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An observational cohort study of 7,423 members of Kaiser Permanente Northern California (KPNC) showed that birth in a state with high stroke mortality was associated with a 28% higher risk of dementia after adjustment for age, sex, and race, according to Rachel Whitmer, PhD, of Kaiser Permanente, in Oakland, Calif, and colleagues.
Black individuals born in states with high stroke mortality had a 67% higher risk of dementia compared with non-blacks born outside of the so-called "stroke belt," the study authors reported online in JAMA Neurology. This was followed by black individuals not born in a high stroke mortality state, who had a 48% increased risk of dementia, and non-black persons born in a high stroke mortality state who had a 46% increased risk.
At age 65, the cumulative 20-year dementia risk for those born inside and outside a high stroke mortality state were 30.13% and 21.8%, respectively. The 9 states with high stroke mortality were Alabama, Alaska, Arkansas, Louisiana, Mississippi, Oklahoma, Tennessee, South Carolina, and West Virginia, many of which are part of the so-called "stroke belt."
"Place of birth has enduring consequences for dementia risk and may be a major contributor to racial disparities in dementia," the study authors said, noting that the latter "may partially reflect geographic patterning of early-life exposures."
"Our findings underscore the importance of examining early life factors when researching dementia risk as these may inform on the timing, populations, and mechanism of interventions," they added. "One's place of birth can have lasting effects by influencing individuals' lifestyle and health behavior throughout adult life, even when residence has changed and access to healthcare is uniform."
In post-hoc analysis controlling only for age, black race was associated with a 56% increased risk of dementia. After adjustment for being born in a high stroke death state, the risk associated with being black "was attenuated by more than a third," Whitmer and colleagues wrote. "This has great public health relevance since many current black elderly individuals were born in the South and moved away during the 'great migration.'"
In an accompanying editorial, Daniel Lackland, DrPH, of the Medical University of South Carolina in Charleston, noted that while the effect of elevated blood pressure on cerebrovascular disease risk -- and the benefit of blood pressure reduction -- is well documented, understanding the influence of geography "remains at or near the same levels as in previous decades."
"The current study contributes a better understanding of the factors associated with the geographic disparity in cerebrovascular disease," Lackland wrote, adding that this is "a critical area of need given the high burden." The study authors have "restated the need for continued assessment of the excess disease burden in risks associated with dementia," he added.
For the study, data on place of birth, race, educational level, and midlife vascular risk factors were collected between 1964 and 1973 when the individuals had a mean age of 43 years. Some 55% were female and 18% were black. Between 1996 and 2015, dementia diagnoses were obtained from electronic health records, when the mean age of participants at the beginning of the 19-year follow-up period was 71 years.
Dementia was diagnosed in 30.4% and was more common among those born in states with high stroke mortality than those born elsewhere (39% versus 28.8%). Birth in the "stroke belt" was 9.6 times more common for black than non-black participants (58.7% versus 6.1%).
These findings are consistent with a previous study showing an elevated risk of dementia-related mortality after examining death records of individuals born in the stroke belt, Whitmer and colleagues noted. The southern U.S. region where many of the high stroke death states are located has been associated with the greatest number of people living in poverty and the largest racial disparities in cognitive outcomes, the investigators pointed out.
"Poverty early in life can reflect a host of factors that could affect brain health and cognitive reserve, such as nutrition, lead exposure, chronic stress, and cognitive stimulation," they said. "Poverty is highly associated with low birthweight and it is very likely that low birthweight disproportionately affected southern black persons in the 1920s, placing them at greater risk for elevated blood pressure, stroke, and cardiovascular disease mortality."
More research is needed to address the geographic patterns of dementia in different cohorts of aging populations to better understand the impact on health of birth place and the social and physical conditions to which individuals are exposed in early life, Whitmer and colleagues said.
Limitations of the study include the fact that the results are susceptible to selection bias, since the entire cohort migrated to California and "may be healthier or more highly educated than those who remained in a high stroke mortality state," the researchers said.
- Reviewed by Henry A. Solomon, MD, FACP, FACC Clinical Associate Professor, Weill Cornell Medical College and Dorothy Caputo, MA, BSN, RN, Nurse Planner
JAMA NeurologySource Reference: Whitmer RA, et al "Association between birth in a high stroke mortality state, race, and risk of dementia" JAMA Neurol 2017; DOI:10.1001/jamaneurol.2017.1553.
JAMA NeurologySource Reference: Lackland DT "Impact of birth place and geographic location on risk disparities in cerebrovascular disease: Implications for future research" JAMA Neurol 2017; DOI: 10.1001/jamaneurol.2017.1560.