Race, Socioeconomic Resources, and Late-Life Mobility and Decline: Findings From the Health, Aging, and Body Composition Study

Abstract

Background.

This study examines the relationship between race and mobility over 5 years in initially well-functioning older adults and evaluates how a broad set of socioeconomic status indicators affect this relationship.

Methods.

Data were from 2,969 black and white participants aged 70–79 from the Health, Aging, and Body Composition study. Mobility parameters included self-reported capacity to walk a quarter mile and climb 10 steps and usual gait speed. Incident mobility limitation was defined as reported difficulty walking a quarter mile or climbing 10 steps at two consecutive semiannual assessments. Gait speed decline was defined as a 4% reduction in speed per year.

Results.

At baseline, even though all participants were free of mobility limitation, blacks had slower walking speed than their white counterparts, which was not explained by poverty, education, reading level, or income adequacy. After 5 years, accounting for age, site, and baseline mobility, blacks were more likely to develop mobility limitation than whites. Adjusting for prevalent conditions at baseline eliminated this difference in women; controlling for education eliminated this difference in men. No differences in gait speed decline were identified.

Conclusions.

Higher rates of mobility loss observed in older blacks relative to older whites appear to be a function of both poorer initial mobility status and existing health conditions particularly for women. Education may also play a role especially for men.

A key objective of the public health agenda is to reduce, if not eliminate, race-related disparities in health and function (1,2). Over the past two decades, little progress has been achieved as black older adults continue to exhibit poorer physical function relative to their white counterparts (2–10). The increasing number of aged minorities and magnitude of these race-related differences (11) demand a greater focus on elucidating factors that underlie race-related inequalities particularly regarding mobility decline, a critical contributor to loss of independence in community-dwelling older adults.

Socioeconomic status (SES) encompassing educational attainment and income is also strongly related to physical function in late life with the economically or educationally disadvantaged consistently showing poorer function (8,9). In the United States, race and SES are highly correlated with blacks having markedly fewer socioeconomic resources than whites (12–15). This strong relationship between race and SES complicates efforts to determine whether race and SES operate independently or jointly to produce disparities in functional status. Although observed race-related differences in physical function diminish after accounting for education and income (16–18), a residual race effect remains. Because educational quality, actual living costs, and accumulated wealth also vary greatly by race, other critical indicators of SES to consider include reading level, perceived income inadequacy, and home ownership among other assets (5,12,15,19–21).

Across the age spectrum in late life, black relative to white adults exhibit poorer health and functional status (20,22,23); yet our understanding of race differences in rates or trajectories of functional decline among older adults remains unclear. Some studies find that blacks have steeper disability trajectories than their white counterparts even after controlling for socioeconomic and health-related factors (7,24). Others find a crossover (16,25), or no differences in disability trajectories between black and whites independent of social and health status in late life (5,26,27). These mixed results may stem from differences in the age groups studied and the impact of selective survival whereby blacks who face adverse social and economic conditions die at younger ages resulting in a very healthy group reaching old age (28,29). It is important to also note that the healthiest individuals enter into and remain in the longitudinal studies (29). The inconsistent findings underscore the need for further research in race differences in functional decline among older adults.

Previously, we examined race differences in functionally limited older women and found blacks to have poorer mobility at baseline but similar rates of mobility decline relative to whites (23). We interpreted these findings as indicating that race- and SES-related disparities occur earlier in the disablement process. To test this presumption, we examine in the present study race-related differences in onset of mobility limitation and mobility decline over 5 years in an initially functionally intact community resident cohort aged 70–79 years and evaluate the degree to which a variety of SES indicators affect this relationship. We hypothesize that black older adults would exhibit poorer mobility at study outset and would also experience greater decline over 5 years than their white counterparts but that consideration of differences in a more comprehensive set of SES parameters would eliminate any observed race-related differences.

METHODS

Study Population

Data were drawn from the Health, Aging, and Body Composition study, a longitudinal cohort study of the interrelationships among body composition, incident and chronic health events, and development of functional difficulties in initially well-functioning black and white men and women. At study inception, a total of 3,075 persons aged 70–79 years, of which 42% were black and 48% were men, were identified from a random sample of white Medicare beneficiaries and all age-eligible black community-dwelling residents in predesignated zip code areas in Memphis, Tennessee, and Pittsburgh, PA. Baseline data were collected between April 1997 and June 1998 and consisted of a face-to-face interview and an extensive clinical evaluation. Participants were considered eligible if they reported no difficulty walking a quarter of a mile or climbing 10 steps without resting. Exclusion criteria included difficulty with activities of daily living, cognitive impairment, inability to communicate with the interviewer, intention of moving within the next 3 years, and receiving active treatment for cancer in the last 3 years. Participants were assessed annually in the clinic and semiannually by telephone. Participants unable to attend their annual clinic visit were offered a home visit or telephone follow-up. Our sample included 2,969 participants who self-reported the capacity to walk a quarter mile, climb 10 steps, and had gait speed assessed at baseline. Five years of follow-up were used in this study. All participants signed an informed written consent approved by the institutional review boards of the study sites.

Measures

Race, the main independent variable, was based on participant self-identification as either white or black.

SES measures included education, reading level, poverty status, perceived income inadequacy, number of financial assets, and home ownership. Education level was classified as less than high school, high-school graduate and some college, and college graduate and beyond. Because quality of education can vary by race and region (21), reading level was also assessed using the Rapid Estimate of Adult Literacy in Medicine tool, which is based on word recognition of 66 health-related terms (30). One point is given for each word correctly pronounced (30). Rapid Estimate of Adult Literacy in Medicine scores between 0 and 18 represent a reading level of third grade or less, scores between 19 and 44 represent a fourth- to sixth-grade reading level, scores of 45 to 60 represent a seventh- to eighth-grade reading level, and scores of 61 and above represent a ninth-grade reading level, which is consistent with the level of most patient education materials (30). For analyses, a binary variable was created that classified individuals as having a reading level below the ninth grade (Rapid Estimate of Adult Literacy in Medicine score <61) or not. Poverty status was determined by applying a set of income thresholds that vary by family size and composition issued in 1998 by the U.S. Census Bureau to reported annual income levels. Every participant was assigned a value representing a percentage of the federal poverty level. Poverty was defined as an income level at or below 150% of the federal poverty level . This indicator is commonly used in determining eligibility for federal and state public assistance programs (31). Because economic need can vary widely, perceived income adequacy was also considered. Participants who reported that their income poorly met their financial needs, they did not have enough money to buy food, or they did not have enough money at the end of the month were considered to perceive their income as inadequate. Number of financial assets comprised another dimension of SES (9,32). Participants reported whether they or a spouse had each of the following: money market account, savings bonds or treasury bills, investment property or housing other than current residence, a business or farm, stock or stock mutual funds, an individual retirement account or KEOGH account, and other investments. Responses were summed, and a dichotomous variable was created to differentiate individuals who owned fewer than three assets from those owning three or more. Because 265 participants (9%) were missing number of financial assets, a third category for missing cases was created to maintain the largest possible sample (32). Participants were also asked whether they owned their house, condo, or apartment. A binary variable was created to identify individuals who did not own a home.

Functional Status Measures

Baseline mobility status was assessed by self-report and observed performance. As a condition of study eligibility, all participants had reported no difficulty walking a quarter mile or climbing 10 steps. To define and differentiate levels of functional capacity, participants were asked how easy (ie, very easy, somewhat easy, or not so easy) it is for them to do each task (ie, walking a quarter mile or climbing 10 steps). Those reporting “not so” or “somewhat” easy were considered to have poorer mobility at baseline.

A performance-based assessment, usual gait speed, was measured by having the participant walk at a normal pace over a 6-m course. Walking aids were permitted at follow-up visits only. The faster of two walking trials constituted usual walking speed in meters per second (m/s).

Mobility Decline

Mobility decline was assessed by self-report and changes in gait speed over 5 years. Persons who reported any difficulty or inability to walk a quarter mile or climb 10 steps during any two consecutive semiannual assessments were considered to have declined and developed mobility limitation. For the analysis of incident mobility limitation, there was near-complete follow-up (only five persons were missing data). A key aspect of the follow-up study design included both telephone interview for persons unable or unwilling to attend a clinic visit and proxy interviews for persons who had died or were otherwise unable to respond for themselves. Decline in gait speed was defined as a 4% reduction in speed per year, which is analogous to the 0.05 m/s decline per year in persons with an average initial gait speed of 1.17 m/s (ie, 0.15 m/s over 3 years or 0.25 m/s over 5 years) (33). A binary variable was created to identify those participants who experienced a decline in gait speed. Over the 5 years of follow-up, 12.3% of participants died and less than 1% (0.40%) were lost to follow-up. Blacks were more likely to die (14.7% vs 10.5%; p < .001) or be lost to follow-up (0.72% vs 0.18%; p = .021) than whites.

Covariates

Demographic variables included age (years) and study site (Memphis or Pittsburgh). Health status included obesity, knee and hip osteoarthritis, diabetes mellitus, coronary heart disease, cerebrovascular disease (CVD), hypertension, and depressive symptoms. Body mass index was calculated (weight (kg)/height (m²) from measured weight and height and categorized as normal (body mass index < 25) overweight (25 < body mass index < 30), or obese (body mass index > 30). Knee and hip osteoarthritis was based on self-report of physician diagnoses and whether they had pain lasting at least 1 month. Diabetes mellitus, coronary heart disease, and cerebrovascular disease were based on self-report of physician diagnoses and use of relevant medications. Hypertension was defined as mean systolic blood pressure ≥140 mm Hg, mean diastolic blood pressure ≥90 mm Hg, or current use of antihypertensive medications. Persons were considered to have depressive symptoms if they scored ≥8 on the Center for Epidemiological Studies–Depression scale short form (10-item scale) (34,35). Each condition was coded as a binary variable (1 = condition present; 0 = condition absent), which was summed to create a variable representing total number of health conditions. Because less than 10% of participants (n = 211) reported three or four chronic conditions (27.1% black vs 26.9% white; p = .925), a dichotomous variable was created to identify participants who reported none or only one condition versus two or more conditions.

Statistical Analyses

Descriptive statistics were used to determine the mean (t tests) and proportional (chi-square tests) differences by race for the demographic, SES, and baseline health and mobility status variables. General linear models were used to estimate the association between race and usual walking speed at baseline. Logistic regression models were used to examine the association between race and ease of walking a quarter mile at baseline and the association between race and mobility limitation and decline in usual walking speed over 5 years. Women report greater mobility difficulty and men exhibit better physical performance for a given level of reported functioning (36–38); therefore, all analyses were stratified by sex. Because SES indicators are likely to be interrelated (12,13), analyses were also conducted using a forward stepwise model-building procedure that included race, age, study site, obesity, knee and hip osteoarthritis, and two or more health conditions in each model. Beta coefficients and corresponding standard errors were reported for linear regression models. Odds ratios (ORs) and associated 95% confidence intervals (CI) were reported for logistic regression models. p Values <.05 were considered statistically significant, and all tests were two sided. Analyses were conducted using SAS version 9.1.3 for windows (SAS Institute, Inc., Cary, NC).

RESULTS

Baseline characteristics by sex and race are shown in Table 1. Black adults exhibited a worse SES profile than white adults with higher rates of poverty, not completing high school, reading level below the ninth grade, perceiving income as inadequate, owning fewer than three assets, and not owning a home. Black adults also displayed poorer baseline health and mobility—a higher proportion reported obesity, two or more health conditions, and poor mobility capacity, and their mean walking speed was slower than white adults. A lower proportion of black women reported hip osteoarthritis relative to white women. There were no race differences observed with respect to knee osteoarthritis or hip osteoarthritis in men.

Table 1.

Distribution of Baseline Characteristics by Sex and Race in the Health, Aging, and Body Composition Study

Characteristic	Black	White
Women	(n = 716)	(n = 813)
Demographic
Age, M ± SD	73.4 ± 2.9	73.6 ± 2.8
Memphis site, %	46.9	56.2***
Socioeconomic
At or below 150% poverty level	82.1	45.3***
Education level, %
Not a high-school graduate	38.6	10.4
High-school graduate and some college	49.1	66.7
College graduate and beyond	12.2	22.7***
Reading level, %
≤6th grade	35.3	13.8
7th–8th grade	16.8	4.9
≥9th grade	47.9	81.3***
Income perceived as inadequate, %	22.7	7.6***
Fewer than three assets, %	80.9	44.9***
Does not own a home, %	38.2	26.2***
Baseline health and function
Obese, %	43.9	17.3***
Knee arthritis, %	11.5	13.4
Hip arthritis, %	4.1	7.5**
Two or more health conditions, %	32.1	16.9***
Walking quarter mile, % other than very easy	41.8	24.9***
Climbing 10 steps, % other than very easy	41.5	28.4***
6 m usual walking speed, m/s, M ± SD	1.04 ± 0.20	1.19 ± 0.22***
Men	(n = 542)	(n = 898)
Demographic
Age, M ± SD	73.5 ± 2.8	73.9 ± 2.9
Memphis site, %	50.7	52.8
Socioeconomic
At or below 150% poverty level	68.1	29.8***
Education level, %	50.1	14.1***
Not a high-school graduate	50.1	14.4
High-school graduate and some college	36.9	46.2
College graduate and beyond	12.9	39.2***
Reading level, %
0–6th grade	45.2	16.5
7th–8th grade	22.1	11.0
≥9th grade	32.7	72.5***
Income perceived as inadequate, %	15.7	6.3***
Fewer than three assets, %	80.4	39.4***
Does not own a home, %	26.2	18.6***
Baseline health and function
Obese, %	25.3	19.7*
Knee arthritis, %	4.7	6.2
Hip arthritis, %	2.1	3.4
Two or more health conditions, %	32.3	26.6*
Walking quarter mile, % other than very easy	26.6	18.8***
Climbing 10 steps, % other than very easy	28.2	19.2***
6 m usual walking speed, m/s, M ± SD	1.13 ± 0.23	1.29 ± 0.23***

Note: *p < .05. **p < .01. ***p < .001.

The relationship between race and baseline functional status by sex is displayed in Table 2. Accounting for age, study site, and health status (Model 1), black women (OR = 1.65, 95% CI: 1.30–2.09) and men (OR = 1.57, 95% CI: 1.21–2.04) had a greater odds of reporting that walking a quarter mile is other than very easy relative to white men and women. After accounting for SES differences (Model 2), blacks had similar odds of reporting poor walking capacity as whites. For both women and men, living at or below 150% of the poverty level was related to reported walking capacity adjusting for race, age, study site, and health status. For women, education was associated with reported walking capacity, whereas for men, reading below ninth-grade reading level was associated with reported walking capacity, independent of potential confounders. In examining baseline walking speed, we observed black women (beta coefficient [β] = −.13, SE: 0.011) and men (β = −.16, SE: 0.013) to have slower walking speeds than their white counterparts independent of age, study site, and health status. Accounting for differences in SES indicators diminished, but did not eliminate, the racial difference. For both men and women, reading below ninth-grade level was negatively associated with walking speed. For women, living at or below 150% of the poverty level and for men not graduating from high school or college and perceiving income as inadequate were also negatively associated with walking speed.

Table 2.

Association Between Race and Mobility Status at Baseline Stratified by Sex in the Health, Aging, and Body Composition Study

Characteristic	Walking ¼ Mile Other Than Very Easy		Year 1 Gait Walking Speed
	OR (95% CI)		β (SE)
	Model 1	Model 2	Model 1	Model 2
Women
Demographic
Black	1.65 (1.30–2.09)	1.31 (0.98–1.74)	−0.13 (0.011)	−0.10 (0.013)
Age	1.07 (1.02–1.11)	1.06 (1.01–1.10)	−0.01 (0.002)	−0.01 (0.002)
Memphis site	0.87 (0.69–1.09)	0.81 (0.63–1.05)	−0.04 (0.010)	−0.03 (0.012)
Health status
Obese	2.13 (1.67–2.72)	1.81 (1.38–2.37)	−0.08 (0.012)	−0.07 (0.013)
Knee arthritis	1.73 (1.19–2.52)	1.86 (1.25–2.77)	−0.01 (0.018)	−0.01 (0.019)
Hip arthritis	0.85 (0.49–1.45)	0.75 (0.42–1.34)	−0.01 (0.025)	−0.01 (0.027)
Two or more health conditions	1.99 (1.55–2.57)	1.92 (1.46–2.53)	−0.06 (0.012)	−0.06 (0.013)
Socioeconomic
At or below 150% poverty level		1.36 (1.00–1.84)		−0.03 (0.013)
Education level
College graduate and beyond		1.0
High-school graduate/some college		1.38 (0.96–2.02)–
Not a high-school graduate		1.82 (1.17–2.85)
Below ninth-grade reading level				−0.04 (0.013)
Income perceived as inadequate
Does not own a home
Men
Demographic
Black	1.57 (1.21–2.04)	1.22 (0.89–1.67)	−0.16 (0.013)	−0.11 (0.015)
Age	1.01 (0.96–1.06)	1.00 (0.95–1.05)	−0.01 (0.002)	−0.01 (0.002)
Memphis site	1.09 (0.84–1.42)	1.02 (0.77–1.36)	−0.07 (0.012)	−0.05 (0.013)
Health status
Obese	1.31 (0.97–1.77)	1.28 (0.93–1.77)	−0.02 (0.014)	−0.02 (0.015)
Knee arthritis	1.22 (0.71–2.12)	1.30 (0.72–2.37)	0.01 (0.027)	−0.003 (0.028)
Hip arthritis	1.94 (0.97–3.90)	2.34 (1.14–4.82)	−0.01 (0.037)	−0.02 (0.037)
Two or more health conditions	1.53 (1.17–2.01)	1.50 (1.13–2.01)	−0.06 (0.013)	−0.05 (0.014)
Socioeconomic
At or below 150% poverty level		1.45 (1.07–1.95)
Education level
College graduate and beyond				−0.05 (0.015)
High-school graduate/some college				−0.09 (0.019)
Not a high-school graduate
Below ninth-grade reading level		1.58 (1.16–2.15)		−0.05 (0.015)
Income perceived as inadequate				−0.07 (0.022)
Does not own a home

Notes: Bolded parameter estimates indicate those that are statistically significant (p < .05). CI = confidence interval; OR = odds ratio.

The association between race and incident mobility limitation over 5 years is shown in Table 3. Among the 2,969 participants, 1,103 (37.1%) developed mobility limitation within 5 years, which varied by race. Fifty-three percent of black women and 33% of black men experienced onset of mobility limitation compared with 40% of white women (p < .001) and 26% of white men (p < .001), respectively. After adjusting for age and study site, black women (OR = 1.69, 95% CI: 1.38–2.07) and men (OR = 1.44, 95% CI: 1.14–1.82) had a greater odds of developing mobility limitation than their white counterparts. In Model 2 after adjusting for baseline health and mobility status, black women (OR = 1.15, 95% CI: 0.91–1.72) had similar odds of developing mobility limitation as white women. However, black men (OR = 1.29, 95% CI: 1.01–1.66) continued to exhibit a greater odds of developing mobility limitation than white men. After accounting for differences in SES indicators in Model 3, the race association with developing mobility limitation in men (OR = 1.08, 95% CI: 0.81–1.43) was eliminated and education level emerged as a contributing factor for developing mobility limitation over 5 years.

Table 3.

Association Between Race and Mobility Limitation Over a 5-Year Period Stratified by Sex in the Health, Aging, and Body Composition Study

Characteristic	OR (95% CI)
	Model 1	Model 2	Model 3
Women
Demographic
Black	1.69 (1.38–2.07)	1.15 (0.91–1.72)	0.92 (0.71–1.28)
Age	1.04 (1.00–1.08)	1.03 (0.99–1.07)	1.04 (0.99–1.08)
Memphis site	0.92 (0.75–1.13)	0.98 (0.79–1.23)	1.06 (0.81–1.38)
Baseline health and function
Obese		2.39 (1.85–3.07)	2.51 (1.90–3.32)
Knee arthritis		1.89 (1.29–2.78)	1.98 (1.30–3.00)
Hip arthritis		1.37 (0.80–2.34)	1.33 (0.75–2.37)
Two or more health conditions		1.53 (1.18–1.99)	1.52 (1.14–2.02)
Walking quarter mile, other than very easy		2.01 (1.54–2.62)	2.15 (1.60–2.88)
Climbing 10 steps, other than very easy		1.80 (1.39–2.33)	1.90 (1.43–2.53)
Socioeconomic
At or below 150% poverty level
Education level
College graduate and beyond			1.0
High-school graduate/some college			1.18 (0.85–1.64)
Not a high-school graduate			1.86 (1.24–2.80)
Below ninth-grade reading level
Income perceived as inadequate
Does not own a home
Fewer than three assets
Men
Demographic
Black	1.44 (1.14–1.82)	1.29 (1.01–1.66)	0.95 (0.71–1.28)
Age	1.04 (1.00–1.09)	1.04 (1.00–1.09)	1.04 (0.99–1.08)
Memphis site	1.07 (0.85–1.35)	1.09 (0.85–1.39)	1.06 (0.81–1.38)
Baseline health and function
Obese		1.83 (1.38–2.41)	1.91 (1.43–2.58)
Knee arthritis		1.54 (0.92–2.55)	1.45 (0.83–2.51)
Hip arthritis		1.55 (0.78–3.10)	1.88 (0.93–3.82)
Two or more health conditions		1.87 (1.45–2.41)	1.73 (1.32–2.26)
Walking quarter mile, other than very easy		1.57 (1.15–2.15)	1.59 (1.14–2.22)
Climbing 10 steps, other than very easy		1.85 (1.36–2.52)	1.65 (1.19–2.29)
Socioeconomic
At or below 150% poverty level
Education level
College graduate and beyond			1.0
High-school graduate/some college			1.18 (0.84–1.64)
Not a high-school graduate			1.68 (1.15–2.47)
Below ninth-grade reading level
Income perceived as inadequate
Does not own a home
Fewer than three assets			1.32 (1.00–1.74)

Notes: Mobility limitation was defined as reporting any difficulty or the inability to walk ¼ mile or climb 10 steps during any two consecutive 6-month assessments. CI = confidence interval; OR = odds ratio.

The association between race and decline in gait speed over 5 years is shown in Table 4. This analysis was restricted to the 2,449 participants who had a clinic or home visit in Year 3 or 5 with an assessment of walking speed of which 749 (31%) experienced a decline in walking speed. Just over one third of black women experienced a decline in walking speed relative to 27% of white women (p < .01), whereas black and white men experienced similar rates of decline in walking speed. Of the 520 participants with no gait speed assessment in exam Year 6, 243 had died, 43 withdrew from the study or were lost to follow-up, and 160 had a telephone interview only. This group was more likely to be black, read below the ninth-grade level, own fewer than three assets, perceive their income as inadequate, and have slower baseline walking speed and less likely to graduate from high school and own a home. Independent of age, study site, and health status, black women (OR = 1.24, 95% CI: 0.96–1.61) and men (OR = 1.29, 95% CI: 0.99–1.68) had similar odds of gait speed decline than white women and men.

Table 4.

Association Between Race and Gait Speed Decline Over 5 Years Stratified by Sex in the Health, Aging, and Body Composition Study

Characteristic	OR (95% CI)
	Model 1	Model 2
Women
Demographic
Black	1.24 (0.96–1.61)	1.04 (0.77–1.41)
Age	1.07 (1.03–1.12)	1.06 (1.02–1.11)
Memphis site	0.86 (0.67–1.10)	0.82 (0.62–1.08)
Baseline health and function
Obese	1.44 (1.10–1.89)	1.36 (1.01–1.83)
Knee arthritis	1.58 (1.07–2.32)	1.78 (1.18–2.69)
Hip arthritis	1.01 (0.59–1.76)	0.93 (0.52–1.68)
Two or more health cond itions	1.28 (0.95–1.71)	1.22 (0.89–1.68)
Socioeconomic
At or below 150% poverty level
Education level
College graduate and beyond
High-school graduate/some college
Not a high-school graduate
Below ninth grade reading level		1.80 (1.29–2.51)
Income perceived as inadequate
Does not own a home		1.60 (1.19–2.14)
Men
Demographic
Black	1.29 (0.99–1.68)	1.23 (0.93–1.62)
Age	1.07 (1.03–1.12)	1.06 (1.02–1.12)
Memphis site	0.80 (0.62–1.04)	0.83 (0.63–1.09)
Baseline health and function
Obese	1.18 (0.87–1.61)	1.13 (0.82–1.57)
Knee arthritis	1.10 (0.63–1.91)	1.22 (0.68–2.20)
Hip arthritis	1.06 (0.51–2.20)	1.04 (0.49–2.20)
Two or more health conditions	1.37 (1.03–1.82)	1.31 (0.97–1.76)
Socioeconomic
At or below 150% poverty level
Education level
College graduate and beyond
High-school graduate/some college
Not a high-school graduate
Below ninth grade reading level
Income perceived as inadequate
Does not own a home		1.49 (1.05–2.11)

Notes: Decline in gait speed was defined as a 4% reduction per year. CI = confidence interval; OR = odds ratio.

DISCUSSION

In initially well-functioning older adults, blacks had poorer mobility status based on walking speed than whites at baseline, but contrary to our hypothesis, although differences were attenuated when a broad variety of SES factors were controlled, a residual race effect remained. Over 5 years, black men experienced greater mobility limitation than white men, but these declines were largely explained by SES factors.

In this study, we investigated the relationship of a broader range of SES indicators with mobility status and decline than most previous work. Despite concerns that previous work overlooked important SES dimensions, findings from the current study support the primacy of education and income as SES indicators associated with mobility. Other indicators including reading level, perceived income inadequacy, and number of financial assets, although less consistently associated with mobility cross-sectionally and longitudinally, appeared to diminish the race association with mobility deficits. The residual association of race with baseline mobility even after accounting for traditional and an expanded set of SES parameters warrants further discussion.

Race and SES are inextricably associated in the United States, which makes it difficult to tease apart the association of each on mobility. When examining race differences, it is also necessary to examine SES because race and SES are strongly correlated (12,13). The residual race effect in baseline walking speed observed in this study may be due to unmeasured factors, such as historical discrimination and intergenerational transfer of wealth (12–14,23), or differential cultural experiences based on the role of SES in access and receipt of medical services (39–41). More recently, investigators have begun to address the confounding of race and SES by addressing it in the design phase of the study (42) or studying within race groups (43,44). The race-related differences in baseline mobility are likely ascribed to accumulated exposure to negative health events and chronic stressors across the life course particularly for blacks (5,43–50). Residential segregation is another possible explanation for the observed race-related deficits in mobility at baseline. Blacks and whites frequently live in different environments (14,42,51), which can affect the resources and opportunities pertinent to maintenance of mobility, such as safe and hospitable walking routes, housing conditions, and air quality. It is possible that race differences in social and environmental conditions resulting from residential segregation could account for the residual race differences in baseline walking speed. Information regarding residential segregation is not available in Health, Aging, and Body Composition. These possibilities underscore the need for further more comprehensive and thoughtful investigations of interrelationships among race, SES, and baseline mobility status using a life course perspective. Future work should also consider a prospective study using a mixed method data collection approach to conduct follow-up interviews as a way of providing additional meaning to some of the quantitative results.

Even after accounting for poorer mobility at baseline, black men still displayed a higher odds of developing mobility limitation than white men over 5 years. However, this residual racial association appears to be largely a function of SES differences. Consistent with our earlier work focused on older functionally limited and disabled women (23), these findings indicate that race-related disparities in functional and mobility decline in older adults largely reflect poorer initial functional status and limited socioeconomic resources. In this study, blacks had slower baseline gait speed, and despite similar rates of decline, the race disparity persisted over 5 years (Figure 1). This suggests that blacks enter old age with poorer mobility status beyond what would be expected by low SES, and this disadvantage continues into old age.

Figure 1.

Mean gait speed by race and sex at the baseline, Year 4, and Year 6 clinic examination assessments in the Health, Aging, and Body Composition study.

Our findings examining baseline differences in mobility status between black and white older adults highlight the importance of using both self-report and objective measures whenever possible. In this study, although we did not observe race differences in reported mobility status at baseline, differences in performance were apparent. In contrast to categorical self-report measures of mobility, performance-based assessments can capture small yet meaningful differences in capacity. Moreover, self-reported mobility status can be influenced by race- and SES-related factors including walking behavior, health care access and utilization, and social and cultural norms and expectations regarding the meaning of difficulty (39,52–54).

A major strength of this study is the ability to examine the association between race, SES, and mobility status and decline over 5 years in an initially well-functioning cohort of black and white older adults. Furthermore, the longitudinal design afforded the opportunity to establish that SES preceded development of mobility decline. There were a number of additional SES measures examined in this study as compared with previous work. Nevertheless, aspects of this study require comment. Selective survival may have affected the association between race and mobility status and limitation. Because Health, Aging, and Body Composition represents a population of older adults who have reached their eighth decade of life, it excludes persons who have died or developed functional limitation or disability at younger ages. The differential loss to follow-up of black individuals and those with low SES from the follow-up measurement of gait speed may have weakened the association between race, SES, and gait speed decline. We conducted a sensitivity analysis for the relationship between race and gait speed decline in which we treated those who died or were lost to follow-up as having experienced decline. After adjusting for age, study site, and baseline health and functional status, black women (OR = 1.45, 95% CI: 1.16–1.81) and men (OR = 1.62, 95% CI: 1.30–2.02) had higher odds of gait speed decline than their white counterparts. However, after accounting for SES differences, the race association with gait speed decline in women (OR = 0.90, 95% CI: 0.69–1.18) and men (OR = 1.03, 95% CI: 0.79–1.33) was no longer significant. Reading level below the ninth grade and not owning a home were the SES indicators associated with gait speed decline. Future work should examine this association in a younger age group (eg, 30–64 years of age) where selective survival is less of an issue. No information was available on earlier life experiences, which have been shown to be related to disparities in late life (55). Earlier life experiences operate in complex ways to influence health at older ages possibly through SES-related cumulative disadvantage (56) and/or the exposure (57,58) and vulnerability hypotheses (59).

With the U.S. population becoming older and more racially and economically diverse, it is important to identify and assess factors that may underlie race-related differences in mobility, a critical stage in the disablement process amenable to preventive interventions. Even within a well-functioning group of older adults, a racial disparity in walking speed at baseline was apparent, which was only partially accounted for by differences in poverty, education, reading level, and perceived income inadequacy. This suggests that other factors beyond SES and health-related measures may contribute to the residual race difference. Thus, disentangling the complex relationships among race, SES, and mobility should remain a key objective in health disparities and aging research.

FUNDING

This study was supported by National Institute on Aging contracts N01-AG-6-2101, N01-AG-6-2103, and N01-AG-6-2106. This research was supported (in part) by the Intramural Research Program of the National Institutes of Health, National Institute on Aging. Research conducted by the first author was supported by a grant from the National Center for Minority Health and Health Disparities (P60MD000214-01).