a Department of Epidemiology & Public Health, Institute of Child Health,30 Guilford Street, London WC1N 1EH, UK.
b School of Public Health and Community Medicine, Hebrew University, Jerusalem, Israel.
Reprint requests to: Dr C Power, Department of Epidemiology & Public Health, Institute of Child Health, 30 Guilford Street, London WC1N 1EH, UK. E-mail: c.power{at}ich.ucl.ac.uk
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Abstract |
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Methods Data were from the 1958 British birth cohort, including the original birth survey and follow-ups at ages 7, 11, 16, 23 and 33 years. Limiting longstanding illness was the outcome at both ages 23 and 33. Potential predictors included childhood health and physical development, socioeconomic conditions in early life and adulthood, and behavioural factors. We estimated the effect of potential explanatory factors using logistic regression, in both univariate and multivariate analyses, separately for limiting illness at 23 and 33 years.
Results Prevalence of limiting illness increased from 5.1% (men) and 4.1% (women) at age 23 to 6% for both sexes at age 33. Risk of limiting illness at age 33 was greater for those reporting an illness at age 23 (29.4%, compared with 4.7% of those without illness), though the majority (66%) of 33-year limiting illnesses had no previous record at age 23 or for childhood. Multivariate analysis of limiting illness at age 23 confirmed the high risk for those with childhood disability and also established two further major predictors, namely, injury (adjusted odds ratio [OR] = 1.42, 95% CI : 1.091.86) and intermediate socio-emotional status (adjusted OR = 1.73, 95% CI : 1.292.31). Additional risks were identified for limiting illness at age 33, including: (i) injury in the preceding 10 years (adjusted OR = 1.55, 95% CI : 1.182.04); (ii) body mass index (BMI), for which the relationship was non-linear, with elevated risks for the underweight (adjusted OR = 1.53, 95% CI : 1.032.26) and overweight (OR = 1.28, 95% CI : 0.871.89); (iii) childhood disadvantage at either or both ages 7 and 11 (adjusted OR = 1.53, 95% CI : 1.072.17); and (iv) height at age 7, with a significant non-linear relationship (the adjusted OR for height less than 15th percentile was 1.43 and for height more than the 85th percentile, 1.30).
Conclusions Both childhood and adult factors predict limiting illness in early adulthood. Childhood is important because some adult illnesses originate in early life, and also because childhood environment influences the risk of adult limiting illness several years later. Our findings suggest that studies seeking to understand the causes of limiting illness, that currently tend to focus exclusively on contemporary factors, need also to consider the contribution of environment in early life.
Keywords Disability, chronic illness, longitudinal cohort
Accepted 22 July 1999
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Introduction |
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Some studies have investigated explanations for chronic illness and disability, but the cross-sectional data available have restricted the conclusions that can be drawn e.g.1,2,7,8 Few studies use longitudinal data, although some exceptions exist, notably for predictors of disability pensions in young men,9 physical disability10 and physical functioning.11 To our knowledge no previous longitudinal analysis has been undertaken for limiting longstanding illness, apart from the basic relationships described for the Health and Lifestyle Survey.12 Longitudinal data are necessary to establish the extent to which biological and social risks experienced at different life stages combine to influence adult disease. Such data can contribute to understanding how risks from early life operate, whether through a pathways model (whereby early life environment affects subsequent life trajectories, that in turn influence adult health) or through a latency model (which emphasizes the impact of influences experienced at particular critical or sensitive periods).13
The purpose of the present study is to identify risk factors for disabling chronic illness, as measured by self-reported limiting longstanding illness, and to consider how these risks operate across the life course. The data used are from birth to age 33 years available from the 1958 British birth cohort study which obtained information on health conditions in childhood and on limiting longstanding illnesses in early adulthood. We investigate, first, the extent to which an individual's rating of limiting illness is stable over a 10-year period, age 2333; second, the relationship between childhood and adult disability; and third, we identify risk factors at different life stages for limiting illness in early adulthood. For the purposes of this third aim, we consider influences on limiting illness at both age 23 and 33, including potential explanatory factors representing childhood health and physical development, socioeconomic conditions in early life and adulthood, and also behavioural factors.
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Methods |
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Outcome measure
Respondents were asked, at both age 23 and age 33, whether they had a longstanding illness, disability or infirmity that limited their daily activities in any way compared to people of the same age. The question is similar to that used in the General Household Survey,4 and similar but not identical to that in the 1991 UK population census.5
Explanatory variables
Physical development and childhood health status
Weight at birth was recorded in ounces. Heights and weights were measured at ages 7 and 33. Body mass index (BMI) was derived as weight (kg)/height (m)2. Although birthweight, height and BMI were analysed as continuous variables, we show data in five percentile groups using cut-offs in Table 1. Disability at age 7 (739/14 627) was identified from (i) school doctor reports of moderate or severe handicap in respect of ordinary schooling for one or more specified abnormal conditions' (such as epilepsy or diabetes), and (ii) mothers' reports of a physical handicap or disabling condition. Disability at age 16 (1249/13 041) was identified from: (i) school doctor reports of anticipated moderate or severe handicap with respect to future employment due to specified abnormal conditions', similar to those specified at age 7, and (ii) a health visitor's assessment that the study subject had a handicapping condition. Enuresis before age 7 was identified from parental reports that the study child was wet during the day after age 3 or at night after age 5, and nocturnal wetting in the preceding month was recorded at age 11. Of 12 288 subjects with information at both ages, 3.5% (436) were wetting at both ages, 8.9% (1098) at age 7 only, and 1.1% (144) at age 11. Socio-emotional status was assessed by teachers using the Bristol Social Adjustment Score (BSAG) at age 7, and the Rutter Behaviour Scale at age 16.16 Total scores on both scales were transformed (square root transformation) and categorized as well adjusted, intermediate and poor. Injuries from infancy to age 16 requiring hospital attendance at casualty or admission were reported by parents. At age 33 subjects reported injuries occurring in the preceding 10 years that had required hospital admission, accident and emergency attendance, or outpatient department attendance.
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Adult socioeconomic conditions and lifestyle
Employment at age 23 was classified as having current paid work versus other (including housewives, students, unemployed and out of the labour force for other reasons). Housing tenure at age 23 was categorized as local authority (or housing association) renter, owner occupier and other. Regular smoking was defined as smoking one or more cigarettes a day for 12 months at both ages 23 and 33. Heavy drinkers had reported consuming >50 units of alcohol/week for men and >35 units/week for women.
Data analysis
Transitions (that is, stability or change) in limiting longstanding illness were assessed first between ages 23 and 33, and second, according to reported childhood disability at ages 7 and 16. Using a likelihood ratio test we considered a number of models, the most parsimonious model which gave a good fit to the observed data was that based on the number of previous reports of a limiting longstanding illness at ages 7 and 16. The effect of potential predictive factors was examined first in univariate analyses separately for limiting illness at age 23 and at age 33. Odds ratios (OR) and associated 95% CI were obtained from logistic regression. For this univariate stage, birthweight, height and BMI are presented as categorical variables, although they were analysed as continuous variables and the appropriateness of linearity in relation to limiting illness was tested by the addition of a quadratic term to the models. Initial analyses had shown generally similar relationships existed for men and women. Therefore, all results are presented for both sexes combined. A few exceptions showing a significant interaction with sex were: (i) for limiting illness at age 23, disability at age 7 and childhood disadvantage (stronger relationships emerged for females); and (ii) for limiting illness at age 33, not having paid work at age 33 (a stronger association for women). All analyses included an adjustment for sex. As evident from the results presented below, reports of limiting illness were strongly associated over the 10-year period. Hence predictors for age 33 are likely to include factors influencing limiting illness previously at age 23. We therefore analysed the effect of potential predictors on limiting illness at age 33 adjusting for limiting illness at age 23, to identify factors that had an additional contribution to 33-year illness.
Preliminary multivariate analyses were undertaken for the repeat measures (such as height and BMI) to identify the ages contributing most to the prediction of limiting illness. Finally, we conducted multivariate analyses that included significant predictors identified from the univariate analyses. Predictors occurring at the same life stage, for example early childhood factors (represented by age 7 variables), were analysed simultaneously and variables that were no longer significant in these analyses were omitted from a final model separately for limiting illness at age 23 and for age 33.
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Results |
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Multivariate analyses
Table 5 presents results from multivariate analyses of 5513 subjects with complete information on all relevant predictors for limiting illness at age 23. Given this reduction in sample size, we repeated the univariate analyses (column 1) to identify whether the results differed from those observed for the full sample used in Figure 1
, and Tables 3 and 4
. Reassuringly, sample biases were small for most factors, although more serious biases were identified for birthweight and enuresis. For example, the OR associated with enuresis (at both age 7 and 11) was 1.47 in the sample available for multivariate analysis (Table 5
), compared with 2.30 in Table 3
. Multivariate analyses were conducted in stages, first of factors occurring at similar life stages (models 13) and second, in a full model. The full model showed that childhood disability, injury and intermediate socio-emotional status at age 16 all remained significant predictors of subsequent limiting illness at age 23 (with adjusted OR respectively of 4.15 for age 7, 1.42 and 1.73). Whilst, models 13 suggested that some factors were markers for other co-occurring predictors. For example, the effect of height at age 7 was weakened after adjusting for concurrent disability and socio-emotional status.
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Discussion |
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Stability of chronic illness from childhood to early adulthood
Three main points emerge in relation to the stability of limiting/disabling conditions. First, reporting of limiting illness within early adulthood shows some stability over time, at least between ages 23 and 33: 29.4% of subjects reporting a limiting illness at age 23 consistently reported an illness 10 years later (compared with only 4.7% of those without a 23-year illness). Understandably, the progression of limiting illnesses is not inevitable, as there is likely to be remission for particular conditions, such as epilepsy.23 Second, there was a strong association between a medical assessment of chronic or disabling conditions (in childhood) and the subject's self-reports (in early adulthood). A related third point, is that limiting illness in early adulthood is strongly associated with that in childhood. This is likely to reflect the persistent nature and associated health consequences of many childhood chronic conditions, including for example, sensory impairment or diabetes. It is reassuring, nonetheless, that self-reported limiting illness shows this expected relationship with medically assessed chronic disability. Comparisons with other studies are limited due to the scarcity of longitudinal data, but our findings are consistent with the 1946 birth cohort in showing strong adult life disability risks for those with serious conditions in childhood.10 The strong associations between childhood and adulthood, and within early adulthood, provide evidence that, as intended, conditions of longstanding are being reported. Despite these associations over time, we cannot exclude the possibility that some misclassification of chronic illness and disability occurs with self-reported limiting illness. Further work will be required on the type of conditions that individuals regard as limiting longstanding illnesses, but meanwhile, the evidence presented here and elsewhere22 is reassuring for reported limiting illness.
Explanations for incident disability
The majority (77%) of cohort members with a limiting illness at age 33 had no such report at age 23. When childhood chronic illnesses and disabilities are discounted there was still a majority with no previously recorded problem. This large proportion of incident cases was anticipated given that it is commonplace to observe increasing prevalence of disability with increasing age. Our analyses contribute to previous studies by suggesting factors that may be responsible for increasing prevalence during early to mid-life.
Accepting that some limiting or chronic problems in adulthood originate in early life, including congenital conditions, the analyses were constructed to establish which subsequent factors contributed to limiting illness in early adulthood. Some factors, such as birthweight and height, are known to be related to childhood disability.24,25 Reassuringly, our data suggest these well-established associations, although unfortunately we were not able to assess the role of birthweight in multivariate analyses given that relevant samples available for analysis underrepresented those weights at birth with the greatest limiting illness risk. As regards other factors, the data appeared to be more robust and plausible associations were demonstrated, for example, with respect to socio-emotional status and injuries. As we have argued elsewhere,19,26 the contribution of injuries to limiting illness is particularly important during early adulthood. For BMI, the strong predictive effect is consistent with the general literature on underweight and obesity,27 and more specifically with higher risk of disability pension among overweight Finnish men and women compared with those who were leaner.28
It is potentially of great interest that some factors may exert a latent effect, in that they do not affect the risk of chronic illness or disability in the short term, but do so some several years later. Childhood disadvantage was notable in this regard, with an OR of 1.52 for limiting illness at age 33 for those experiencing disadvantage at either or both age 7 and 11. This excess risk was established in an analysis that simultaneously took account of several other factors, including disability at earlier ages. Disadvantage in the childhoods of the 1958 cohort cannot be compared directly with that experienced currently, but it is nonetheless a concern that, in 1992 in the UK, 29% of children were defined as living in poverty.29 Our results suggest that there may be long-term health effects arising from the adverse circumstances experienced by these children. Support for this conclusion is provided by other studies showing that economic hardship in childhood increases the risk of physical and mental health problems several years later in adulthood.30,31 An additional implication of a latency effect of childhood disadvantage is that socioeconomic status effects on chronic illness/ disability may be less pronounced in childhood, and then strengthen over time. This could in part explain why social class gradients in limiting illness appear to emerge in early adulthood, whilst being of lesser importance at younger ages.32
Long-lasting effects of height were also found in this analysis of the 1958 birth cohort, particularly in relation to limiting illness at age 33, and with a stronger effect of height in childhood than that achieved in adulthood. The relationship between height and limiting illness was non-linear with the greatest risks for the shortest individuals, but with elevated risks also for the tallest group. Growth impairment is a well-recognized correlate of chronic illness in childhood25 and hence part of the effect of height operates through the persistence of disability originating in childhood. (Hence, the relationship between height and limiting illness attenuates with adjustment for childhood disability, particularly for limiting illness at age 23.) However, our analyses suggest an additional, and possibly once again latent, effect in that childhood height was associated with 33-year limiting illness even when allowance was made for earlier chronic illness and disability. The predominant effect of height at age 7 suggests that slow growth is more relevant than final height, which may be a less sensitive measure of growth deficits in early life because of a later compensating effect of catch-up growth. Our results accord with studies of later life outcomes in which shorter leg length in childhood was associated with elevated risks of coronary heart disease.33 It is also of interest that tallness increases the risk of limiting illness in early adulthood. This may reflect the propensity of taller individuals to musculoskeletal problems, especially back pain, which are an important cause of disability at this life stage, although the evidence linking stature and back pain is inconsistent.34 However, taller stature is associated with other health risks later in life, including cancer, particularly among men.33 Our findings for both height and disadvantage in childhood need to be confirmed in other studies and will be of particular interest in future follow-ups of this cohort. From the evidence presented here, it would appear that early life factors are important in relation to adult health status, as suggested by a growing number of studies.35,36
Indicators of health status
Few studies are able to collect the breadth of data on health status that is available within the 1958 birth cohort. Consequently, there has been increasing reliance on proxies, such as height, on the grounds that such indicators predict mortality. Our analysis of limiting illness suggests that commonly used indicators of health status, including birthweight, height, BMI, are also useful indicators of chronic illness and disability. Indeed, for the age group examined it is notable that many of the major risk factors for limiting illness are similar to those identified in relation to mortality. However, this pattern could change with increasing age with the onset of chronic adult conditions that are associated with a differing set of predictors.
To conclude, this paper demonstrates a contribution of childhood factors to limiting illness in adult life. This contribution appears to be due to differing processes. The first involves strong associations over time in the experience of chronic illness/ disability either from a persisting condition or consequences related to that condition. Because most subjects with a limiting illness at age 33 had no prior chronic illness or disability, this first pathway may not prove to be the major explanation for a link between child and adult limiting illness or disability. A second possible process is also suggested however, whereby childhood factors, such as poor childhood growth, exert a latent effect on adult health. This link between childhood and adult limiting illness is not yet widely appreciated. As a consequence, the importance of childhood environment for adult disability will be understated in many studies. This does not detract from the importance of adulthood predictors of limiting illness, which in the 1958 cohort included injury and BMI. Our findings suggest, however, that studies seeking to understand the causes of limiting illness, that currently tend to focus exclusively on contemporary factors, need also to consider the contribution of factors occurring in earlier life.
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Acknowledgments |
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References |
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