Abstract
Objectives. We investigated trends in disability among older Americans from 1988 through 2004 to test the hypothesis that more recent cohorts show increased burdens of disability.
Methods. We used data from 2 National Health and Nutrition Examination Surveys (1988–1994 and 1999–2004) to assess time trends in basic activities of daily living, instrumental activities, mobility, and functional limitations for adults aged 60 years and older. We assessed whether changes could be explained by sociodemographic, body weight, or behavioral factors.
Results. With the exception of functional limitations, significant increases in each type of disability were seen over time among respondents aged 60 to 69 years, independent of sociodemographic characteristics, health status, relative weight, and health behaviors. Significantly greater increases occurred among non-Whites and persons who were obese or overweight (2 of the fastest-growing subgroups within this population). We detected no significant trends among respondents aged 70 to 79 years; in the oldest group (aged ≥ 80 years), time trends suggested lower prevalence of functional limitations among more recent cohorts.
Conclusions. Our results have significant and sobering implications: older Americans face increased disability, and society faces increased costs to meet the health care needs of these disabled Americans.
The impact on society of the health and health care needs of older adults has been the subject of growing debate in the United States. This population is expanding more rapidly now that baby boomers (born in 1946–1964) are beginning to reach their 60s.1–3 Heightened concern with our aging population was highlighted by a recent Institute of Medicine report, Retooling for an Aging America: Building the Health Care Workforce.4 Questions about potential burdens of disability are salient because increased disability in our rapidly growing population of older adults may exert enormous strains on available human and financial resources.
Concerns about levels of disability were allayed somewhat by encouraging evidence from the 1980s and 1990s of downward trends in disability.5–13 More recent findings are mixed: data from the National Long-Term Care Survey showed declines,14–16 but other studies suggest that these trends may be reversing, with newer cohorts (including the oldest of the baby boomers) reporting worse health status and more disability than did their earlier counterparts.9,17,18
The current epidemic of obesity has been suggested as a contributor to current and future increases in disability. Obesity among US adults has increased dramatically, rising from 11% to 16% in the early 1960s to 28% to 34% by 2000,19,20 resulting in rapidly increasing proportions of overweight, obese, and severely obese members of cohorts now reaching their 60s.21,22 Worse still, forecasts are for levels of obesity as high as 45.4% within 20 years if trends persist.23 Evidence also suggests that the disability risks associated with obesity may be greater than those experienced 15 to 20 years ago,9,21,24 possibly because of earlier onset of obesity (and thus longer lifetime exposure).25,26
The changing racial/ethnic composition of cohorts now reaching their 60s is another potential contributor to changing disability trends. The most rapid growth is projected to be among Blacks and Hispanics,27,28 groups with significantly higher rates of obesity (45% of non-Hispanic Blacks and 36.8% of Hispanics versus 30% non-Hispanic Whites20) and disproportionately lower socioeconomic status—both factors associated with increased risks for functional limitations and disability.29,30
We used data from the National Health and Nutrition Examination Survey (NHANES) for 1988 to 1994 and 1999 to 2004 to examine trends in the prevalence of reported disability for adults aged 60 to 69 years, 70 to 79 years, and 80 years and older, with particular attention to whether trends differed between the youngest and the older 2 groups. For respondents aged 60 to 69 years, the more recent NHANES data included individuals born just prior to the baby boomer generation (1930–1944), providing potential clues to likely trends in the large generation that will immediately follow. We assessed the extent to which differences in reported disability between the 2 survey periods could be explained by changes in the sociodemographic composition of the population, changes in the prevalence of overweight and obesity, or changes in lifestyle or other aspects of health status.
We analyzed responses from adults aged 60 years and older from NHANES 1988 to 1994 (n = 4688) and 1999 to 2004 (n = 4239). Disability data for persons aged younger than 60 years was not collected in NHANES for these periods. Each survey provided a nationally representative sample of the US population for the period, and each included interview, clinical exam, and laboratory components.31 The surveys were conducted by the Centers for Disease Control and Prevention with appropriate institutional review board approval and participant written consent. We excluded respondents with missing data for model covariates (sociodemographic characteristics, health status, and health behaviors). Rather than exclude for missing education or income data (for which missing data were more common), we included indicator variables in our models, flagging those with missing data. We separately excluded respondents with missing data for a given disability outcome (n = 4 for basic activities of daily living [ADL]; n = 46 for instrumental activities of daily living [IADL]; n = 42 for mobility; n = 5 for functional limitations). Participants with missing data tended to be older and to report having less education and poorer health status (e.g., more chronic conditions and extremes of relative weight).
We assessed disability by responses to questions asked consistently across the 2 periods. Responses of “do not do” to items asking about performance of certain activities/behaviors (available to respondents only as of NHANES 2003–2004) were coded as missing because such responses did not indicate whether the activity was not done because of disability or for other reasons; supplementary analyses coding these responses as indicative of either disability or no disability did not alter findings (data not shown). We chose items that are commonly used to assess each of 4 types of disability.32
The 4 ADL disability questions asked about difficulty walking from room to room on the same level, getting in and out of bed, eating, and dressing. The 3 IADL disability questions asked about difficulty doing chores around the house, preparing own meals, and managing money. We assessed mobility disability from 2 questions about difficulty walking a quarter of a mile and walking up 10 steps without rest. Three functional limitation items asked about difficulty stooping, crouching, or kneeling; lifting or carrying 10 pounds; and standing from an armless chair. Response categories for all items were no difficulty, some difficulty, much difficulty, or unable to do. For each outcome, we defined presence of each disability as a report of some or greater difficulty on 1 or more relevant items for that disability.
Covariates included standard sociodemographic characteristics, measures of relative weight, indices of health status, and health behaviors. Age was the reported age at the NHANES screening interview. We assessed socioeconomic status (SES) by the highest level of education completed (grade school, some high school, complete high school, some college, or college or higher [reference group]) and the poverty income ratio, an index reflecting the ratio of household income to the household poverty level, determined by area of residence and household size (categorized as < 1, 1–1.99, 2–2.99, 3–3.99, 4–4.99 or ≥ 5 [reference group]).33–36 We derived race/ethnicity from self-report (categorized as non-Hispanic Black, Mexican American, other [including other Hispanic groups], or non-Hispanic White [reference group]).
We assessed relative weight by body mass index (BMI; defined as weight in kilograms divided by height in meters squared): obesity (BMI ≥ 30 kg/m2), overweight (BMI = 25–29 kg/m2), normal weight (BMI = 18.5–24 kg/m2 [reference group]), or underweight (BMI < 18.5 kg/m2). We included waist circumference as a continuous measure and as a binary variable by National Cholesterol Education Program criteria (waist circumference > 88 cm [versus ≤ 88] for women and > 102 cm [versus ≤ 102] for men).37
Indices of health status were self-rated health (categorized as excellent, very good, good, fair, or poor) as well as reported doctor-diagnosed prevalence of major chronic conditions (myocardial infarction, stroke, diabetes, congestive heart failure, arthritis, bronchitis, emphysema, or asthma). We created a summary index of cumulative biological risks and dysregulation (allostatic load),38 reflecting a count across 9 biological parameters (systolic and diastolic blood pressure, pulse, glycosylated hemoglobin level, total and high-density lipoprotein cholesterol level, waist–hip ratio, C-reactive protein level, and albumin level) of the number of parameters that met clinical criteria for high risk.39
Our measures of health behavior were smoking status (current smokers, former smokers [having ever smoked 100 or more cigarettes], or never smokers [reference group]) and total physical activity (a summary score derived from reported moderate and vigorous activities, with activities weighted by their associated metabolic equivalent rating, as defined by Pate et al.,40 multiplied by the reported monthly frequency).
We compared reported prevalence for each type of disability across the 2 NHANES periods for each age group (60–69, 70–79, and ≥ 80 years). Because those born between 1931 and 1934 fell into the youngest age group in both surveys (they were aged 60–63 years in 1988–1994 and 66–69 years in 1999–2004), we also ran separate analyses for respondents aged 60 to 64 years (i.e., where years of birth for those aged 60–69 years in the NHANES 1999–2004 sample did not overlap with those aged 60–69 in the NHANES 1988–1994 sample).
We first analyzed descriptive measures for all variables of interest by age group and NHANES period. For each type of disability, we then fit logistic regression models for each age group to assess the relative odds of each type of disability as a function of time (i.e., comparing reported prevalence in 1999–2004 with 1988–1994). Models were initially adjusted for age (continuous age within age group), gender, race/ethnicity, poverty income ratio, and education, and then in incremental models, additionally adjusted for obesity, overweight, underweight (versus normal weight), BMI, and waist circumference; self-rated health, prevalence of health conditions, and allostatic load; and health behaviors.
We also examined possible differences in disability time trends by major demographic characteristics other than age (race/ethnicity, gender, and SES) by including interaction terms in the unadjusted models. We ran similar tests for obesity in response to recent evidence of an increased effect of obesity on disability among persons aged 60 years and older.24 We then assessed significant interaction terms in fully adjusted models; for significant effects, we stratified the fully adjusted models by the relevant factor.
We estimated population-level values from the study sample with NHANES mobile examination center weights to take into account selection probability and nonresponse. These weights accounted for oversampling of older, Black, and Mexican American participants.41 To achieve results representative of the US population in each survey period, we used period-specific sampling weights and robust error variance estimation with clustering at the primary sampling unit level (with the cluster option in Stata). We used Stata version 9 (StataCorp LP, College Station, TX) for all analyses. We considered a 2-tailed P < .05 statistically significant.
Table 1 presents descriptive statistics for the 2 survey periods for each of the age groups. As expected, educational attainment increased significantly over time for all age groups. In the youngest group, there was some evidence of the anticipated increases in the percentage of non-Whites,28 although differences did not reach statistical significance. Average BMI and waist circumference increased significantly, as did percentage of respondents meeting criteria for obesity (BMI ≥ 30 kg/m2). This was particularly evident among persons aged 60 to 69 years, in whom prevalence increased from 27.4% to 37.6% (P < .001). Reported prevalence of chronic conditions—including diabetes, asthma, and arthritis—also increased, most notably among respondents aged 60 to 69 years. All age groups showed striking declines in reported levels of physical activity.
TABLE 1 Descriptive Statistics by Age Group: National Health and Nutrition Examination Surveys (NHANES), 1988–1994 and 1999–2004
| Age 60–69 Years (n = 4070) | Age 70–79 Years (n = 2930) | Age ≥ 80 (n = 1927) | |||||||
| NHANES 1988–1994, % or Mean (SD) | NHANES 1999–2004, % or Mean (SD) | P | NHANES 1988–1994, % or Mean (SD) | NHANES 1999–2004, % or Mean (SD) | P | NHANES 1988–1994, % or Mean (SD) | NHANES 1999–2004, % or Mean (SD) | P | |
| Age, y | 64.4 (3.1) | 64.2 (3.0) | .09 | 73.8 (2.8) | 74.0 (2.9) | .3 | 83.7 (2.5) | 83.0 (1.7) | <.001 |
| Gender | |||||||||
| Men | 45.7 | 47.0 | .5 | 42.7 | 42.9 | .9 | 35.2 | 36.4 | .7 |
| Women | 54.3 | 53.0 | .5 | 57.4 | 57.1 | .9 | 64.8 | 63.6 | .7 |
| Race/Ethnicity | |||||||||
| Non-Hispanic White | 82.3 | 79.9 | .4 | 88.1 | 83.4 | .09 | 89.6 | 89.1 | .8 |
| Non-Hispanic Black | 8.3 | 9.2 | .6 | 7.1 | 6.8 | .9 | 5.9 | 5.1 | .6 |
| Mexican American | 2.9 | 3.9 | .3 | 1.5 | 2.8 | .06 | 1.5 | 1.9 | .5 |
| Other | 6.6 | 7.0 | .8 | 3.4 | 7.0 | .04 | 3.1 | 3.9 | .5 |
| Education | |||||||||
| < High school | 35.4 | 24.7 | <.001 | 43.8 | 32.5 | .001 | 49.4 | 32.7 | <.001 |
| High school | 34.7 | 28.6 | .01 | 27.7 | 30.2 | .3 | 24.0 | 29.8 | .08 |
| > High school | 29.6 | 46.6 | <.001 | 28.2 | 37.3 | .006 | 25.6 | 36.9 | .002 |
| Missing | 0.3 | 0.1 | .2 | 0.2 | 0.04 | .2 | 1.0 | 0.6 | .4 |
| Poverty income ratioa | 3.6 (1.8) | 3.7 (1.8) | .2 | 3.1 (1.6) | 3.2 (1.7) | .5 | 2.8 (1.2) | 2.9 (1.3) | .6 |
| Relative weight | |||||||||
| BMI, kg/m2 | 27.4 (5.7) | 29.1 (6.0) | <.001 | 26.7 (5.0) | 27.8 (5.3) | <.001 | 25.4 (3.4) | 26.1 (4.0) | .008 |
| Underweight (BMI < 18.5) | 1.9 | 0.7 | .009 | 1.4 | 1.8 | .5 | 3.5 | 2.2 | .2 |
| Normal weight (BMI = 18.5–24) | 30.7 | 23.8 | <.001 | 38.4 | 27.8 | <.001 | 43.1 | 42.2 | .7 |
| Overweight (BMI = 25–29) | 39.9 | 38.0 | .3 | 38.3 | 41.0 | .2 | 40.6 | 39.0 | .6 |
| Obese (BMI ≥ 30) | 27.4 | 37.6 | <.001 | 22.0 | 29.4 | .001 | 12.9 | 16.7 | .05 |
| Waist circumference, cm | 97.8 (14.5) | 101.7 (15.3) | <.001 | 96.2 (13.0) | 99.7 (13.6) | <.001 | 93.6 (9.0) | 95.5 (10.9) | .001 |
| Large waistb | 59.0 | 67.0 | <.001 | 55.2 | 64.8 | <.001 | 51.3 | 56.4 | |
| Chronic conditions | |||||||||
| Myocardial infarction | 8.7 | 8.6 | .98 | 13.0 | 11.0 | .2 | 14.5 | 12.7 | .3 |
| Stroke | 3.6 | 4.2 | .4 | 6.9 | 7.0 | .9 | 10.0 | 10.4 | .8 |
| Diabetes | 15.3 | 19.1 | .02 | 16.6 | 17.9 | .5 | 15.0 | 13.7 | .5 |
| Asthma | 8.4 | 11.1 | .03 | 7.0 | 10.1 | .02 | 4.7 | 7.8 | .04 |
| Emphysema | 5.3 | 3.3 | .03 | 6.7 | 6.6 | .97 | 4.8 | 5.2 | .7 |
| Bronchitis | 9.3 | 8.1 | .4 | 10.6 | 9.8 | .6 | 7.3 | 7.2 | .9 |
| Congestive heart failure | 5.0 | 4.0 | .2 | 8.3 | 7.8 | .7 | 7.4 | 10.7 | .06 |
| Arthritis | 40.7 | 46.0 | .02 | 45.7 | 52.5 | .01 | 49.1 | 56.3 | .04 |
| Self-rated health | |||||||||
| Excellent | 16.1 | 17.9 | .3 | 11.3 | 13.0 | .3 | 11.7 | 13.9 | .3 |
| Very good | 24.9 | 28.3 | .1 | 25.4 | 26.0 | .8 | 24.8 | 25.0 | .9 |
| Good | 33.2 | 30.3 | .1 | 34.6 | 34.9 | .9 | 32.1 | 34.4 | .4 |
| Fair | 19.4 | 17.2 | .3 | 21.5 | 20.7 | .7 | 25.1 | 21.7 | .2 |
| Poor | 6.3 | 6.3 | .99 | 7.1 | 5.4 | .1 | 6.4 | 4.9 | .3 |
| Allostatic load | 2.4 (1.5) | 2.1 (1.4) | <.001 | 2.5 (1.3) | 2.1 (1.4) | <.001 | 2.6 (1.0) | 2.1 (1.0) | <.001 |
| Smoking status | |||||||||
| Current | 20.1 | 16.9 | .07 | 11.1 | 8.6 | .08 | 5.2 | 3.7 | .2 |
| Pastc | 40.2 | 41.3 | .6 | 43.1 | 42.8 | .9 | 32.7 | 39.1 | .02 |
| Never | 39.7 | 41.8 | .4 | 45.8 | 48.6 | .2 | 62.1 | 57.2 | .1 |
| Physical activity, METS/mo | 109.9 (135.6) | 71.8 (147.5) | <.001 | 107.3 (127.5) | 64.4 (116.3) | <.001 | 83.0 (90.3) | 48.0 (80.3) | <.001 |
Note. BMI = body mass index. Population-level values derived from the study sample with NHANES mobile examination center weights. P values are for comparison of the 2 survey periods. All sample sizes are unweighted.
a Missing values for the 1988–1994 NHANES were: aged 60–69 years, n = 222; aged 70–79 years, n = 162; aged ≥ 80 years, n = 141. Missing values for the 1999–2004 NHANES were: aged 60–69 years, n = 212; aged 70–79 years, n = 131; aged ≥ 80 years, n = 90.
b For women, waist circumference greater than 88 cm; for men, greater than 102 cm.
c Having ever smoked 100 or more cigarettes.
Time trends for each type of disability by age group are illustrated in Figure 1. For ADL disability (Figure 1a), reported prevalence increased significantly over time only among respondents aged 60 to 69 years. For IADL (Figure 1b) and mobility disability (Figure 1c), respondents aged 60 to 69 years and 70 to 79 years showed significantly increased prevalence of reported disability over time; for mobility disability, persons aged 80 years and older also showed significant decline. For functional limitations (Figure 1d), the prevalence increased significantly over time among those aged 60 to 69 years, but decreased significantly among respondents aged 80 years and older.
FIGURE 1 Time trends by age group between National Health and Nutrition Examination Surveys (NHANES) in 1988–1994 and 1999–2004 in prevalence of (a) basic activities of daily living disability, (b) instrumental activities of daily living disability, (c) mobility disability, and (d) functional limitations.
*P ≤ 0.05 for difference between NHANES cohorts.
Table 2 presents results of logistic regression models estimating the log odds of disability as a function of time (i.e., NHANES 1999–2004 versus 1988–1994), with sequential controls for possible explanatory variables (see the appendix for complete model results, available as a supplement to the online version of this article at http://www.ajph.org). Controls for time trends in sociodemographic characteristics did not alter the unadjusted findings for respondents aged 60 to 69 years (Figure 1). Reported prevalence for all types of disability was 40% to 70% higher in 1999 to 2004 than in 1988 to 1994 (Table 2, model 1). Controls for measures of obesity, actual BMI, and waist circumference reduced these differences only modestly, and all remained significant. With further controls for health status and health behaviors, differences in functional limitations were reduced to nonsignificance; however, reported prevalence for all other types of disability remained significantly higher among respondents aged 60 to 69 years in 1999 to 2004 (Table 2, models 3 and 4). Analyses of respondents aged 60 to 64 years yielded parallel and somewhat stronger findings in the fully adjusted models (ADL, odds ratio [OR]TIME = 1.7; 95% confidence interval [CI] = 1.2, 2.5; IADL, ORTIME = 1.8; 95% CI = 1.2, 2.7; mobility, ORTIME = 1.6; 95% CI = 1.1, 2.2; Table 2).
TABLE 2 Results of Logistic Regression Models, Comparing Log Odds of Disabilities, With Stepwise Adjustment for Time Trends in Sociodemographic Characteristics, Health Status, and Health Behaviors: National Health and Nutrition Examination Surveys, 1988–1994 and 1999–2004
| Age Group, y | Model 1,a OR (95% CI) | Model 2,b OR (95% CI) | Model 3,c OR (95% CI) | Model 4,d OR (95% CI) |
| Basic activities of daily living | ||||
| 60–64 (n = 2195) | 1.7 (1.3, 2.3) | 1.5 (1.1, 2.1) | 1.8 (1.3, 2.5) | 1.7 (1.2, 2.5) |
| 60–69 (n = 4067) | 1.7 (1.4, 2.2) | 1.5 (1.2, 2.0) | 1.6 (1.2, 2.1) | 1.6 (1.2, 2.1) |
| 70–79 (n = 2930) | 1.1 (0.9, 1.4) | 1.0 (0.8, 1.3) | 1.0 (0.7, 1.3) | 0.9 (0.7, 1.2) |
| ≥ 80 (n = 1926) | 1.1 (0.9, 1.5) | 1.1 (0.8, 1.4) | 1.0 (0.8, 1.4) | 1.0 (0.8, 1.3) |
| Instrumental activities of daily living | ||||
| 60–64 (n = 2189) | 1.6 (1.1, 2.2) | 1.6 (1.1, 2.2) | 2.0 (1.4, 2.9) | 1.8 (1.2, 2.7) |
| 60–69 (n = 4057) | 1.7 (1.3, 2.2) | 1.6 (1.2, 2.1) | 1.7 (1.3, 2.3) | 1.6 (1.2, 2.2) |
| 70–79 (n = 2919) | 1.4 (1.1, 1.8) | 1.3 (1.0, 1.7) | 1.3 (1.0, 1.7) | 1.2 (0.9, 1.5) |
| ≥ 80 (n = 1905) | 1.2 (0.9, 1.6) | 1.2 (0.9, 1.6) | 1.1 (0.9, 1.5) | 1.0 (0.8, 1.3) |
| Mobility disability | ||||
| 60–64 (n = 2192) | 1.7 (1.2, 2.2) | 1.5 (1.1, 2.0) | 1.8 (1.3, 2.5) | 1.6 (1.1, 2.2) |
| 60–69 (n = 4059) | 1.5 (1.2, 1.9) | 1.3 (1.1, 1.7) | 1.4 (1.1, 1.8) | 1.3 (1.0, 1.7) |
| 70–79 (n = 2913) | 1.3 (1.1, 1.7) | 1.2 (0.9, 1.4) | 1.2 (0.9, 1.5) | 1.1 (0.9, 1.4) |
| ≥ 80 (n = 1913) | 1.1 (0.8, 1.4) | 1.0 (0.8, 1.3) | 1.0 (0.8, 1.3) | 0.9 (0.7, 1.2) |
| Functional limitations | ||||
| 60–64 (n = 2195) | 1.4 (1.1, 1.9) | 1.3 (1.0, 1.7) | 1.3 (1.0, 1.9) | 1.3 (0.9, 1.8) |
| 60–69 (n = 4067) | 1.4 (1.1, 1.7) | 1.2 (1.0, 1.5) | 1.2 (1.0, 1.5) | 1.2 (0.9, 1.5) |
| 70–79 (n = 2930) | 1.2 (1.0, 1.5) | 1.0 (0.8, 1.3) | 1.0 (0.8, 1.3) | 1.0 (0.8, 1.2) |
| ≥ 80 (n = 1925) | 0.8 (0.6, 1.0) | 0.7 (0.5, 0.9) | 0.7 (0.5, 0.9) | 0.6 (0.4, 0.8) |
Note. CI = confidence interval; OR = odds ratio.
a Variables included age, gender, race/ethnicity, and socioeconomic status (education and poverty income ratio).
b Added controls for relative weight (body mass index, obesity, overweight, underweight, and waist circumference) to model 1.
c Added controls for health status (self-rated health; reported myocardial infarction, stroke, diabetes, congestive heart failure, arthritis, bronchitis, emphysema, and asthma; and a summary biological risk index) to model 2.
d Added controls for smoking and physical activity to model 3.
We found similar though somewhat weaker patterns among persons aged 70 to 79 years. Reported IADL disability was significantly higher in 1999 to 2004 than in 1988 to 1994, independent of sociodemographic characteristics (ORTIME = 1.4; 95% CI = 1.1, 1.8; Table 2) as was mobility disability (ORTIME = 1.3; 95% CI = 1.1, 1.7); functional limitations were also marginally higher (ORTIME = 1.2; 95% CI = 1.0, 1.5). However, controls for relative weight reduced the mobility and functional limitation differences to nonsignificance, and the IADL difference was reduced to nonsignificance with further controls for health status and health behaviors. For respondents aged 80 years and older, only functional limitations differed significantly over time, with decreased odds of reporting functional limitations in 1999 to 2004 (ORTIME = 0.6; 95% CI = 0.4, 0.8).
Because the demographic profile of the older population is changing, we tested for differences in time trends by race/ethnicity, gender, and SES. In fully adjusted models, for respondents aged 60 to 69 years, the increase in ADL disability prevalence was significantly greater among non-Hispanic Blacks (ORTIME = 3.3; 95% CI = 2.1, 5.2; P = .001 for interaction) and marginally greater among Mexican Americans (ORTIME = 1.9; 95% CI = 1.2, 3.1; P = .09 for interaction) than among non-Hispanic Whites (ORTIME = 1.4; 95% CI = 1.0, 1.9). Among participants aged 70 to 79 years, the increase in prevalence of functional limitations was also significantly greater among non-Hispanic Blacks (ORTIME = 1.3; 95% CI = 0.8, 1.9; P = .04 for interaction) and marginally greater among Mexican Americans (ORTIME = 1.6; 95% CI = 0.9, 3.0; P = .07 for interaction) than among non-Hispanic Whites (ORTIME = 0.9; 95% CI = 0.7, 1.2). Among respondents aged 80 years and older, women had a significantly greater reduction in functional limitations (ORTIME = 0.4; 95% CI = 0.3, 0.7) than did men (ORTIME = 1.0; 95% CI = 0.7, 1.4; P = .009 for interaction). Similarly, the reduction in functional limitations was significantly greater among respondents with less than high school education (ORTIME = 0.4; 95% CI = 0.2, 0.6) than among those with more than high school education (ORTIME = 0.8; 95% CI = 0.5, 1.3; P = .005 for interaction).
Paralleling earlier reports from NHANES,24 we found significant interactions for obesity status (and in our analysis for overweight) with time. Among respondents aged 60 to 69 years, both obese and overweight persons reported significantly greater increases in IADL disability (obese, ORTIME = 1.8; 95% CI = 1.2, 2.8; P = .03 for interaction; overweight, ORTIME = 2.8; 95% CI = 1.8, 4.5; P = .006 for interaction) than did persons of normal weight (ORTIME = 0.9; 95% CI = 0.5, 1.6). For the oldest age group (aged 80 years or older), respondents of normal weight were less likely to report mobility disability over time (ORTIME = 0.6; 95% CI = 0.4, 1.0) while overweight respondents showed a non-significant trend toward increased mobility disability over time (ORTIME = 1.3; 95% CI = 0.9, 2.0; P = .03 for interaction).
By contrast with earlier reports of declining prevalence in disability among older Americans,5–9 our comparison of prevalence of disability across 2 NHANES periods (1988–1994 versus 1999–2004) revealed increased reporting of all types of disability for all but the very oldest of these Americans. For respondents aged 60 to 69 years, reported prevalence of ADL, IADL, and mobility disability increased significantly over time, most starkly among non-Whites and obese and overweight persons. We observed similar, though nonsignificant, trends for reported functional limitations. For participants aged 70 to 79 years, we detected no significant differences in reported prevalence of disability over time, independent of differences in relative weight, health status, and health behaviors. Participants aged 80 years and older were the only group to show evidence of disability declines, with lower reported prevalence of functional limitations over time.
Somewhat unexpectedly, differences in sociodemographic, health status (chronic conditions and biological risk parameters), and health behavior profiles did not fully explain the increase in disability prevalence over time among respondents aged 60 to 69 years. Similarly, for persons aged 80 years and older, differences in these factors did not fully explain the lower prevalence of reported functional limitations in the more recent NHANES. Several factors could explain why our controls, particularly for health status and health behaviors, did not eliminate the differences in reported disability.
First, measures of health conditions reflected self-reports of the presence of certain conditions, along with information on a limited set of biological parameters; NHANES provided no information on duration or severity of reported conditions. For respondents aged 60 to 69 years, among whom we observed increases in disability, it is possible that more recent cohorts included individuals who survived with more severe disease or with disease of longer duration and that our controls for the presence or absence of disease did not fully capture cohort differences in disease burdens. Indeed, although declining mortality rates have contributed to overall growth in the population of older adults, most notable are the increased numbers of non-Whites now reaching older age. To the extent that non-Whites have lower SES, they may experience more severe disease stemming from their higher risk for a range of chronic health conditions and their potential reduced access to high-quality care.42
Access to care itself was less likely to be a factor because all participants were aged older than 60 years and therefore likely to be eligible for Medicare. Indeed, the NHANES data contained only a very small percentage of respondents reporting no regular health care provider, and controls for this factor did not alter our findings. For respondents aged 80 years and older, among whom we observed a decline in reported prevalence of functional limitations, it is possible that increased availability of a broader range of assistive devices contributed to lower reporting of functional limitations during more recent periods. However, item wording (participants were asked about difficulty “without use of aids”) in both surveys rendered this explanation less likely. Similarly, changes in lifestyle behaviors such as physical activity did not explain the observed trends.
NHANES collected data on current weight but not on duration of obesity or overweight status. Obesity rates among US adults have been rising since at least the 1960s (with successive cohorts exhibiting higher age-specific rates than their predecessors21,22,25), so it is not only plausible but highly likely that more recent cohorts of adults are entering older adulthood with longer duration of obesity or overweight. If longer exposure to excess relative weight contributes to increased rates of disability, our controls for current status would not fully capture such duration effects. The survey responses on physical activity also reported current levels of activity but not past levels, and these data were likely highly confounded by current disability status. Also, although NHANES asked about a range of activities from lighter to more vigorous, the data might nonetheless have underestimated differences related to more subtle changes over time in levels of physical exertion associated with everyday activities (e.g., greater use of remote control devices and greater availability of elevators, moving walkways, and other labor-saving devices). To the extent that more recent cohorts were exerting less energy for everyday activities, our controls would fail to fully capture cohort differences in physical activity.
Although these measurement issues may have contributed to our inability to fully explain observed trends in disability prevalence, it is also possible that there were additional unmeasured factors contributing to the observed time trends in disability (e.g., unmeasured lifestyle changes). It is also possible that people's definition of what constitutes difficulty in performing activities may have changed over time and had unknown effects on answers to the questions used to assess disability.
Our analyses also indicated that certain subgroups within the older population experienced significantly greater changes in disability over time. Among respondents aged 60 to 69 years, non-Hispanic Blacks (and Mexican Americans to a lesser extent) reported significantly greater increases in ADL disability over time than did non-Hispanic Whites. Similarly, obese and overweight persons exhibited greater increases than persons with normal weight in reported IADL disability. The latter findings extend a recent report in JAMA showing larger increases over time in ADL disability and functional limitations for obese individuals aged 60 years and older.24 Our results suggest that these trends are particularly stark among persons aged 60 to 69 years and hold true for overweight as well as obese individuals in this age group. Among those aged 80 years and older, women enjoyed significantly greater reductions in functional limitations than did men, as did those with less than high school education (compared with respondents with high school or higher education), but overweight participants did not exhibit the reduction in mobility disability found among normal-weight persons.
Our findings are sobering, because US adults just entering the senior population face significantly increased prevalence of multiple types of disability. Furthermore, these trends are significantly stronger among the fastest-growing subgroups of this population (non-Whites and obese and overweight persons).21,22,25,27,28 Increased burdens of disability for new cohorts of older Americans could have significant effects on the health of the US population as a whole, manifested not only in direct effects on persons moving into older age but also in indirect effects on younger cohorts. Clearly, the growing numbers of individuals aged 60 years and older, with their increased prevalence of disability, will place ever-growing demands on the health care system. Increased levels of disability (particularly among the youngest of the older adults) may also negatively affect economic productivity because individuals burdened by disability are more likely to reduce or eliminate their involvement in the labor market, reducing their contributions to tax revenues and simultaneously placing greater demands on the health care system. Disability trends may also affect younger age groups if their members have to compete with older people for scarce resources in an overburdened health care system.
In light of these potential consequences of increasing trends in disability, it will be important both to confirm these initial findings and to delve further into their sources if we are to avoid overwhelming burdens of disability among future generations of older Americans. Greater attention to the broader societal effects of such disability is needed as well: for example, we need to quantify not only direct health care costs but also the potential consequences of those expenditures, such as corresponding reductions in other areas of government spending as well as disability-related lost economic productivity and associated reduced tax revenues. Such economic data would illuminate the broader reach of disability trends—far beyond their consequences for affected individuals and their immediate families.
Indeed, to the extent that persons currently aged 60 to 69 years are harbingers of likely disability trends for the massive baby boomer generation, the health care and assistance needs of disabled older Americans could, in the not-so-distant future, impose heavy burdens on families and society. Current economic conditions only serve to underscore the imperative for the United States to undertake efforts to better delineate and understand not only disability trends and their sources but also the broader societal costs of such disability. Such data could underpin efforts to develop more effective policies and to invest in primary and secondary prevention programs to reduce disability and its associated costs.
Acknowledgments
This research was supported by the National Institute on Aging (grants R01 AG 023347 and P30 AG172365).
Human Participant Protection
No protocol approval was needed for this study because data were obtained from secondary sources.
