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Effect of Social Determinants of Health on Cognition and Risk of Alzheimer Disease and Related Dementias

      Abstract

      Social determinants of health are the conditions in which people are born, work, live, and age and the wider set of forces and systems that shape the conditions of daily life. They affect every area of life, particularly health and health care. There is increasing focus on modifiable factors that affect cognition and risk of Alzheimer disease and related dementias (ADRDs). This article examines the impact of various social determinants of health, which are potentially reversible, on the incidence, prevalence, and risk of ADRDs and cognition. Various social determinants of health affect cognition and risk of ADRDs. Lower socioeconomic status (SES) and less education are associated with a higher incidence of ADRDs, whereas higher SES and education level appear to be protective, leading to a deceleration of time to diagnosis. In terms of employment, manual labor is associated with a higher risk of ADRDs. Higher body mass index in midlife and a decreasing body mass index in old age are associated with a higher risk of ADRDs. Furthermore, lower food security in early and late life is associated with a higher risk of ADRD diagnosis. Neighborhoods that are economically disadvantaged with fewer physical resources are associated with a higher risk of ADRDs. Higher levels of social engagement have a protective effect on diagnosis of ADRDs. Higher levels of stress are associated with a higher likelihood of developing ADRDs. Early-life adversity is associated with an increased risk of ADRDs, and further work in this area will be illuminating. Racial discrimination also leads to higher risk of ADRDs through the direct effect of discrimination and indirectly through lower SES, educational level, employment, and residential segregation. With an aim of reducing of ADRDs, future work in enhancing education, improving socioeconomic conditions, work, and neighborhood environments, and eliminating racial discrimination could potentially have a drastic impact.

      Key words

      Introduction

      Alzheimer disease and related dementias (ADRDs) refer to the cumulative of most common forms of neurodegenerative illnesses that present with dementia and cognitive impairment. Illnesses included under this umbrella term include Alzheimer disease, frontotemporal lobar degeneration, dementia with Lewy bodies, vascular dementia, and multifactorial dementias.

      NINDS. Focus on alzheimer's disease and related dementias. 2020; Available from: https://www.ninds.nih.gov/Current-Research/Focus-Disorders/Alzheimers-Related-Dementias.

      The prevalence of ADRDs is estimated to be 5 million adults older than 65 years in the United States, and these numbers are set to increase to an estimated 13 million in 2060.
      • Matthews K.A.
      • et al.
      Racial and ethnic estimates of Alzheimer's disease and related dementias in the United States (2015–2060) in adults aged ≥65 years.
      With these rapidly increasing numbers, there has been a significant focus on the modifiable factors that may cause or affect the prevalence of ADRDs.
      Social determinants of health (SDHs) are defined by the World Health Organization as “conditions in which people are born, work, live, and age, and the wider set of forces and systems shaping the conditions of daily life.”

      WHO. Social determinants of health. 2011; Available from: https://www.who.int/health-topics/social-determinants-of-health#tab=tab_1.

      These determinants are the nonmedical factors that affect and influence health outcomes. These factors include forces and systems such as political structures, socioeconomic constructs, and cultural norms. The Healthy Population initiative by the US Department of Health and Human Services conceptualized SDHs to have 5 main categories: economic stability, education, social and community context, health and health care, and neighborhood and built environment.

      U.S. Department of health and human services, o.o.d.p.a.h.p. Healthy people 2030. 2020; Available from: https://health.gov/healthypeople/objectives-and-data/social-determinants-health.

      The aim of formulating the concept of SDHs was to create a standardized framework of assessing the various social dimensions that affect the health outcomes for all the different populations.
      There is a considerable body of literature on the effect of SDHs. SDHs have been conceptualized as being upstream or downstream in nature. Downstream SDHs are factors that are temporally close to health effects and include health care, health-related attitudes, beliefs, and behaviors.
      • Braveman P.
      • Egerter S.
      • Williams D.R.
      The social determinants of health: coming of age.
      Upstream SDHs are the fundamental factors that set causal pathways in motion, leading to downstream health effects. These factors include educational level, early-life stress, intergenerational transfer of advantage and health, employment situation, as well as neighborhood conditions.
      • Braveman P.
      • Egerter S.
      • Williams D.R.
      The social determinants of health: coming of age.
      The categorization based on immediate and distant causality leads to a broader understanding of SDHs and their effect on health. This understanding then can lead to interventions at various levels to address the upstream and downstream SDHs.

      SDHs and ADRDs

      In this general overview, we summarize the evidence on the effect of individual SDHs on the incidence, prevalence, and risk of ADRD. For this review, studies that addressed cognitive function and rate of cognitive decline in response to various SDHs were also included. The SDH categories included here are economic stability, educational level, social and community context, health and health care, and neighborhood and built environment. A deeper understanding of SDHs in relation to cognition and ADRD risk is vital because these are potentially modifiable factors at systemic, community, and individual levels.
      The relationships among various SDHs and their effect on health outcomes are complex and highly interlinked.
      • Braveman P.
      • Egerter S.
      • Williams D.R.
      The social determinants of health: coming of age.
      Educational attainment, for example, affects health outcomes through multiple pathways. It can lead to varying levels of health-related behaviors; affect employment, working conditions, income, social standing, and social support; and cause stress. All these various pathways affect health outcomes. Hence, the ultimate effect on health outcomes of each factor is nuanced and complex. A full discussion of the causality and relationships is beyond this overview. However, the overall principles of interlinkage of SDHs also apply to the study of the effect of all individual SDHs on ADRD diagnosis.

      Economic Stability

      Socioeconomic Status

      Socioeconomic status (SES) is an SDH with a significant effect on ADRDs, with lower levels of wealth and SES being associated with a higher incidence of ADRD. Cadar et al
      • Cadar D.
      • et al.
      Individual and Area-Based Socioeconomic Factors Associated With Dementia Incidence in England: Evidence From a 12-Year Follow-up in the English Longitudinal Study of Ageing.
      determined that the incidence of dementia was 1.68 times higher for those in the lowest quintile of wealth compared with those in the highest quintile.
      • Cadar D.
      • et al.
      Individual and Area-Based Socioeconomic Factors Associated With Dementia Incidence in England: Evidence From a 12-Year Follow-up in the English Longitudinal Study of Ageing.
      In addition, lower SES is associated with an accelerated cognitive decline. Ajnakina et al
      • Ajnakina O.
      • Cadar D.
      • Steptoe A.
      Interplay between Socioeconomic Markers and Polygenic Predisposition on Timing of Dementia Diagnosis.
      found that intermediate and low levels of wealth were associated with an accelerated time to dementia diagnosis by 12 and 18 months, respectively. In a cross-sectional study in the United States, a clear relationship was observed between annual household income and subjective cognitive decline. Those with a higher annual income had considerably lower odds of subjective cognitive decline than those with an income >$10,000.
      • Peterson R.L.
      • et al.
      State inequality, socioeconomic position and subjective cognitive decline in the United States.
      Analysis of one longitudinal cohort study estimated the effect of SES during childhood and early and later adulthood. The study found that stable, high SES is associated with a better memory function at baseline and slower decline compared with low SES at all 3 points in life.
      • Marden J.R.
      • et al.
      Contribution of Socioeconomic Status at 3 Life-Course Periods to Late-Life Memory Function and Decline: Early and Late Predictors of Dementia Risk.
      Hence, lower SES (as measured by household income) is associated with a higher incidence of ADRD, whereas higher SES appears to be protective and leads to a deceleration of time to diagnosis.

      Employment

      Employment as an SDH seems to modify ADRD risk through the type of work. The literature suggests an association between less intellectually intensive employment and higher risk and incidence of ADRD. A systematic review in 2016 by Gracia Rebled et al
      • Gracia Rebled A.C.
      • et al.
      [Occupation and Risk of Cognitive Impairment and Dementia in People in over 55 Years: A Systematic Review, Spain].
      reported on longitudinal studies and concluded that individuals with manual occupations have a higher risk of developing cognitive impairment than others who had occupations with more intellectual workload. One longitudinal cohort study
      • Richards M.
      • et al.
      Identifying the lifetime cognitive and socioeconomic antecedents of cognitive state: Seven decades of follow-up in a British birth cohort study.
      from the United Kingdom found that increasing complexity of employment in midlife was modestly, but independently, associated with better cognitive functioning in later life. A retrospective case-control study
      • Nakahori N.
      • et al.
      A pathway from low socioeconomic status to dementia in Japan: results from the Toyama dementia survey.
      from Japan found that the odds of dementia were higher for those with a history of so-called blue-collar jobs than for white-collar jobs. Multiple studies have replicated this association with primary lifetime occupation, suggesting that occupations that require manual labor may be associated with worse cognitive performance, whereas more complex occupations with higher intellectual demands are associated with a lower risk of dementia.
      • Hughes T.F.
      • Ganguli M.
      Modifiable Midlife Risk Factors for Late-Life Cognitive Impairment and Dementia.
      A twin case-control study found that having a job with higher requirements of reasoning, mathematics, and language was associated with lower dementia risk.
      • Potter G.G.
      • et al.
      Job demands and dementia risk among male twin pairs.

      Food Insecurity

      Food insecurity, defined generally as limited access to food because of a lack of resources,
      • Na M.
      • et al.
      Food Insecurity and Cognitive Function in Middle to Older Adulthood: A Systematic Review.
      affects cognitive function across the lifespan. In a systematic review, Na et al
      • Na M.
      • et al.
      Food Insecurity and Cognitive Function in Middle to Older Adulthood: A Systematic Review.
      found that early-life food insecurity doubled the odds of dementia. A longitudinal study
      • Wong J.C.
      • et al.
      Food Insecurity Is Associated with Subsequent Cognitive Decline in the Boston Puerto Rican Health Study.
      found baseline food insecurity to be associated with a 2-year decline in cognitive functioning, with worse food security leading to accelerated decline in cognitive function.

      Education

      Educational level affects ADRDs in various studies and settings. The most consistent findings are an association between low educational level and higher risk of ADRDs and lower baseline cognitive scores. A systematic review by Sharp et al
      • Sharp E.S.
      • Gatz M.
      Relationship between education and dementia: an updated systematic review.
      found that, of the 88 studies included, 51 studies (58%) reported a significant effect of lower educational level on risk of dementia. The greatest risk of dementia was found in those with the lowest levels of education.
      • Sharp E.S.
      • Gatz M.
      Relationship between education and dementia: an updated systematic review.
      Many studies have also found a protective effect of higher educational levels. The Chicago Health and Aging Project (CHAP), a longitudinal, population-based study, found that more years of formal education were associated with a higher baseline global cognitive score as well as a lower risk of Alzheimer disease (AD).
      • Weuve J.
      • et al.
      Cognitive Aging in Black and White Americans: Cognition, Cognitive Decline, and Incidence of Alzheimer Disease Dementia.
      Studies also suggest that education may be protective against cognitive decline. The Analysis of the Health and Retirement Study found that higher educational level decreases the probability of cognitive impairment, with highly educated individuals living up to 1.6 to 1.9 years less with cognitive impairment compared with less well-educated peers.
      • Reuser M.
      • Willekens F.J.
      • Bonneux L.
      Higher education delays and shortens cognitive impairment: a multistate life table analysis of the US Health and Retirement Study.
      The literature indicates a clear association between lower educational level, quantified in number of years, with higher risk of ADRDs, whereas more education is associated with higher baseline cognitive functions and a deceleration to the time of ADRD diagnosis.

      Health and Health Care

      Body Mass Index

      The relationship between weight and ADRDs is complex, with evidence of associations with low or high body mass index (BMI) in different stages of life. Midlife obesity, defined as a BMI of >30 kg/m2, has been found in several large meta-analyses to be associated with increased risk of developing dementia.
      • Anstey K.J.
      • et al.
      Body mass index in midlife and late-life as a risk factor for dementia: a meta-analysis of prospective studies.
      ,
      • Kivimäki M.
      • et al.
      Body mass index and risk of dementia: Analysis of individual-level data from 1.3 million individuals.
      However, the relationship between ADRD risk and BMI changes in late life. In a longitudinal cohort of older clergy (>70 years of age), a decrease of one unit in BMI was associated with a 35% increase in the risk of AD compared with persons with no change in BMI.
      • Buchman A.S.
      • et al.
      Change in body mass index and risk of incident Alzheimer disease.
      Other studies also reported similar findings with additional reports of faster rate of cognitive decline in those with decreasing BMI.
      • Arvanitakis Z.
      • et al.
      Body Mass Index and Decline in Cognitive Function in Older Black and White Persons.
      ,
      • Wagner M.
      • et al.
      Evaluation of the Concurrent Trajectories of Cardiometabolic Risk Factors in the 14 Years Before Dementia.

      Stress

      Stress is an important SDH and is postulated to affect health via its affects on the hypothalamus-pituitary-adrenal axis as well as via inflammation.
      • Braveman P.
      • Egerter S.
      • Williams D.R.
      The social determinants of health: coming of age.
      Knowledge about the impact of stress, particularly through the impact of glucocorticoids on the hippocampus and frontal lobes,
      • Lupien S.J.
      • et al.
      The effects of stress and stress hormones on human cognition: Implications for the field of brain and cognition.
      has led to interest in interventions aimed at addressing the overall long-term physical effects of stress on the body.
      • Wilkinson R.G.
      • et al.
      Social determinants of health: the solid facts /edited by Richard Wilkinson and Michael Marmot.
      The effect of stress on ADRD diagnosis has been studied, with higher stress levels correlated with poor baseline cognition and higher ADRD risk. In a 6-year longitudinal cohort study, higher levels of perceived stress were related to a lower baseline cognitive score as well as a faster rate of cognitive decline.
      • Aggarwal N.T.
      • et al.
      Perceived stress and change in cognitive function among adults 65 years and older.
      A longitudinal cohort study in the African American population found that higher perceived stress is associated with a faster rate of cognitive decline.
      • Turner A.D.
      • et al.
      Perceived Stress and Cognitive Decline in Different Cognitive Domains in a Cohort of Older African Americans.

      Neighborhood and Built Environment

      ADRDs are also affected by a neighborhood's environmental quality and local resources, which are often studied by measuring the socioeconomic status of the neighborhood demographics. A systematic review by Wu et al
      • Wu Y.T.
      • Prina A.M.
      • Brayne C.
      The association between community environment and cognitive function: a systematic review.
      found that most studies had associations between local deprivation and poor cognitive function, cognitive decline, and higher risk of cognitive impairment. Local deprivation was measured by community SES and deprivation indexes. In a retrospective cohort study based in Hong Kong,
      • Guo Y.
      • et al.
      Neighborhood environment and cognitive function in older adults: A multilevel analysis in Hong Kong.
      neighborhood economic disadvantage was associated with cognitive decline. The association between economically disadvantaged neighborhood and cognitive decline was also seen in a systematic review by Besser et al.
      • Besser L.M.
      • et al.
      Neighborhood Environment and Cognition in Older Adults: A Systematic Review.
      Clarke et al
      • Clarke P.J.
      • et al.
      Cognitive decline and the neighborhood environment.
      investigated the effect of neighborhood environment, stratified by the presence of community centers, vicinity to public transport, and neighborhood walkability, as well as quality of public spaces. The presence of neighborhood resources was associated with a slower rate of cognitive decline.
      • Clarke P.J.
      • et al.
      Cognitive decline and the neighborhood environment.
      A longitudinal study
      • Luo Y.
      • Zhang L.
      • Pan X.
      Neighborhood Environments and Cognitive Decline Among Middle-Aged and Older People in China.
      over four years in China found that older adults living in neighborhoods with better social and physical resources had better cognitive function and slower rate of cognitive decline.
      The environmental attributes of neighborhoods, such as air quality, may also affect ADRD. In a retrospective cohort study
      • Carey I.M.
      • et al.
      Are noise and air pollution related to the incidence of dementia? A cohort study in London, England.
      in the United Kingdom, higher levels of residential exposure to air pollutants, such as nitrogen dioxide and particulate matter, were associated with a higher risk of dementia. In a systematic review,
      • Peters R.
      • et al.
      Air Pollution and Dementia: A Systematic Review.
      13 studies from various parts of the world confirmed findings of a higher risk of dementia in settings of higher air pollution.

      Social and Community Context

      Social Engagement

      Low levels of social engagement (social exclusion or isolation) have also been viewed as an important SDH that affects cognition. One narrative review
      • Peterson R.L.
      • et al.
      The role of social and behavioral risk factors in explaining racial disparities in age-related cognitive impairment: a structured narrative review.
      found an overall association between social isolation and poor cognitive function in a range of studies. There are extensive data on social engagement that point to a protective effect of higher levels of middle- and late-life social engagement, reducing the risk of ADRD diagnosis in later life.
      • Livingston G.
      • et al.
      Dementia prevention, intervention, and care: 2020 report of the <em>Lancet</em>Commission.
      In a longitudinal, population-based, cohort study,
      • Krueger K.R.
      • et al.
      Social engagement and cognitive function in old age.
      increased levels of social engagement, as measured by social activity and support, were associated with improved cognitive functions at baseline. A retrospective analysis also found that higher numbers of social networks and levels of social engagement were associated with better baseline cognitive function. Furthermore, longitudinally, high numbers of social networks and higher social engagement reduced the rate of subsequent cognitive decline by 39% and 91%, respectively.
      • Barnes L.L.
      • et al.
      Social resources and cognitive decline in a population of older African Americans and whites.
      These findings were also supported by subsequent meta-analyses.
      • Zhou Z.
      • Wang P.
      • Fang Y.
      Social Engagement and Its Change are Associated with Dementia Risk among Chinese Older Adults: A Longitudinal Study.
      ,
      • Penninkilampi R.
      • et al.
      The Association between Social Engagement, Loneliness, and Risk of Dementia: A Systematic Review and Meta-Analysis.

      Early-Life Adversity

      Early-life adversity is another SDH that may have relevance for ADRD. Analysis from CHAP found that early-childhood adversity was associated with poorer baseline cognitive performance in African American participants. This difference was not mediated by years of education.
      • Barnes L.L.
      • et al.
      Effects of early-life adversity on cognitive decline in older African Americans and whites.
      A cross-sectional study from Australia used the Childhood Trauma Questionnaire (CTQ) to assess for early-life adversity in the Aboriginal Australian population. The authors found that higher CTQ scores were associated with a higher risk of AD in later life. This association remained even when depression and anxiety as well as posttraumatic stress disorder were controlled for in their analysis.
      • Radford K.
      • et al.
      Childhood Stress and Adversity is Associated with Late-Life Dementia in Aboriginal Australians.

      Racial Discrimination

      Discrimination has been conceptualized as a significant SDH.

      WHO. Social determinants of health. 2011; Available from: https://www.who.int/health-topics/social-determinants-of-health#tab=tab_1.

      Discrimination is defined as the differential treatment of a group or individual, which is unfair or unjustified, on the basis of differences (race, ethnicity, sex, sexuality) that lead to disadvantages or harm to the group or individual.

      Services, U.S.D.o.H.a.H. Healthy people 2020-discrimination. 2020 [cited 2021; Available from: https://www.healthypeople.gov/2020/topics-objectives/topic/social-determinants-health/interventions-resources/discrimination.

      ,

      Council, N.R., Measuring Racial Discrimination, ed. R.M. Blank, M. Dabady, and C.F. Citro. 2004, Washington, DC: The National Academies Press. 334.

      In addition to differential treatment, discrimination also includes a disparate effect, wherein rules or procedures lead to some groups or individuals being favored over others.
      • Pager D.
      • Shepherd H.
      The Sociology of Discrimination: Racial Discrimination in Employment, Housing, Credit, and Consumer Markets.
      It is well known that minority ethnic populations, including African American and Hispanic populations, have a higher incidence and prevalence of dementia compared with their white counterparts.
      • Matthews K.A.
      • et al.
      Racial and ethnic estimates of Alzheimer's disease and related dementias in the United States (2015–2060) in adults aged ≥65 years.
      ,
      • Gurland B.J.
      • et al.
      Rates of dementia in three ethnoracial groups.
      ,
      • Mayeda E.R.
      • et al.
      Inequalities in dementia incidence between six racial and ethnic groups over 14 years.
      Minority ethnic populations have also been shown to present with greater levels of cognitive impairment and have a faster decline in cognition.
      • Cooper C.
      • et al.
      A systematic review and meta-analysis of ethnic differences in use of dementia treatment, care, and research.
      ,
      • Rajan K.B.
      • et al.
      Race, APOEɛ4, and Long-Term Cognitive Trajectories in a Biracial Population Sample.
      Given the considerable racial disparities, we focus on the effect of racial discrimination on ADRDs. The effect of racial discrimination can be viewed at 2 different levels: macro (structural racism or discrimination) and individual. Residential segregation and inequitable distribution of health care and education are all examples of structural discrimination.

      Services, U.S.D.o.H.a.H. Healthy people 2020-discrimination. 2020 [cited 2021; Available from: https://www.healthypeople.gov/2020/topics-objectives/topic/social-determinants-health/interventions-resources/discrimination.

      ,
      • Williams D.R.
      • Lawrence J.A.
      • Davis B.A.
      Racism and Health: Evidence and Needed Research.
      Individuals also face discrimination, which has been studied by the effect of perceived racial discrimination.
      Perception of discrimination itself seems to affect cognitive function, which may affect the risk of dementia. Data on the effect of perceived discrimination on ADRD risk are limited, and most available data address cognitive function. A first of its kind cross-sectional study by Barnes et al
      • Barnes L.L.
      • et al.
      Perceived discrimination and cognition in older African Americans.
      found that perceived discrimination was associated with poor global cognitive function. A retrospective analysis of the Health and Retirement Study cohort found that non-Hispanic Black people had the highest perceived discrimination, followed by Hispanic people. Non-Hispanic Black people had worse memory at initial presentation, via impact of depressive symptoms and loss of external locus of control on memory. They also had a faster rate of cognitive decline, which appeared to be due to perceived discrimmination.
      • Zahodne L.B.
      • Sol K.
      • Kraal Z.
      Psychosocial Pathways to Racial/Ethnic Inequalities in Late-Life Memory Trajectories. The journals of gerontology.
      Another prospective cohort study
      • Coogan P.
      • et al.
      Experiences of racism and subjective cognitive function in African American women.
      of African American women found that the experiences of both individual and institutional racism were associated with worse subjective cognitive function. Racial discrimination is generally associated with poor mental and physical health, which themselves appear to increase the risk of dementia.
      • Paradies Y.
      • et al.
      Racism as a Determinant of Health: A Systematic Review and Meta-Analysis.
      Structural racial discrimination acts through various SDHs, such as educational level, SES, employment, and neighborhood status, and can affect cognition and risk of ADRDs. Educational attainment has racial differences: only 22% of Latino adults (≥25 years of age) had an associate's degree or higher compared with non-Hispanic white adults (46%) and black adults (31%) according to the US Census Bureau.

      Santiago DA, C.-G.E., Taylos M. The condition of latinos in education: 2015 factbook. 2015 [cited 2020; Available from: https://www.nccpsafety.org/assets/files/library/The_Condition_of_Latinos_in_Education.pdf.

      Generally, Black study participants have fewer years of formal education compared with their White counterparts.
      • Weuve J.
      • et al.
      Cognitive Aging in Black and White Americans: Cognition, Cognitive Decline, and Incidence of Alzheimer Disease Dementia.
      ,
      • Sisco S.
      • et al.
      The role of early-life educational quality and literacy in explaining racial disparities in cognition in late life. The journals of gerontology.
      In a longitudinal study, African Americans with high educational levels had almost twice higher risk of dementia than White people with high educational levels. In addition, the risk of highly educated Black individuals was the same as that of White individuals with low educational levels.
      • Shadlen M.F.
      • et al.
      Education, cognitive test scores, and black-white differences in dementia risk.
      The decreased protective effect of higher educational level in the African American population may be attributable to poorer quality of education, as found in a longitudinal cohort study
      • Sisco S.
      • et al.
      The role of early-life educational quality and literacy in explaining racial disparities in cognition in late life. The journals of gerontology.
      in which accounting for early-life educational quality moderated the racial disparity in cognitive function.
      Racial differences in SES also affect ADRD diagnosis. The distribution of annual household median incomes by race is variable: non-Latino White people earn $62,950, Black people earn $36,898, and Hispanic people earn $45,148.

      Proctor, B.D., J.L. Semega, and M.A. Kollar, Income and poverty in the United States: 2015. US Census Bureau, Current Population Reports, 2016: p. P60–256.

      One longitudinal study
      • Yaffe K.
      • et al.
      Effect of socioeconomic disparities on incidence of dementia among biracial older adults: prospective study.
      found significant racial differences in rates of dementia development. However, once socioeconomic differences were accounted for, the differences became insignificant. Another prospective cohort study
      • George K.M.
      • et al.
      Life-Course Individual and Neighborhood Socioeconomic Status and Risk of Dementia in the Atherosclerosis Risk in Communities Neurocognitive Study.
      found that a higher level of SES across the lifespan was associated with lower risk of dementia, with the effect being more pronounced for African American populations than for non-Hispanic White populations. This positive impact was also shown in another cohort study from Baltimore.
      • Schwartz B.S.
      • et al.
      Disparities in cognitive functioning by race/ethnicity in the Baltimore Memory Study.
      Racial discrimination in employment has also been studied. Data indicate that African American males spend more time in search of employment, gain less work experience, and have less stable employment compared with their white counterparts.
      • Tomaskovic-Devey D.
      • Thomas M.
      • Johnson K.
      Race and the accumulation of human capital across the career: A theoretical model and fixed-effects application.
      Furthermore, African American men and women are paid less than their White counterparts.
      • Cancio A.S.
      • Evans T.D.
      • Maume Jr, D.J.
      Reconsidering the declining significance of race: Racial differences in early career wages.
      Closely related to SES and employment is the concept of intergenerational drag, which refers to the passing of social assets and liabilities to descendants. Hence, the effects of educational attainment and wealth of one generation can affect the SES of the next generation, leading to the perpetuation of existing disparities.
      • Gee G.C.
      • Ford C.L.
      Structural Racism and Health Inequities: Old Issues, New Directions.
      Racial residential segregation, defined as “the occupancy of different neighborhood environments by race,”
      • Williams D.R.
      • Lawrence J.A.
      • Davis B.A.
      Racism and Health: Evidence and Needed Research.
      is an important manifestation of systemic discrimination that affects overall health. Previous discussion indicates that neighborhood status can affect the risk and diagnosis of ADRDs. Residential segregation was instituted in the early 20th century by policies such as discriminatory zoning, mortgage discrimination, red-lining, and restrictive covenants.
      • Williams D.R.
      • Lawrence J.A.
      • Davis B.A.
      Racism and Health: Evidence and Needed Research.
      ,
      • WILLIAMS D.R.
      Race, Socioeconomic Status, and Health The Added Effects of Racism and Discrimination.
      Although recently decreasing in some areas, the levels of residential segregation among minority populations, particularly for African American populations in the United States, remain high.
      • White K.
      • Borrell L.N.
      Racial/ethnic residential segregation: framing the context of health risk and health disparities.
      Residential segregation frequently leads to underresourced neighborhoods, substandard educational opportunities, and decreased socioeconomic mobility.
      • Williams D.R.
      • Lawrence J.A.
      • Davis B.A.
      Racism and Health: Evidence and Needed Research.
      ,
      • Williams D.R.
      • Mohammed S.A.
      Racism and Health I: Pathways and Scientific Evidence.
      The deleterious effect of residential segregation has been quantified in many studies using the Area Deprivation Index to measure the neighborhood's socioeconomic disadvantage.
      • White K.
      • Borrell L.N.
      Racial/ethnic residential segregation: framing the context of health risk and health disparities.
      For example, a cross-sectional imaging study, using a mediation analysis, found that the Area Deprivation Index accounted for almost all the higher risk of AD found in African American participants.
      • Meeker K.L.
      • et al.
      Socioeconomic Status Mediates Racial Differences Seen Using the AT(N) Framework.

      Discussion

      The aim of this overview was to outline the effect of various SDHs on cognition and ADRDs, with a goal of ultimately highlighting areas for future work, innovation, and intervention. We found that various categories of SDHs affect cognitive function and risk of ADRDs. With the evidence of the significant effect of SDHs, policies and programs should be initiated and tailored to address the various disparities that lead to a higher risk of ADRDs. Focusing on education and providing good quality education early in life could potentially lead to a decrease in the risk of ADRDs in later life. Ensuring food security programs while addressing the obesity epidemic would have important downstream effects on ADRDs. Similarly, addressing neighborhood conditions and providing social and intellectual engagement programs from middle life onwards may prove to be protective against ADRDs. Socioeconomic inequality, possibly the most important factor tying together all the various SDHs, must take center stage. On a policy level, a more equitable distribution of wealth and resources would have a considerable direct and downstream effect on ADRDs. Structural racial discrimination needs to be addressed by legal mandates that expand early-life education, neighborhood resources, and access to employment opportunities in underprivileged areas with significant minority ethnic populations. More minority ethnic individuals need to be involved in future research as well.
      This overview attempts to present an up-to-date review of some SDHs and their effect on cognition and ADRDs. Although the commentary may be limited by choice of search terms for literature review and author bias, these issues may be addressed by future systematic reviews on the topic. There is also an overall dearth of data that involve some SDHs and ADRDs, such as early-life adversity. Furthermore, most of the studies included data on individual SDHs without extensive investigation into other, potentially confounding SDHs. The list of SDHs is exhaustive, and only some of the major and most pertinent factors have been highlighted in this review. Although there is evidence linked to individual SDHs, no SDH can be fully studied in isolation. The interactions between the various SDHs are dynamic and complex, requiring further research.

      Acknowledgments

      Primary literature review was performed by Dr Muniza Majoka with significant contributions to writing review by Dr Corbett Schimming.

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