1 Department of Community Medicine, The University of Hong Kong, Pokfulam, Hong Kong Special Administrative Region, China
Correspondence: Dr Gabriel M Leung, Department of Community Medicine, 5/F Academic and Administrative Block, Faculty of Medicine Building, University of Hong Kong, 21 Sassoon Road, Hong Kong Special Administrative Region, China. E-mail: gmleung{at}hku.hk
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Abstract |
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Methods We analysed, using logistic regression, the risk of self-reported diabetes, hypertension, hyperlipidaemia, and ischaemic heart disease in a population-based sample of 3643 Chinese men and 3778 Chinese women some of whom had experienced a change to more favourable economic conditions at different life stages through migration from mainland China to Hong Kong.
Results Adjusting for socio-economic status, risk behaviours, and family history, the development of diabetes was associated with migration from China to Hong Kong in the first two decades of life, albeit with a decreasing intensity of effect (OR = 2.02, 95% CI: 1.18, 3.45, OR = 1.84, 95% CI: 1.27, 2.66, and OR = 1.72, 95% CI: 1.21, 2.45 for migration at ages 07, 817, and 1824, respectively). The development of hypertension was mostly susceptible to environmental change during the growth spurt and puberty (migration at ages 817 OR = 1.56, 95% CI: 1.22, 1.99). The development of heart disease was associated with a sex-specific critical period in early childhood for men (migration at ages 07 OR = 3.17, 95% CI: 1.70, 5.91).
Conclusion Environmental change by migration throughout the first two decades of life can affect the development of diabetes, hypertension, hyperlipidaemia, and ischaemic heart disease, although adverse childhood conditions alone may not be a risk factor. Our results suggest that specific life course pathways may pre-dispose to these conditions and could be relevant to their aetiology in populations undergoing rapid development.
Accepted 2 April 2004
Genetic, environmental, and adult lifestyle or behavioural factors alone do not fully explain the secular trends and variations in population risk profiles nor individual differences in cardiovascular diseases and associated conditions such as diabetes mellitus, hypertension, and hyperlipidaemia.13 Life course theory suggests there are critical or sensitive periods that are key to the development and subsequent manifestation of most chronic diseases. In Western populations it has been shown that environmental factors play a definite role throughout the life course in the development of coronary heart disease and other chronic conditions; this may be the detrimental effect of cumulative disadvantage4,5 or of particular life course pathways, involving poor fetal growth, poor pre-pubertal growth,69 or catch-up growth.1017 Environmental factors over the life course could be particularly relevant for Asian populations, which are at an earlier stage in the epidemiological transition, have a different pattern of cardiovascular disease,18 and are undergoing a more compressed transition to a westernized lifestyle. There are few long-established studies in Asia that could investigate these issues directly. One study from Korea suggests that, unlike in the West, for men early life experiences (measured by height) are not associated with coronary heart disease mortality,19 but we do not know more generally about the effect of life course pathways in Asia.
In Hong Kong rapid economic growth started in the first half of the 20th century; by 1952 per capita GDP was about half that of Western Europe and by 1995 it had overtaken Western Europe and was one of the highest in Asia.20,21 Economic growth took off later in China. Since the establishment of the People's Republic of China in 1949 per capita GDP has reached levels which took centuries to achieve in Western Europe20 and living standards have improved immensely, but Hong Kong still has a better record on common health benchmarks such as infant mortality and life expectancy.22
Hong Kong has been largely an immigrant community, populated mostly with migrants from southern China. Hong Kong Chinese have a common ethnic background with life histories determined by their age on arrival in Hong Kong, when they may have experienced a sudden change in environmental conditions. Accelerated growth has been associated with childhood migration to more affluent countries23 and with improved childhood nutrition.24,25 We hypothesized that people who migrated from mainland China to Hong Kong during childhood or adolescence might have experienced accelerated growth and hence could have a higher risk of cardiovascular diseases and diabetes. As growth is not uniform we also postulated that sensitivity to environmental change might vary with different stages of growth and development at the time of migration. The objective of the present study was to examine the relationship between migration to a more advantaged environment during various phases of life and the subsequent development of type II diabetes, hypertension, hyperlipidaemia, and ischaemic heart disease (IHD).
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Methods |
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Measurement of exposures and outcomes
The exposure was the timing of migration in relation to life stage. Age at migration from mainland China to Hong Kong was constructed from place of birth, age, and length of stay in Hong Kong; excluding those born outside China and those missing such information. Growth phases potentially mediated by different hormonal influences are infant (01 years), childhood (27 years), and pubertal (817 years).30,31 This sample was not collected with the specific objective of looking at the effect of migration. Therefore, for this analysis, the sample was grouped to represent as many potential stages as possible, to avoid small groups and to obtain similar sized groups, giving migration age-bands of 07 years, 817 years, 1824 years, and 25 years.
The outcomes were the presence of type II diabetes, hypertension, hyperlipidaemia, and heart disease, from self-reports of diagnosis by a doctor. Angina was from the Rose angina questionnaire, translated into Chinese and validated in a study of elderly Chinese.26 Heart disease and angina were combined into one outcome: IHD. Anyone reporting diabetes diagnosed after the age of 20 was assumed to have type II diabetes. Type I diabetes is rare in Chinese, and young subjects with diabetes usually have type II.
Statistical Analysis
Risk of each condition in relation to migration age band, adjusted for potential confounding factors, was assessed using logistic regression. A two-tailed P-value <5% was considered statistically significant. Men and women were analysed together, unless there was evidence that the effect of migration on these conditions differed between the sexes (i.e. there was a significant interaction between migration age band and gender). Model fit was checked using the Hosmer-Lemeshow test; a P-value >5% was taken to indicate adequate fit. Missing data were excluded in the multivariate procedures.
Potential confounding socio-economic factors considered were a measure of early life advantage (highest level of education attained as none/kindergarten, primary, secondary, and tertiary) and current advantage (type of dwelling considered as public housing, home ownership scheme, private [whole], private [shared], temporary housing, quarter, or others, and marital status considered as married or single/divorced/widowed).
Confounding risk behaviours considered were alcohol consumption (non-drinker or drinker), smoking (never, ex-smoker, or current smoker) and exercise (none or some exercise). As some of these may be on the causal pathway they were added in the order they would occur in a lifetime. The baseline model (model 1) examines differences in risk of reporting conditions adjusted for age, sex (if data pooled), and family history (where availablediabetes, hypertension and IHD). Education was added in Model 2. Current advantage and current risk behaviours were added in Model 3. Acculturation in Hong Kong (measured by more or less than 25 years in Hong Kong) was added in Model 4. Finally adjustment was made for the other medical conditions in Model 5. Analyses of diabetes, hypertension and hyperlipidaemia were adjusted for the other two conditions. Analysis of IHD was adjusted for diabetes, hypertension and hyperlipidaemia.
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Discussion |
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Until recently interest has focused on fetal and infant growth. Fewer studies have examined the relative effect of later growth phases, so their effect and the mechanisms by which they operate are not well understood. However, the growth spurt and puberty are accompanied by metabolic changes, such as insulin resistance, changes in lipid levels and changes in blood pressure, which might result in particular sensitivity to environmental influences. On the other hand, some previous experience, such as low birthweight or poor infant growth, may have made migrants vulnerable at this stage. Specifically, the change in living conditions could have promoted potentially detrimental overweight, catch-up growth or altered growth trajectories, such as increased nutrition promoting earlier puberty.32 Although IHD is an epidemic particularly affecting men33 why some childhood conditions should have a detrimental effect on males but not females and what the protective mechanism is for females is not clear. The increased IHD risk for early childhood migrants could be related to more advantaged living conditions enhancing growth on an inadequate constitution; one childhood feeding experiment found that supplemented children grew longer legs, but did not have lower CHD mortality rather it was (non-significantly) higher.24
Alternatively or in addition to these broad environmental effects, child migrants may have found the move stressful, even within the Chinese culture and tradition common to Hong Kong and the mainland. Stress in childhood is associated with poorer adult mental health34 and could impact adult cardiovascular health,35 though the evidence is sparse. Child migrants may also have been preferentially the survivors of early childhood adversity, with a constitution that increased their chance of survival and their subsequent risk. However risk varies with childhood life stage at migration and there is no additional risk for adult migrants who might have been subject to the same pressures. Therefore, our results are also potentially compatible with the interpretation that early life experience may not necessarily be a risk factor per se for cardiovascular diseases and associated metabolic conditions in this population, as for IHD mortality in Korean men.19 People born in Hong Kong and older migrants from mainland China have similar risks, despite different early life environments. Retaining their birthplace diet could have protected the adult migrants, or the Chinese diet in Hong Kong may buffer risk such that early life experiences are much attenuated. Alternatively there may be other differences between growing up in mainland China and Hong Kong, such as higher levels of physical activity promoting better cardio-protection.36
Strengths and weaknesses of this study
This study used broad proxies as a new way to investigate the role of environmental change at different stages in a population where long-term records are not available. Studies of migration usually investigate the socio-cultural context, acculturation, the healthy migrant effect, or the relative role of early life and later life factors. Instead, we considered migration from the perspective of an individual's life course and the trajectories generated by migration. Such a perspective has rarely been used before in migration studies, although one study considered age at migration to Israel in relation to adult blood pressure.37 Our study was also able to take into account (and discount) a range of other possible explanatory factors: childhood and adult advantage, adult risk behaviours and family history.
There are several limitations of this study. First, we are assuming that migration from China to Hong Kong provided economic advantage. However, this exposure is suboptimally characterized, which would potentially have diluted the observed effect as a result of random misclassification. Second, we are using self-reports. However there is little reason why there should be lower levels of doctor diagnosed type II diabetes, hypertension, hyperlipidaemia and IHD among those who migrated young, especially when social circumstances and risk behaviours are taken into account. Younger migrants did not generally perceive their health differently (Table 6), and were unlikely to over-report the outcomes being studied any more than those who migrated later. Third, the increase in risk of these conditions for younger migrants is not large. However, all probably experienced some improvement in living standards in line with economic growth, so it is not a comparison between change and no change, but between different amounts of change. Fourth, family history may have been less well known to the migrants (if they did not have such close contact with their families or their families had not lived long enough to develop these conditions or had not had them diagnosed). However re-analysis without family history showed similar results (not shown).
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KEY MESSAGES
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Acknowledgments |
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References |
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