Childhood migration and cardiovascular risk

Mary Schooling1, Gabriel M Leung1, Edward D Janus1,2, Sai Yin Ho1, Anthony J Hedley1 and Tai Hing Lam1

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


    Abstract
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 Abstract
 Methods
 Results
 Discussion
 Conclusion
 References
 
Background Childhood living conditions have been hypothesized to be associated with the development of cardiovascular diseases and diabetes mellitus in adult life.

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 0–7, 8–17, and 18–24, respectively). The development of hypertension was mostly susceptible to environmental change during the growth spurt and puberty (migration at ages 8–17 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 0–7 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.


Keywords Life course epidemiology, economic conditions, cardiovascular disease, diabetes mellitus, hypertension, migration

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.1–3 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,6–9 or catch-up growth.10–17 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).


    Methods
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 Abstract
 Methods
 Results
 Discussion
 Conclusion
 References
 
Sources of data
Data for the present paper were drawn from the Hong Kong Cardiovascular Risk Factor Prevalence Study of 1995–1996, which is a randomly selected, high response (78%), random digit telephone survey of a large (7174), representative sample of the Hong Kong Chinese population aged between 25 and 74.26 It was supplemented by a relatively small number (556) of specifically recruited subjects aged 55–74 years, because there were fewer subjects of this age in the general population.27 Those with serious diseases such as cancer or who were hospitalized were excluded. A standardized questionnaire was used to collect information on demographic characteristics, smoking, drinking, physical activity, angina, hyperlipidaemia, personal history and family history of coronary heart disease, diabetes mellitus, hypertension, and self-perceived general health status. Telephone ownership is near universal (>99%) in Hong Kong households. This holds true even for more recent immigrants because family reunion has always been the predominant reason for granting residency status to mainland Chinese, where most new arrivals live with relatives and therefore would have had telephone access during study recruitment. Recent illegal migrants might been concealed from an interviewer, however, illegal immigration has been rare and difficult since the early 1980s.28 The method of telephone interview was validated in Hong Kong for a morbidity survey.29 The study complied with the Declaration of Helsinki.

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 (0–1 years), childhood (2–7 years), and pubertal (8–17 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 0–7 years, 8–17 years, 18–24 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 available—diabetes, 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.


    Results
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 Abstract
 Methods
 Results
 Discussion
 Conclusion
 References
 
Almost all of the subjects came from southern China; born in Hong Kong (51.9%) or the adjacent province of Guangdong (36.6%) (Table 1). Migrants were fairly evenly divided into before the age of 18 (34%), between the ages of 18 and 24 (28%), and ≥25 years (38%). Consistent with Hong Kong migration history most moved during 1955–1980 (57%), 30% before 1955, and 13% since 1980 (Table 2). Table 3 shows the reported prevalence of each outcome.


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Table 1 Place of birth

 

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Table 2 Age, years in mainland China, and time of migration by migration age band (n = 7421a)

 

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Table 3 Prevalence of diabetes mellitus, hypertension, hyperlipidaemia, and ischaemic heart disease

 
Diabetes, hypertension, and hyperlipidaemia
Table 4 shows odds ratios (OR) and 95% CI for adjusted analysis of all outcomes. Adjusting for age, sex, and family history (model 1), people who migrated before age 25, but not later, showed increased risk of type II diabetes compared with the Hong Kong born, with greater risk for earlier migration (OR = 1.98, 95% CI: 1.16, 3.37, OR = 1.85, 95% CI: 1.29, 2.67, OR = 1.75, 95% CI: 1.23, 2.48 for migration age bands 0–7, 8–17 and 18–24 respectively). People who migrated between 8 and 17, but not at other ages, showed higher risk of hypertension (OR = 1.53, 95% CI: 1.20, 1.94). People who migrated between 8 and 17 and possibly before age 8 showed higher risk for hyperlipidaemia (OR = 1.36, 95% CI: 1.01, 1.83 for migration age band 8–17). Further adjustment for an increasing number of potential confounders (models 2–4) and for the other two conditions (model 5) made little difference, although the result for hyperlipidaemia was no longer significant. In the analysis of diabetes, hypertension, and hyperlipidaemia there were no significant interactions between migration age band and sex. Most models were adequate fits (Hosmer-Lemeshow test).


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Table 4 Migration status and risk of type II diabetes, hypertension, hyperlipidaemia, and ischaemic heart disease for men and women pooled

 
Ischaemic heart disease
Adjusted for age, sex, and family history, people who migrated before age 8, but not later, were at greater risk of IHD (OR = 1.96, 95% CI: 1.27, 3.03) compared with the Hong Kong born (Table 4). In the analysis of IHD there was a significant interaction between migration age band and sex (P = 0.008). Table 5 shows separate analyses for men and women. Men who migrated before age 8, but not later, were at greater risk of IHD (OR = 3.16, 95% CI: 1.71, 5.84). Women who migrated before age 25 did have higher risks, but not statistically significantly so. Adjusting for an increasing number of potential confounders (models 2–4) and for diabetes, hypertension, and hyperlipidaemia (model 5) made little difference. The models showed adequate fits (Hosmer-Lemeshow test).


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Table 5 Migration status and risk of ischaemic heart disease for men and women separately

 
Table 6 shows that there were no significant relationships found between migration age-bands and self-rated health.


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Table 6 Migration status and perceived healtha men and women together (n = 7218)

 

    Discussion
 Top
 Abstract
 Methods
 Results
 Discussion
 Conclusion
 References
 
Moving to a more advantaged environment at some time before age 25 is associated with an increased risk of developing type II diabetes, hypertension, hyperlipidaemia, and IHD, potentially mediated by environmental change or other concomitant factors. The dose–response relationship for diabetes suggests environmental influences at work throughout the first two and a half decades of life for men and women, albeit with a decreasing intensity of effect. The development of hypertension for men and women was mostly susceptible to environmental changes during the growth spurt and puberty. The development of hyperlipidaemia could be associated with environmental change before the age 18, but the results are less consistent. Hyperlipidaemia might also be the least likely of these conditions to be reliably diagnosed, as testing for lipids was less common than blood pressure measurement. The development of IHD shows a sex-specific critical period in childhood where an abrupt environmental change for men before the growth spurt and puberty has an impact on subsequent IHD risk.

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).


    Conclusion
 Top
 Abstract
 Methods
 Results
 Discussion
 Conclusion
 References
 
Our study provides preliminary evidence from Hong Kong and China that there is a likely impact of environmental factors at different stages of growth on the risk of diabetes, hypertension, hyperlipidaemia, and IHD in migrants. Our results emphasize the importance of understanding the impact over the life course of environmental conditions during growth and development, particularly for Asian populations. These findings also highlight potential differences between Asian and western populations, which deserve further investigation and confirmation, as they could reveal important protective factors.


KEY MESSAGES

  • Environmental change through migration in the first two decades of life is associated with increased risk of diabetes, hypertension, and hyperlipidaemia for men and women, and such change in early childhood is associated with increased risk of ischaemic heart disease in men. Environmental change in adulthood is not associated with increased risk of these conditions.
  • Our results suggest that in this population there are childhood life-stage-specific vulnerabilities to environmental change, although adverse childhood conditions alone may not be a risk factor. This indicates the importance of specific life course pathways on the development of these conditions for populations undergoing rapid transition, the further understanding of which may reveal protective factors.

 


    Acknowledgments
 
The Hong Kong Cardiovascular Risk Factor Prevalence Study Steering Committee consisted of the following members: ED Janus (Chairman), CS Cockram, R Fielding, AJ Hedley, P Ho, CP Lau, M Lo, SL Lo, PL Ma, JRC Maserei, YT Tai, B Tomlinson, SP Wong, JLF Woo. Funding came from the Hong Kong Health Services Research Committee (HSRC# 411026), the University of Hong Kong Committee on Research and Conference Grants, the Hong Kong Research Grants Council (grant 407/94 m), and the Hong Kong Society for the Aged. We would like to thank the late MR Janus, survey centre nurse co-ordinator, SF Chung for her assistance in recruitment and telephone interview co-ordination, STS Siu for assistance in data processing and all the interviews, and all the people who took part.


    Notes
 
2 Present address: Wimmera Base Hospital, Baillie Street, Horsham, VIC, Australia 3400 Back


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