A prospective study of limiting longstanding illness in early adulthood

Chris Powera, Leah Lia and Orly Manorb

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


    Abstract
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Background Chronic illness and disability are of increasing public health importance but little is known about the lifetime influences involved in their onset and progression. We aim to (i) establish whether an individual's rating of limiting illness is stable over a 10-year period from age 23 to 33; (ii) assess the relationship between childhood and adult disability; and (iii) identify lifecourse influences on limiting illness in early adulthood.

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 : 1.09–1.86) and intermediate socio-emotional status (adjusted OR = 1.73, 95% CI : 1.29–2.31). Additional risks were identified for limiting illness at age 33, including: (i) injury in the preceding 10 years (adjusted OR = 1.55, 95% CI : 1.18–2.04); (ii) body mass index (BMI), for which the relationship was non-linear, with elevated risks for the underweight (adjusted OR = 1.53, 95% CI : 1.03–2.26) and overweight (OR = 1.28, 95% CI : 0.87–1.89); (iii) childhood disadvantage at either or both ages 7 and 11 (adjusted OR = 1.53, 95% CI : 1.07–2.17); 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


    Introduction
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
With increasing life expectancy, chronic illness and disability have become an important focus of public health. The primary reason for this is that chronic conditions require considerable health and social service resources. Hence, information is now collected routinely in many countries in order to obtain prevalence estimates of chronic illness and disability. One of the most common indicators of chronic illness is limiting longstanding illness, which has been used extensively for example in demonstrating international health differences1,2 and time trends.3 Various measures of limiting longstanding illness, including that recorded in the British General Household Survey4 and 1991 Census,5 have been investigated to establish their usefulness in the planning of health services and resource allocation.6–8 Given the public health importance of chronic illness and disability, a more in-depth understanding is required of its causes. Furthermore, a particular focus on limiting longstanding illness is justified because of its widespread use as a morbidity index in national health surveys throughout Europe.1,2

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 23–33; 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.


    Methods
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Study sample
The 1958 birth cohort includes all children born in England, Wales and Scotland during the 3–9 March 1958. Information was obtained for 98% of births totalling 17 414. Follow-up of survivors was undertaken at ages 7, 11, 16, 23 and 33, with 11 405 subjects re-interviewed most recently in 1991.14 Those remaining in the study have been found to be generally representative of the original birth sample.14,15 Data sources include parents, teachers, study subjects and a school medical examination for ages 7, 11 and 16; and a personal interview for ages 23 and 33.

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 1Go. 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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Table 1 Percentile categories for birthweight, height at 7 years and33 years, and body mass index (BMI) at 23 years
 
Early life socioeconomic conditions
Disadvantage was defined as having one of the following, as reported by parents for ages 11 and 16: single parent or large family, overcrowded home or lack of hot water, free school meals or supplementary benefit.17 The definition for age 7 was similar except contact with the National Assistance Board was used rather than free school meals/supplementary benefit.18 Of the subjects, 15.2% (1496/9832) were categorized as disadvantaged in childhood (age 7 or 11) and 27.2% (2667/9823) as disadvantaged at age 16 or earlier.

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.


    Results
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Prevalence and stability of limiting illness
The prevalence of limiting illness increased from 5.1% and 4.1% for men and women respectively at age 23 to 6% for both sexes at age 33. Hence, women had a lower risk of limiting illness than men only at the younger age (OR = 0.78, 95% CI : 0.66–0.93). Prevalence rates in early adulthood disguise the extent of change in limiting illness status, with more than two-thirds of those reporting a limiting illness at age 23 having no such illness 10 years later (Table 2Go). Conversely, the majority (77%) of cohort members with a limiting illness at age 33 had no such report at age 23 and this percentage remains substantial at 66% when those with a childhood disability are discounted. Risk of limiting illness at age 33 is, nonetheless, elevated for those reporting an illness 10 years earlier (29.4% compared with only 4.7% of those without a limiting illness at age 23). Moreover the stability of limiting illness in early adulthood varied according to whether a disability had been identified previously in childhood. Notably, the stability of limiting illness from age 23 to 33 was greatest for those with a disability at both ages 7 and 16, and least stable for those without a prior disability (Table 2Go). These data suggest a persistence of chronic illness and disability from childhood to adulthood, as would be expected for several conditions, such as sensory impairment or diabetes.


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Table 2 Transitions of limiting longstanding illness (ages 23 and 33) according to childhood disability status
 
Predictors of limiting illness
Physical development and childhood health status
Birthweight was significantly related to limiting illness (adjusted for sex), although this was a non-linear association for limiting illness at age 23 and linear for age 33 (Figure 1Go). For limiting illness at age 23 the association approximated a reverse J-shape, with little differentiation of risk among those with higher and medium weights at birth, but greater risks for the lowest weights. Subjects who were below the 10th percentile had an OR of 1.62 (95% CI : 1.22–2.17) compared with those between the 25th and 75th percentiles. A linear relationship for limiting illness at age 33 was not significant after adjusting for limiting illness at age 23. Height was also associated with limiting illness in early adulthood, with significant non-linear relationships for height at both ages 7 and 33 (Figure 1Go). Short stature at age 7 was associated with increased risk (OR = 1.87, 95% CI : 1.40– 2.50) of limiting illness at age 23; an elevated OR of 1.31 (95% CI : 0.98–1.77) was also observed for taller children. An effect of height at age 7 on limiting illness at age 33 persisted after adjusting for limiting illness at age 23, suggesting that height in childhood has a predictive effect on limiting illness over the long term. There was also a significant non-linear relation-ship with limiting illness for BMI at age 23, with increased risks among both the leanest and fattest subjects (Figure 1Go). Men and women below the 15th BMI percentile had an OR of limiting illness at age 23 of 1.42 (95% CI : 1.10–1.84), while the OR for those above the 85th percentile was 1.69 (95% CI : 1.33–2.15). Similar risks were evident for limiting illness at age 33, with an excess risk remaining for both the leanest and fattest subjects after adjusting for 23-year limiting illness.



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Figure 1 Odds ratio (OR) (sex adjusted) for limiting illness at ages 23 and 33 years* for percentiles{dagger} of birthweight, height and BMI.

* OR for age 33 years is adjusted for sex and limiting illness at 23 years.

{dagger} Using the middle category as the reference group. (Birthweight, height and BMI were modelled as continuous variables and non-linearity tested with a quadratic term.)

 
Table 3Go presents simple analyses in which child to adult health predictors of limiting illness are examined with adjustment for sex only. The increased risk of limiting illness associated with a childhood chronic condition or disability is evident, as mentioned above in relation to Table 2Go. Here we see that childhood disability increases the risk of limiting illness at both ages in early adulthood, and that this risk persists for age 33 after allowing for 23-year limiting illness. Table 3Go also shows increased risk of limiting illness for subjects with enuresis recorded at both 7 and 11 years, with OR of 2.30 and 1.55 for illness at ages 23 and 33, respectively. The increased risk associated with childhood enuresis was also evident for limiting illness at age 33 after adjusting for 23-year illness. For childhood socio-emotional status an increased risk of subsequent limiting illness was found for both the poorly adjusted and intermediate groups (Table 3Go). As this finding applied to socio-emotional status at both ages 7 and 16 years, we analysed both ages simultaneously and found that each contributed to the prediction of limiting illness. An effect of childhood injury also emerged, although the increased risk for limiting illness at age 23 did not persist to age 33 (Table 3Go). Injury in early adulthood increased the risk of 33-year limiting illness and this was evident before and after adjustment for limiting illness at age 23.


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Table 3 Odds ratios (OR) (sex adjusted) for limiting longstanding illness at ages 23 years and 33 years for indicators of child to adult health status
 
Early life socioeconomic conditions
Table 4Go shows that disadvantage either in childhood or in adolescence increased the risk of limiting illness in early adulthood (OR ranged from 1.29 to 1.60). An exception was for disadvantage, age 7 and/or 11, in relation to limiting illness at age 23.


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Table 4 Odds ratios (OR) (sex adjusted) for limiting longstanding illness at ages 23 years and 33 years for child to adult socioeconomic conditions and lifestyles
 
Adult socioeconomic conditions and lifestyle
Risk of limiting illness at age 33 was raised for those not living in owner-occupied accommodation previously at age 23 (Table 4Go). Similarly, being without paid employment at age 23 was predictive of limiting illness subsequently at age 33, even after allowing for prior illness. The results for housing tenure and employment status suggest that risk of limiting illness increases for those with prior socioeconomic disadvantage. In relation to adulthood lifestyles, OR for limiting illness at age 33 were raised for regular smokers, but not for heavy drinkers at age 23 (Table 4Go).

Multivariate analyses
Table 5Go 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 1Go, and Tables 3 and 4GoGo. 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 5Go), compared with 2.30 in Table 3Go. Multivariate analyses were conducted in stages, first of factors occurring at similar life stages (models 1–3) 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 1–3 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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Table 5 Child to adult predictors of limiting longstanding illness at age 23 (n = 5513). Regression parameters (ß and SE) adjusted for sex
 
Table 6Go shows a parallel multivariate analysis for limiting illness at age 33, but with an adjustment for 23-year limiting illness, so that factors could be identified that predict later illness over and above any effect on prior illness. Comparing univariate relationships (column 1) for the restricted sample (n = 4592) with those for the full sample, again suggested that sample biases were minimal, except for birthweight. Two interesting observations can be made from Table 6Go. First, more recent factors appear to be exerting an effect, notably injuries between ages 23 and 33 (adjusted OR = 1.55) and BMI. For BMI the association remained quadratic as in Figure 1Go, reflecting elevated risks for both under- and overweight subjects, with adjusted OR respectively of 1.53 and 1.28. Second, the results suggest a re-emerging influence of some factors, notably childhood disadvantage at either or both ages 7 and 11 (adjusted OR = 1.53) and disability at age 7 (adjusted OR = 1.90). Height in childhood could also be regarded as having a similar latent effect, with a quadratic association with limiting illness. The OR for height less than 15th percentile was 1.43 and for height more than the 85th percentile it was 1.30 (adjusted for all factors except height at age 33). Finally, model 3 demonstrates that prior housing tenure, employment status and smoking behaviour were no longer significant when analysed with other early adulthood factors.


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Table 6 Child to adult predictors of limiting longstanding illness at age 33 (n = 4592). Regression parameters (ß and SE) adjusted for sex and for limiting illness at age 23
 

    Discussion
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
In this large national study, chronic illness and disability in early adulthood was predicted by risks from early life as well as by those occurring more recently in adulthood. Major risk factors included poor adolescent socio-emotional status, disadvantaged socioeconomic circumstances in childhood, injury, and poor physical development (specifically, low birthweight, short stature in childhood, and either underweight or overweight in early adulthood). The important contribution of injuries has been demonstrated in previous work,19 but so far the role of other factors for disability in early adulthood has not been investigated in detail. Disability at young ages has a large impact on health expectancy (that is, disability-free life expectancy) and so explanations of such illnesses are of particular interest. Clearly our results apply to a general measure of chronic illness and disability and further examination of specific conditions will be of interest, especially as these develop with increasing age of the cohort. With regard to self-reported limiting illness, concern exists as to whether it is a reliable indicator of morbidity.7,20,21 Elsewhere we assess reliability more directly within this cohort, by comparing responses for limiting illness with those for specific conditions, such as diabetes, epilepsy and hayfever.22 But the results presented here also have some bearing on the issue of reliability, particularly in relation to the stability of chronic illness over time.

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.


    Acknowledgments
 
Chris Power and Leah Li are grateful for financial support from the Department of Health for England and Wales. The Canadian Institute for Advanced Research supports CP as a Weston Fellow. Data acknowledgement: Centre for Longitudinal Studies, Institute of Education, National Child Development Study Composite File including selected Perinatal Data and sweeps one to five [computer file]. National Birthday Trust Fund, National Children's Bureau, City University Social Statistics Research Unit [original data producers]. Colchester Essex: The Data Archive [distributor], 21 June 1994. SN:3148.


    References
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
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