by midlife,” and positive changes in lifestyle factors can make a difference as people age.
She said it would be helpful for clinicians to ask questions that provide insight into the environments in which their patients live because it could help inform possible intervention strategies, such as lifestyle changes and staying physically and socially active.
Lisa Barnes, PhD, the Alla V. and Solomon Jesmer Professor of Gerontology and Geriatric Medicine in the Rush Alzheimer's Disease Center at Rush University, said the CARDIA study used for the racial residential segregation analysis is a good cohort to examine mid-life risk factors associated with cognition because it is large, diverse, started during early adulthood and collected a wealth of data on many aspects of the participants' lives, including where they lived over the years. She said racial residential segregation “onto itself may not be a problem.”
“No one is saying it's a bad thing to live with people of your own culture, but in the US, racial residential segregation represents clustering of adversity and systematic disinvestment,” said Dr. Barnes, who is a cognitive neuropsychologist.
She said “the ability to engage in health-promoting behaviors is often lacking in minority neighborhoods that are highly segregated,” due to a lack of green space and gyms for physical activity, grocery stores with fresh fruits and vegetables, as well as exposure to pollutants and possibly higher crime.
“There is a lot of research to show that businesses do not stay in African-American communities,” even middle-class ones, Dr. Barnes said.
She conducts health disparities research among older adults that examines modifiable risk factors related to cognitive decline and Alzheimer's, and not all of them are obvious—factors like being treated unfairly, social isolation, loneliness, and lack of purpose.
“We really need to dismantle the policies that lead to racial residential segregation and cause (health) disparities,” Dr. Barnes said.
Drs. Cuanca and Barnes had no disclosures. Dr. Hu has consulted for ViveBio, LLC., AARP, Inc., and Biogeni, Inc; he has also received research support from Fujirebio USA.