Background Persons with lower health risks tend to live longerthan those with higher health risks, but there has been concernthat greater longevity may bring with it greater disability.We performed a longitudinal study to determine whether personswith lower potentially modifiable health risks have more orless cumulative disability.
Methods We studied 1741 university alumni who were surveyedfirst in 1962 (average age, 43 years) and then annually startingin 1986. Strata of high, moderate, and low risk were definedon the basis of smoking, body-mass index, and exercise patterns.Cumulative disability was determined with a health-assessmentquestionnaire and scored on a scale of 0 to 3. Cumulative disabilityfrom 1986 to 1994 (average age in 1994, 75 years) or death wasthe measure of lifetime disability.
Results Persons with high health risks in 1962 or 1986 had twicethe cumulative disability of those with low health risks (disabilityindex, 1.02 vs. 0.49; P<0.001). The results were consistentamong survivors, subjects who died, men, and women and for boththe last year and the last two years of observation. The onsetof disability was postponed by more than five years in the low-riskgroup as compared with the high-risk group. The disability indexfor the low-risk subjects who died was half that for the high-risksubjects in the last one or two years of observation.
Conclusions Smoking, body-mass index, and exercise patternsin midlife and late adulthood are predictors of subsequent disability.Not only do persons with better health habits survive longer,but in such persons, disability is postponed and compressedinto fewer years at the end of life.
The compression-of-morbidity hypothesis1,2,3 suggests that itmay be possible to reduce cumulative lifetime morbidity. Sincechronic illness and disability usually occur late in life, cumulativelifetime disability could be reduced if primary-prevention measurespostponed the onset of chronic illness. However, decreases inhealth risks may also increase the average age at death.4,5,6,7,8,9The compression-of-morbidity hypothesis predicts that the ageat the time of initial disability will increase more than thegain in longevity, resulting in fewer years of disability anda lower level of cumulative lifetime disability.
Some contend that healthier lifestyles may actually increasemorbidity (and health expenditures) late in life by increasingthe number of years of chronic illness and disability.9,10,11,12Thus, cumulative lifetime disability could actually be greaterin persons with lower health risks than in those with higherhealth risks.4,5,6,7,8,9
The data on cumulative lifetime disability are limited, particularlydata on cumulative disability in persons with different levelsof health risk. National trends in health behavior have beeninconsistent. In recent years, for example, the prevalence ofsmoking has decreased, but the prevalence of obesity has increased.13,14Hence, it is difficult to evaluate the effect of changes inhealth behavior on the onset of disability, life expectancy,and cumulative disability. Recent studies suggest a decreasein cumulative disability over time.15,16,17,18 In addition,persons with higher socioeconomic status19 and higher educationallevels20 and those who engage in regular aerobic activity21,22,23,24have substantially better health,25 suggesting the possibilityof a compression of morbidity in such persons.
We tested the compression-of-morbidity hypothesis by performinga longitudinal study of cumulative disability in relation tothree levels of health risk. Our hypothesis was that personswith lower health risks will have disability later in life,will have less disability at any given age, and will have lesscumulative disability than persons with greater health risks.
Methods
In 1986, investigators for the Arthritis, Rheumatism, and AgingMedical Information System acquired access to a unique set ofdata from persons attending the University of Pennsylvania in1939 and 1940, who were studied at that time and were subsequentlysurveyed in 1962, 1976, and 1980.26,27,28,29,30 Since 1986,2841 alumni have been studied. Mailed health-assessment questionnaireshave been used to obtain information on disability, medicalhistory, and health habits in this population on an approximatelyannual basis. Seven surveys were performed between November1986 and March 1994.
The survey participants were included in this study if theymet three criteria. First, they had to have completed at leasttwo consecutive questionnaires, so that at least two consecutiveyears of disability data would be available preceding any deaths;397 subjects were excluded because they did not meet this criterion.Second, the subjects had to have completed all survey questionnairessince 1986 or, in the case of those who died, all questionnairespreceding their deaths; 306 participants (11 percent) were lostto follow-up and it was not known whether they were dead oralive. Third, the participants had to be between 63 and 72 yearsold in 1986; an additional 397 subjects did not meet this criterion.The resulting study population consisted of 1741 subjects; 77percent were men, and 99 percent were white. Complete data ondisability were available for more than 97 percent of the subjects.The mean (±SD) age in 1986 was 67.0±2.8 years.
Variables of interest included age, sex, body-mass index (theweight in kilograms divided by the square of the height in meters),smoking habits, exercise, number of chronic conditions, useof medical services, and disability index. Exercise was definedas the number of minutes per week during which the subject engagedin physical activity and became short of breath or worked upa sweat. Body-mass index was calculated by dividing the weightin kilograms by the square of the height in meters. Chronicconditions included arthritis, back pain, osteoporosis, heartdisease, high blood pressure, stroke, diabetes, cancer, andlung diseases.
The health-assessment questionnaire, used to measure the dependentvariable, assesses activities of daily living. Each of eightactivities (dressing, rising, eating, walking, grooming, reaching,gripping, and performing errands) is scored as 0 (no difficulty),1 (some difficulty), 2 (much difficulty), or 3 (unable to do),and the scores are averaged across the eight activities. A cumulative-disabilityindex of 3.0 could represent the inability to perform any oneof the eight activities for one year, some difficulty performingall eight activities for three years, or the inability to performtwo activities for four years. The average disability indexis 1.2 per year for patients with rheumatoid arthritis and 0.8per year for those with osteoarthritis. Minimal disability (anindex of 0.1) represents some difficulty in performing one ofthe eight activities. This disability measure has been widelyused, is sensitive to change, and has been extensively validated.31,32,33,34,35
The three end points of the study were disability in the lastyear of observation, disability in the last two years of observation,and cumulative disability, which was calculated as the sum ofthe disability indexes since 1986. Cumulative disability wasused as a surrogate measure for total disability. For the deceasedparticipants, cumulative disability was estimated as the sumof the disability values from 1986 until the last completedquestionnaire. The last disability index was the index basedon the last survey before death, for the subjects who died,and the index in 1994, for the survivors. Disability in thelast two years was calculated as the sum of the last two indexes.
Body-mass index, smoking, and exercise were selected as stratificationvariables on the basis of the known associations between thesevariables and disability.21,23,24,36,37,38 Subjects were assignedscores of 0 to 3 for body-mass index (<22.5, 22.5 to <24.0,24.0 to <26.0, or >26.0), number of cigarettes per day(0, 1 to 20, 21 to 30, or >30), and minutes of vigorous exerciseper week (>240, 120 to 239, 1 to 119, or 0), as determinedin 1986 and also in 1962. Body-mass index had a J-shaped relationto subsequent disability in this data set, and although thepower of the study may have been decreased as a result, we selectedcutoff points for stratification to be consistent with thosein other studies.22,23,24,36,37,38 The scores for the threerisk factors were totaled, and the subjects were assigned alevel of risk according to the total score (low risk, 0 to 2;moderate risk, 3 or 4; high risk, 5 to 9).
To eliminate the effect of base-line disability, parallel analyseswere performed with data from participants without initial disability.The postponement of disability was assessed by comparing theaverage age at the onset of minimal disability (score, 0.1)in the three strata. We repeated the main analyses with stratadetermined on the basis of the 1962 data, when the average ageof the participants was 43 years, allowing an assessment ofthe association between disability and the health-risk scoreat midlife, 32 years before the last observation (i.e., at atime when minimal disability would be expected).
Analyses were performed with SAS software.39 Pearson's correlationcoefficients were calculated with cumulative disability as thedependent variable. Two-tailed t-tests were performed for thethree disability end points, with comparisons of the low- andmoderate-risk groups and the low- and high-risk groups. Testsfor trend were performed by fitting the dependent variablesto the risk-factor strata, coded as 1, 2, and 3.
Results
The mean disability index in 1986 was low (0.06±0.16),and 74 percent of the subjects had no disability, suggestingthat the disability index in 1962 (not available) would havebeen close to zero.37 Univariate correlations between cumulativedisability and the independent variables, as determined in 1986,were analyzed; initial values for the disability index had thehighest correlation (r = 0.64, P<0.001). Female sex and agreater number of chronic conditions were also correlated withcumulative disability (r = -0.16 and r = 0.20, respectively;P<0.001 for both comparisons). Age and educational levelwere not significantly associated with cumulative disability(r = 0.02 for both comparisons), probably because of the nearlyuniform age and educational level in this population. Greatercumulative disability was associated with a larger number ofcigarettes smoked per day (r = 0.09, P<0.001), less exerciseper week (r = -0.11, P = 0.001), and a higher body-mass index(r = 0.11, P = 0.001), suggesting approximately equal contributionsfrom each of the three risk factors.
The results were similar in the group of subjects who were excludedfrom our study, although they were slightly older (70.0 vs.68.2 years) and had a higher initial disability index (0.10vs. 0.08) and lower exercise levels (111 vs. 133 minutes). Asignificantly larger proportion of deceased subjects were men(84 percent, vs. 77 percent of survivors) with a higher initialdisability index (0.15 vs. 0.06), a larger number of cigarettessmoked per day (3.3 vs. 1.6), and a larger number of chronicconditions (0.66 vs. 0.46).
The disability outcomes are shown in Table 1 according to riskstrata (based on body-mass index, exercise, and smoking). Theinitial age and the number of questionnaires completed weresimilar in the low-, moderate-, and high-risk groups, with anaverage of 6.94, 6.99, and 6.99 questionnaires completed bythe surviving subjects, respectively, and 4.6, 4.5, and 4.2completed by those who died, respectively. The subjects in thehigh-risk group had an average cumulative-disability index of1.02, and the values for the subjects in the moderate- and low-riskgroups were 0.71 and 0.49, respectively (P for trend, <0.001).There were similar differences among the three risk groups fordisability in the last year and the last two years of observation.For subjects with no initial disability, the cumulative valuesshowed a similar trend (P<0.01). For all comparisons, therewas substantially more disability with higher risk than withlower risk. For the total study population, the disability indexesin the high- and low-risk groups differed by approximately afactor of two. Mortality rates also differed among the threegroups, although the differences were smaller: 11.9 percentin the high-risk group, 9.9 percent in the moderate-risk group,and 7.9 percent in the low-risk group. Thus, the high-risk grouphad approximately a 50 percent higher mortality rate and approximately100 percent greater disability than the low-risk group.
Table 1. Disability According to Health Risk in 1986.
Surviving subjects in the high-risk group had a cumulative-disabilityindex of 0.91, whereas those in the moderate- and low-risk groupshad values of 0.68 and 0.45, respectively (P for trend, <0.001).For the subjects without initial disability, the differencespersisted but were somewhat less striking. Among deceased subjects,the high-risk group had a cumulative-disability index of 1.8,whereas the moderate- and low-risk groups had average valuesof 0.93 and 0.86, respectively (P for trend, <0.05). In thegroup of deceased subjects without initial disability, the moststriking difference in cumulative disability was in the high-riskgroup as compared with the other two groups.
Since men were overrepresented in the high-risk group, sex-specificanalyses were performed. Women had substantially more disabilityby all measures; however, the differences among the three health-riskgroups were similar for men and women (Table 1).
Table 2 shows the results of analyses of disability accordingto the health risk in 1962 (similarly determined according tobody-mass index, exercise, and smoking in subjects for whomthese data were available). These data, obtained when the averageage of the subjects was 43 years, are assumed to represent minimaldisability, given an average disability index of only 0.06 in1986, and to reflect health risks at midlife. The results ofthese analyses are consistent with those shown in Table 1, withdisability scores in the high-risk group that were approximatelytwice those in the low-risk group (P<0.001).
Table 2. Disability According to Health Risk in 1962.
Figure 1 shows the disability index in the three risk groupsaccording to the year. The rates of progression were similarin the three groups although slightly more rapid in the high-riskgroup than in the other two groups. The onset of minimal disability(an index of 0.1) was postponed by approximately five yearsin the low-risk group as compared with the high-risk group.An analysis of disability according to age (Figure 2) showedthat the onset of minimal disability was postponed by approximatelyseven years in the low-risk group as compared with the high-riskgroup.
Figure 1. Disability Index from 1986 to 1994 in 1741 Subjects, According to the Year and Level of Health Risk in 1986.
The rate of progression in average disability as assessed on the basis of the disability index (on a scale of 0 to 3) was similar in the three risk groups, but the progression to a particular level of disability was postponed by approximately five years in the low-risk group as compared with the high-risk group. The horizontal line indicates a disability index of 0.1, which corresponds to minimal disability (some difficulty in performing one of eight activities of daily living, such as walking).
Figure 2. Disability Index According to Age at the Time of the Last Survey and Health Risk in 1986.
Average disability increased with age in all three risk groups, but the progression to a given level of disability was postponed by approximately seven years in the low-risk group as compared with the high-risk group. The horizontal line indicates a disability index of 0.1, which corresponds to minimal disability.
Figure 3 shows the cumulative disability in all subjects, thosewithout disability in 1986, those who survived, and those whodied, according to the level of risk. The results are robustand consistent for the study end points and for the subgroupsof subjects.
Figure 3. Mean (±SE) Cumulative Disability According to Health Risk in 1986.
Cumulative disability was assessed after an average age of 67 years. The data represent approximately eight years of follow-up for subjects who were alive at the end of the study period and five years of follow-up for subjects who died. The disability index in the high-risk group was approximately twice that in the low-risk group for all subjects, those with no disability in 1986, those who survived, and those who died. A score of 1.0 corresponds to moderate disability in performing two of eight activities of daily living, such as walking and rising from a chair, for two years or mild disability in performing all eight activities for one year.
Discussion
This study shows that persons with lower health risks (definedin terms of smoking, body-mass index, and exercise) have initialdisability at an older age and have lower levels of cumulativedisability and disability at any given age than do persons withhigher health risks. These findings suggest that for the averageperson, efforts to reduce modifiable health risks may resultin a postponement of initial disability and decreased lifetimedisability.
The main results of the study were similar in men and women,in persons with no initial disability and those with initialdisability, and in persons who survived and those who died duringthe study period. The marked differences in the degree of disabilityamong the three risk groups persisted throughout nearly eightyears of follow-up, to an average age of 75 years. The resultswere similar whether the risk strata were defined on the basisof midlife (1962) data or 1986 data. Disability indexes in thelow- and high-risk groups differed by a factor of approximatelytwo, and the differences were statistically significant. Disabilitywas postponed by more than five years in the low-risk groupas compared with the high-risk group. Among the subjects whodied, both cumulative disability and disability in the one ortwo years before death were much lower in the low- and moderate-riskgroups than in the high-risk group. Similarly, among the survivors,cumulative disability was much lower in the low-risk group thanin the other two groups.
Caveats apply to the results. The study population had a higheducational level, was relatively homogeneous in terms of ageand socioeconomic status, and was almost entirely white. Overthree fourths of the subjects were men, but separate analysesof men and women had similar results. In addition, since thestudy end points were determined on the basis of responses toa questionnaire, there is the possibility of bias. However,the health-assessment questionnaire used to determine disabilityhas been repeatedly validated.31,32,33,34,35 In a study of runnersand controls, for example, we found no differences between thetwo groups in reliability or in correlations with spousal estimatesof disability.21 However, for the subjects in our study, thetime of greatest disability (after the age of 85 years) is stillin the future. As the study continues, it will be possible toassess the effects of changes in specific risk factors suchas cessation of smoking.
Eleven percent of the subjects were lost to follow-up and theirstatus (dead or alive) was not known, which is another potentialsource of bias. These subjects were slightly older and moredisabled and had more hospitalizations in 1986 than the subjectswho were followed, and it is possible that many of them diedor were institutionalized. If more low-risk persons than moderate-or high-risk persons were lost to follow-up, the imbalance couldproduce a bias. However, the proportions of persons lost tofollow-up were similar in the low-, moderate-, and high-riskgroups and in the groups of patients with and without disabilityin 1986.
This study documents a strong association between the levelof health risk and subsequent disability but does not provecausality. It is possible that other, unmeasured variables arecorrelated both with risk-factor scores and with cumulativedisability. However, with age, education, and race essentiallyheld constant in our study, and with prior studies indicatingthat smoking, obesity, and level of exercise are independentlyrelated to disability, it is difficult to think of additionalcausal variables.
Initial disability might have been a confounding variable, sinceit is strongly associated with cumulative disability and sincehealth habits might have been modified in response to earlydisability. In the group of subjects without initial disability,the results were less robust. It is more likely, however, thatthe early disability was the result of a high health risk beforethe study began. We examined this issue both by performing aseparate analysis of the subjects without initial disabilityand by assigning the subjects to risk groups at an average ageof 43 years, when disability should have been minimal. In bothinstances, the results were consistent with those in the overallstudy population.
Our data base contains serial data on disability, which areavailable in few other data bases. Previous longitudinal studieshave not examined the relation between cumulative disabilityor mortality and health risks, despite the relevance of suchstudies to health policy issues.23,40,41 None of these previousstudies have reported on disability until the end of life, whichpermits a direct estimation of total lifetime disability.
Our study provides evidence that persons with lower health riskshave less lifetime disability, as well as less disability atany given age. The results are consistent with the hope thatfuture increases in the number of elderly persons will be offsetin part by greater vitality on the part of the elderly, if thereare improvements in modifiable health risks. Thus, the implicationsfor public health are important. Primary prevention is a positiveapproach to reducing mortality and morbidity.42,43 Some havefeared that preventive measures may lead to an expanding populationof frail, disabled persons. This study suggests instead thatlower health risks will result, on average, in less lifetimedisability.
Supported by grants from the National Institute of Child Healthand Human Development (R01 HD35641-01), the National Instituteof Arthritis and Musculoskeletal and Skin Diseases (AR43584),and the Donald and Delia Baxter Foundation.
We are indebted to Daniel A. Bloch, Ph.D., and Raymond Balise,Ph.D., for statistical assistance and to Dena Ramey for editorialassistance.
Source Information
From the Department of Medicine, Stanford University School of Medicine, Stanford, Calif.
Address reprint requests to Dr. Fries at 1000 Welch Rd., Suite 203, Palo Alto, CA 94304-1808.
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