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. Author manuscript; available in PMC: 2024 Mar 1.
Published in final edited form as: J Am Geriatr Soc. 2023 Jan 11;71(3):765–773. doi: 10.1111/jgs.18140

Social Isolation and 9-year Dementia Risk in Community-Dwelling Medicare Beneficiaries in the United States

Alison R Huang b, David L Roth a,b, Tom Cidav a, Shang-En Chung a, Halima Amjad a, Roland J Thorpe Jr a,b, Cynthia M Boyd a,b, Thomas KM Cudjoe a
PMCID: PMC10023331  NIHMSID: NIHMS1851189  PMID: 36628523

Abstract

BACKGROUND:

Social isolation can influence whether older adults develop dementia. We examine the association between social isolation and incident dementia among older adults in a nationally representative sample of community dwelling older adults in the United States (U.S.). We also investigate whether this association varies by race and ethnicity.

METHODS:

Data (N=5,022) come from the National Health and Aging Trends Study, a longitudinal and nationally representative cohort of older adults in the U.S. A composite measure of social isolation was used to classify older adults as socially isolated or not socially isolated at baseline. Demographic and health factors were measured at baseline via self-report. Dementia was measured at each round of data collection. Discrete-time proportional hazard time-to-event models were used to assess the association between social isolation and incident dementia over 9 years (2011–2020).

RESULTS:

Of 5,022 older adults, 1172 (23.3%) were socially isolated and 3,850 (76.7%) were not socially isolated isolated. Adjusting for demographic and health factors, being socially isolated (vs. not socially isolated) was associated with a 1.28 (95% CI: 1.10 – 1.49) higher hazard of incident dementia over 9 years. There was no statistically significant difference by race and ethnicity.

CONCLUSION:

Social isolation among older adults is associated with greater dementia risk. Elucidating the pathway by which social isolation impacts dementia may offer meaningful insights for the development of novel solutions to prevent or ameliorate dementia across diverse racial and ethnic groups.

Keywords: Social isolation, dementia, race, disparities

INTRODUCTION:

There is mounting evidence linking social isolation in older adults to greater risk of dementia. Social isolation is defined as an “objective state of having few social relationships or infrequent social contact with others.”1 Social isolation is a multi-dimensional construct characterized by structural (e.g. existence of social connections), functional (e.g. social support, resource sharing), and quality factors (e.g. relationship strain).1 In the United States, social isolation impacts approximately 1 in 4 older adults.2,3

Prior studies suggest socially isolated older adults have higher risk of incident dementia,4–7 but no study, to our knowledge, has described this longitudinal association in a nationally representative cohort of older adults in the United States (U.S.). Prior studies of U.S. older adults found associations between certain aspects of social isolation (e.g. poor social engagement, poor social support) and dementia, but these studies focused on older adults living in specific metropolitan areas8–11 and/or older adults with specific demographic characteristics (e.g. older women).12

Our study was informed by the conceptual foundation that upstream social and personal resources are linked to downstream health outcomes including cognitive health and function13,14 as well as the National Institutes of Aging Health Disparities Research Framework.15 Studies (within and outside the U.S.) have also yet to assess race and ethnicity group specific associations between social isolation and dementia. The U.S. population of older adults is racially and ethnically diverse and will continue to become more diverse over the next decades.16 It is important to understand how risk factors may impact dementia risk differently across race and ethnicity, especially in the U.S. The social isolation-dementia association may differ by race and ethnicity given disparities in both dementia incidence and prevalence of social isolation. Dementia incidence is higher in African American, Hispanic, American Indian or Alaska Native older adults compared to White older adults.17,18 Some studies have also found that social isolation is more prevalent in African American and Hispanic older adults (vs. White)19 but findings are mixed.1,2

In the current study, we examine the association between social isolation and incident dementia over 9 years in a large and nationally representative sample of Medicare beneficiaries in the US. We also assess differences in the social isolation – incident dementia association by race and ethnicity. This longitudinal study is the first, to our knowledge, to investigate potential racial and ethnic disparities in the association between social isolation and incident dementia.

Methods:

Subjects and Study Design

The National Health and Aging Trends Study (NHATS) is a nationally-representative, longitudinal cohort of U.S. Medicare beneficiaries aged 65 and above (N=8,245). The NHATS sample was drawn from the Medicare enrollment file. A stratified, three-stage sampling design was used: 95 counties were sampled from the contiguous U.S., then 655 zip codes were sampled from the counties, then 12,411 beneficiaries were sampled from the zip codes. Beneficiaries were sampled if they were age 65 or older as of September 30, 2010 with no date of death and resided within the 655 sampled zip codes. NHATS also included an oversample of older adults and Black individuals to maintain representation of these groups over time and to allow for adequate power to conduct subgroup analyses of these groups. Baseline data collection began in 2011 and is ongoing. Participants complete annual, two-hour, in-person interviews including assessments of function, economic and health status, and well-being.20,21 The current analysis uses data from 2011–2020. The analytic sample includes 5,022 community-dwelling older adults who attended, at minimum, both the baseline and first follow-up visits. Participants who, at baseline, were classified as having probable dementia and those who were missing social isolation data were excluded. Participants were also excluded if they were not community dwelling (e.g. institutionalized) or responded via a proxy respondent.

Measures

The National Health and Aging Trends Study includes domains that are relvant for the characterization of social isolation. Further, we utilize a previously characterized typology of structural social isolation that is informed by the Berkman Syme Network Index.2,22 These domains (living arrangement, discussion networks, and participation) are clinically relvant, practical and components of a comprehensive social history. The cut points employed are informed by previous studies that examine the association between social connections and morbidity/mortality.22,23 Individuals classified as socially isolated; often live alone, have no one or only one person that they can rely upon to discuss important matters, and they have no or limited engagement in social or religious groups. Furthermore, leveraging existing measures in NHATS, social isolation was characterized utilizing questions about: living with at least one other person, talking to 2 or more people about “important matters” in the past year, attending religious services in the past month, and participating in other activities (clubs, meetings, or group activities, or doing volunteer work) in the past month.2 Study participants received 1 point for each item/domain. Individuals who had a sum score of zero or one were classified as socially isolated and those with sum scores of two or more as not socially isolated.

NHATS developed a validated algorithm to identify dementia at each round of data collection. Participants are classified as having probable dementia if they meet any of the following three criteria: (1) self or proxy reported dementia diagnosis, (2) cognitive test performance ≤1.5 standard deviation below the mean of self-respondents in at least 2 of 3 cognitive domains (memory [immediate and delayed 10-word recall], orientation [date, month, year, day of the week, naming the President and Vice President], and executive function [clock drawing test] are assessed), or (3) score indicating probable dementia (score ≥ 2) on the AD8 Dementia Screening Interview for participants with a proxy respondent.24 This empirical classification of dementia applied in NHATS is widely used. Compared to clinical diagnosis of dementia in the The Aging, Demographics, and Memory Study (ADAMS), specificity of “probable dementia” is 87.2% and sensitivity is 65.7%.25

Covariates included demographic factors (age, sex [female, male], race/ethnicity [White, Black, Hispanic, Other (American Indian, Alaska Native, Asian, Native Hawaiian, Pacific Islander)], education [none or less than high school [H.S.], H.S. graduate/equivalent or vocational/trade diploma, and some college and beyond]) and health factors (number of chronic conditions [0, 1 or 2 conditions or ≥ 3 conditions], body mass index, and current smoking status [not smoking, currently smoking]).21 Covariates were chosen based on prior evidence of a confounding relationship or if there is statistical support for inclusion. All covariates were measured at baseline.

Statistical Analysis

The distribution (mean [standard deviation] or frequency [proportion]) of baseline participant characteristics was calculated for the total sample and by social isolation status (no social isolation, social isolation). Using a discrete time proportional hazards model, we modeled the association between social isolation and incident dementia over 9 years of follow-up. The time-scale was study time with the origin modeled at Round 1 (study initiation) and the interval between round 1 and round 2 was scaled as year 1. Individuals lost-to-follow-up or deceased prior to the end of the study period were censored. Data were fit to 3 models using a nested model building approach: (1) unadjusted; (2) adjusted for age, sex, education, and race/ethnicity; and (3) additionally adjusted for number of medical conditions, body mass index, and current smoking status. An interaction term between social isolation and race/ethnicity was added in secondary analyses to assess differences in the social isolation – incident dementia association by race/ethnicity. Survey weights were used in discrete time proportional hazard analyses to account for the complex sampling survey design. Additionally, to aid in the interpretation of the estimated cause-specific relationship between social isolation and incident dementia, we conducted a competing risks sensitivity analysis using Cox proportional hazards models to compare risk of non-dementia death (dementia prior to death treated as a censoring even) in persons with and without social isolation.26 All analyses were conducted in SAS 9.4.

RESULTS

Characteristics of baseline participants for the total sample and by social isolation status are shown in Table 1. The analytic sample at baseline included 5,022 older adults who had a mean age of 76.41 years (SD: 7.32) and included mostly individuals who reported being female (57.21%) and White, Non-Hispanic (71.71%), with 42.35% having more than a college education. Overall, 23.3% percent of the sample was classified as socially isolated and 21.1% developed dementia by the end of the follow-up period. Socially isolated older adults were more likely to develop dementia by the end of the follow-up period (25.9%) compared to older adults not socially isolated (19.6%) (Table 1, Supplemental Table 1) Average follow-up time was 5.1 years and similar by social isolation status (social isolation: 4.5 years, no social isolation: 5.3 years).

Table 1.

Select characteristics of baseline participants in the National Health and Aging Trends Study

Variable Total sample (n=5022) Social Isolation (n=1172) No Social Isolation (n=3850)
Age at baseline, mean (SD) 76.41 (7.32) 77.39 (7.81) 76.11 (7.14)
Sex, n(%)
 Female 2873 (57.21) 589 (50.26) 2284 (59.32)
 Male 2149 (42.79) 583 (49.74) 1566 (40.68)
Race, n(%)
 White, non-Hispanic 3574 (71.71) 839 (72.14) 2735 (71.58)
 Black, non-Hispanic 1041 (20.89) 243 (20.89) 798 (20.88)
 Hispanic 257 (5.16) 49 (4.21) 208 (5.44)
 Other 112 (2.25) 32 (2.75) 80 (2.09)
Education, n(%)
 Less than high school 1129 (22.62) 392 (33.68) 737 (19.25)
 High school graduate, equivalent, or vocational/trade 1749 (35.04) 412 (35.40) 1337 (34.93)
 Some college or beyond 2114 (42.35) 360 (30.93) 1754 (45.82)
Number of chronic conditions, n (%)
 0 conditions 451 (8.98) 94 (8.02) 357 (9.27)
 1 or 2 conditions 2236 (44.52) 478 (40.78) 1758 (45.66)
 3 or more conditions 2335 (46.50) 600 (51.19) 1735 (45.06)
Body mass index, mean (SD) 27.80 (5.56) 27.56 (5.74) 27.88 (5.51)
Smoking status, n (%)
 Currently smoking 390 (7.77) 159 (13.57) 231 (6.00)
 Not smoking 4632 (92.23) 1013 (86.43) 3619 (94.00)
Probable dementia by end of follow-up 1058 (21.07) 303(25.85) 755(19.61)
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Notes:

The social isolation measure includes the following domains (1 point each): living with at least one other person, talking to 2 or more people about “important matters” in the past year, attending religious services in the past month, and participating in other activities (clubs, meetings, or group activities, or doing volunteer work) in the past month.2 A sum score of zero or one = socially isolated; sum scores of two or more = not socially isolated.

Other race includes American Indian, Alaska Native, Asian, Native Hawaiian, Pacific Islander

Chronic conditions include heart disease, hypertension, arthritis, osteoporosis, diabetes, lung disease, stroke, broken bone, and cancer

Over 9 years of follow-up, social isolation was significantly associated with 1.33 (95% CI: 1.13 – 1.56) higher hazard of incident dementia after adjusting for demographic factors. The association persisted after further adjustment for health factors; social isolation was significantly associated with a higher hazard (1.27 95% CI: 1.08– 1.49) of incident dementia after full adjustment shown in Table 2 and the Forest plot shown in Figure 1).

Table 2.

Association between social isolation and incident dementia over 9 years in 5022 participants in the National Health and Aging Trends Study (2011–2020)

Variable Hazard Ratio (95% CI)
Model 1 Model 2 Model 3
Social isolation (ref: No Social isolation) 1.53 (1.30,1.81)*** 1.33 (1.13,1.56)*** 1.27 (1.08, 1.49)**
Age 1.11 (1.10,1.12)*** 1.11 (1.10, 1.12)***
Race (Ref: White, non-Hispanic)
 Black, non-Hispanic 1.63 (1.35, 1.98)*** 1.61 (1.34, 1.94)***
 Hispanic 2.05 (1.51, 2.79)*** 2.11 (1.54, 2.87)***
 Other 1.84 (1.13, 2.98)* 1.61 (0.97, 2.67)
Education(Ref: Some college or beyond)
 Less than high school 2.17 (1.86, 2.53)*** 2.08 (1.77, 2.45)***
 High school graduate/equivalent 1.29 (1.09,1.51)** 1.27 (1.07, 1.49**
Sex (Ref: Female)
 Male 0.94 (0.82, 1.07) 0.99 (0.86, 1.13)
Smoking (Ref: Not smoking)
 Currently Smoking 1.48 (1.12,1.95)*
Body mass index 0.99 (0.98,1.01)
Number. of chronic conditions (Ref: 0 conditions)
 1–2 conditions 1.13 (0.82,1.55)
 3 or more conditions 1.41 (1.01,1.96) *
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Notes:

Other race includes American Indian, Alaska Native, Asian, Native Hawaiian, Pacific Islander

Chronic conditions heart disease, hypertension, arthritis, osteoporosis, diabetes, lung disease, stroke, broken bone, and cancer

Model 1: unadjusted

Model 2: adjusted for age, race, education and sex

Model 3: adjusted for age, race, education, sex, smoking status, Body mass index, and number of chronic conditions

*

p < 0.05.

**

p <0.01.

***

p<0.001

Figure 1.

Figure 1.

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Forest plots of hazards risk between social isolation and incident dementia over a 9-year period in the National Health and Aging Trends Study (2011–2020)

With respect to examining the association between social isolation and incident dementia by race and ethnicity, we observed no statistically significant difference in the social isolation – incident dementia association by race and ethnicity (results not shown). To aid in the interpretation of our findings, we estimated the cause-specific hazard of death before dementia. Overall, 18% of participants died prior to dementia over follow-up. The cause-specific hazard ratio of death before dementia associated with social isolation was 1.28 (95% CI: 1.12, 1.47) (Supplemental Table 2).

DISCUSSION

In this nationally-representative sample of community dwelling older adults, social isolation is common, affecting nearly 1 in 4 older adults, and associated with increased risk of developing dementia over 9 years. The social isolation – incident dementia association did not significantly vary by race and ethnicity in this cohort; additional research is needed to further investigate this association among unique race and ethnicity groups.

This study is the first, to our knowledge, to examine the longitudinal association between social isolation and incident dementia in a nationally representative sample of U.S. older adults. Prevalence of social isolation was 23.3%, similar to prevalence reported in the Health and Retirement Study (19.0%), a nationally representative study of individuals over 50 years in the U.S.27 Our primary results are consistent with prior research on social isolation and dementia. A meta-analysis of 19 longitudinal cohort studies reported that low social participation and less frequent social contact were associated with a 41% and 57% higher risk of dementia,4 respectively. A larger meta-analysis of 33 longitudinal studies similarly found that having a poor social network and poor social support were associated with a 59% and 28% higher risk of dementia, respectively.28 The Lancet Commission also reported that, assuming a causal relationship between social isolation and dementia, 4% of dementia cases can be attributed to social isolation in later life.29,30

The null finding regarding potential difference in the social isolation - incident dementia association by race is potentially due to smaller sample sizes of participants who identified as Hispanic (n=257) and Other race (n=112). Additional research in this particular area and within racial and ethnic groups is needed. Older adults from certain racial and ethnic subgroups are historically underrepresented in dementia research.31 While modest progress has been made to increase recruitment and retention of older adults from underrepresented groups in dementia research,31 this work continues to advocate for greater representation of diverse racial and ethnic subgroups in longitudinal studies of aging in order to understand the critical implications of potential group differences in the social isolation – incident dementia association.32,33

The primary analyses estimated the cause-specific hazard of dementia before death by social isolation status. These estimates cannot be interpreted as the absolute risk of dementia over time associated with social isolation because dementia incidence depends on dementia risk as well as survival. So, to aid in the interpretation of our findings, we assessed the cause-specific hazard of death without dementia in secondary analysis. Estimated associations for social isolation were strong for both dementia and death, suggesting that differential survival by social isolation status may impact the time participants with social isolation were at risk for dementia compared with participants with no social isolation.

Several potential mechanisms may underly the observed association between social isolation and dementia. First, there is a strong link between social isolation and several physical and mental health risk factors for dementia (e.g. hypertension, coronary heart disease, depression). Social isolation may increase occurrence and severity of these risk factors which can subsequently lead to elevated dementia risk.1,6,34,35 Secondly, lack of social engagement can lead to reduced cognitive activity and lower cognitive reserve.36,37 The cognitive reserve hypothesis suggests individuals with lower levels of cognitive functioning and reserve are more susceptible to pathology associated with dementia. Finally, socially isolated older adults may fail to benefit from social resources (e.g. social support, information sharing, coordination of caregiving, health care access) that contribute to maintaining health and preventing poor health outcomes.1

Study Limitations

While we are unable to comment on other functional and quality components of social isolation, such as social support, loneliness, and relationship quality, our findings complement findings from other studies focusing on other aspects of social isolation. Future longitudinal studies of older adults should consider inclusion of multiple indicators of social isolation to thoroughly capture the various components of social isolation and their relationship to cognitive health.38 Additionally, as with all longitudinal studies, attrition occurred over time; participants who dropped out (30%) or transitioned to a residential care/nursing home (<1%) were censored. Participants who dropped out or transitioned to residential care/nursing home in this sample were slightly older and had lower levels of education (both dementia risk factors), thus selection of these participants out of the risk sets may also result in potential underestimation of dementia risk. Attrition was similar by social isolation status. Lastly, as the analytic sample includes only community-dwelling older adults, findings are not generalizable to institutionalized older adults, such as those residing in nursing homes or residential care facilities. Given high prevalence of both social isolation and dementia among residential/nursing home residents,39–43 our estimate of the social isolation-dementia association may be an underestimate of the association in the general population of older adults.

Implications

Social isolation for decades has been understood as an issue that impacts the wellbeing of individuals across the life course influencing risk for cardiovascular disease, mental health, and cognition. More recently, with the COVID-19 pandemic and investigations that have directly examined the health implications of social isolation, practitioners and policymakers are increasingly understanding the burdensome nature of this problem. Moving forward, from clinical and intervention perspectives, screening for social isolation in clinical settings may be valuable for maintaining health and mental health. Social isolation is also potentially modifiable and may be an additional focus for dementia prevention interventions. Several interventions have been designed to prevent social isolation among older adults44,45; however, no interventions, to our knowledge, have sought to increase social connectedness to prevent or delay dementia.

Continued progress in research and study design is needed to further understand social isolation and it’s association with dementia risk in different race and ethnicity groups. This is especially important in the U.S., given high and increasing racial and ethnic diversity within the population of older adults.16 In nationally representative cohort studies, recruitment and retention of large and diverse samples, with sufficient sample size maintained within certain racial and ethnic groups, is needed to effectively assess group-specific differences in longitudinal studies of older adults. Investigating social isolation and dementia in smaller diverse or racial and ethnic group-specific cohorts also provides opportunity to further our understanding of this association.

In a nationally representative study of US older adults, social isolation was associated with higher risk of dementia over 9 years. Future research should investigate racial and ethnic disparities in this association as well as assess the benefit of clinical screening and intervention as a potential approach for reducing risk of dementia among older adults.

Supplementary Material

supinfo

Supplemental Table 1: Characteristics of dementia classification among dementia cases, National Health and Aging Trends Study

Supplemental Table 2. Association between social isolation and death before dementia over 9 years in 5022 participants in the National Health and Aging Trends Study (2011–2020)

Key Points.

  • Among older adults in the Untied States, social isolation is common (1 in 4 adults experience social isolation) and associated with higher hazard of incident dementia over 9 years.

  • There were no observed differences in the association between social isolation and dementia by race and ethnicity.

Why Does This Matter?

Social isolation may be a valuable and modifiable risk factor to target in interventions for reducing dementia risk across diverse racial and ethnic groups.

Acknowledgements

Our families, patients, colleagues, and funders for their continued commitment to our interest and careers.

Disclosures:

Dr. Huang was supported by a National Institute of Aging Grant #1F31AG072746.

Dr. Cudjoe was supported by an National Institute on Aging Grant # 5R03AG064253, the Secunda Family Foundation, and the Patient-Centered Care for Older Adults with Multiple Chronic Conditions: Research and Mentoring Program Alzheimer’s Disease and Related Dementias (ADRD) supplement 3K24AG056578-02S1. Dr. Boyd was supported by the National Institute on Aging/National Institutes of Health Grant number K24 AG056578 and 1P30AG066587-01. Dr. Thorpe was supported by the National Institute on Aging Grant Numbers K02AG059140 and P30AG059298 and National Institute on Minority Health and Health Disparities Grant Number U54MD000214. Dr. Amjad was supported by NIA K23 AG064036.

Footnotes

Conflict of Interest

Authors of this manuscript have no relevant conflict of interest.

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Associated Data

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Supplementary Materials

supinfo

Supplemental Table 1: Characteristics of dementia classification among dementia cases, National Health and Aging Trends Study

Supplemental Table 2. Association between social isolation and death before dementia over 9 years in 5022 participants in the National Health and Aging Trends Study (2011–2020)

NIHMS1851189-supplement-supinfo.pdf (153.2KB, pdf)

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