MRC Unit, Social and Public Health Sciences Unit, University of Glasgow, Lilybank Gardens, Glasgow, G12 8RZ, UK. E-mail: seeromanie.harding{at}mrc.msoc.ac.uk
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Abstract |
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Method Study members in a national cohort, aged 2554 years in 1971, were followed up from 1971 to 2000. There were 1540 migrant Caribbeans amongst whom there were 329 deaths during follow-up. Cox regression models were used to analyse mortality from cardiovascular disease and cancers. All results were adjusted for sex and socioeconomic position.
Results All-cause mortality was not related to duration of residence or age at migration at ages 2534 or 3544 years in 1971. At ages 4554 years a pattern of increasing mortality with each additional year of residence prior to 1971 (hazard ratio [HR] = 1.07, 95% CI: 0.95, 1.20, 144 deaths) and with each additional year of age at migration (HR = 1.09, 95% CI: 0.97, 1.22) was observed. Circulatory disease mortality, accounting for 40% of all deaths, contributed to this pattern. At ages 4554 years, both duration of residence (HR = 1.21, 95% CI: 1.01, 1.44, 62) and age at migration (HR = 1.25, 95% CI: 1.06, 1.49) increased per year of each. Of these deaths, stroke mortality was positively associated with both predictors (HR = 1.38, 95% CI: 1.10, 1.74 for duration of residence and HR = 1.44, 95% CI: 1.15, 1.80 for age at migration), a pattern due to effects at ages 4554 years. Deaths from coronary heart disease showed similar trends in the oldest age cohort. No significant trends were observed for deaths from cancers.
Conclusion Circulatory disease mortality in Caribbean migrants increased with increasing duration of residence and age at migration in the oldest age cohort, primarily due to the effects from stroke mortality.
Accepted 22 October 2003
The conventional wisdom from migration studies is that migration is selective of healthier and socioeconomically advantaged people1,2 but a shift in disease patterns towards that of the host population is expected, as migrants adopt new health-related attitudes and behaviours such as tobacco use and the local diet.37
Compared with the general population, migrant Caribbeans in the UK have higher rates of hypertension, stroke, and type 2 diabetes mellitus,810 yet up to 50% lower mortality from coronary heart disease (CHD) with or without diabetes.11,12 Greater deprivation explains some of the excess stroke mortality.13 The paradoxical feature of a deficit in CHD but an excess of hypertensive-related conditions is also seen in Caribbean migrants in the US14 and in West African-heritage populations in the Caribbean and in West Africa.15,16 In contrast, African Americans, of similar ancestry even if of different colonial history, experience very high rates of CHD and other cardiovascular-related disorders relative to White Americans,14 but up until the 1960s the rates were lower.17,18
Little is known about the processes of acculturation among migrant Caribbeans and its impact on their disease risk. In much of the available evidence on health-related behaviours, migrant Caribbeans are not identified separately from British-born Caribbeans. Even if the distinction is made, health-related behaviours are very rarely examined in relation to duration of residence. Smoking, a risk factor for both lung cancer and cardiovascular disease, is now similarly prevalent among younger Black Caribbeans and Whites in the UK but alcohol use remains low relative to Whites.19,20 Some changes in diet have been observed in young Caribbeans, with greater energy intake from fat among 2534 year olds compared with Whites, but older migrant Caribbeans continue to eat traditional diets associated with a protective effect for CHD and cancer.21 The prevalence of obesity is greater in Caribbean women than White women but metabolic profiles are generally favourable in both men and women.22,23
Longitudinal studies on migrants are rare but health patterns of migrants in their new environments have been compared with those left behind.24,25 This study uses a national cohort study in England and Wales to investigate whether mortality of Caribbean migrants is associated with duration of residence or age at migration.
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Country of birth in the 1971 Census was used to identify migrants born in the Caribbean Commonwealth countries. Sample members who have not been found at a subsequent census or by registration of a vital event are assumed to be lost to follow-up.27 For migrants, loss to follow-up was mainly due to return migration or re-migration. About one-third of the survivors over age 54 years in 1971 were lost to follow-up. This age group was excluded from the analysis. Migrants under age 25 years were also excluded because the number of deaths was too small for analysis. Thus, the final sample for analysis consisted of migrants ages 2554 in 1971.
Of the 1941 Caribbean migrants aged 2554 years in 1971, 401 were excluded because of emigration. These people did not contribute any person-days at risk to the study. It was not possible to censor person-days at risk at date of exit as the date of emigration was not known. This left 1540 Caribbean migrants eligible for analysis at the start of follow-up in 1971.
Duration of residence was derived using year of arrival. This was treated as a continuous variable. The mean year of arrival was 1961 and mean age of migration was 21 years for ages 2534 years, 1958 and 27 for ages 3544 years, and 1956 and 34 for ages 4554 years. These measures were related to mortality from all causes (International Classification of Diseases, Ninth Revision [ICD-9] 000999), circulatory diseases (ICD-9 390459) including CHD (ICD-9 410414) and cerebrovascular disease (stroke) (ICD-9 430438), and all malignant neoplasms (ICD-9 140208) for the period 19712000 using Cox regression models. Predictors were fixed at the start of follow-up in 1971 so that duration of residence refers to up to 1971. After 1971, duration of residence was part of the person-days at risk outcome variable. Person-days at risk from 1971 until death or the end of study were calculated using a survival analysis approach. All hazard ratios (HR) were adjusted for sex, 5-year age bands, and socioeconomic position in 1971. The latter adjustment was important because, in the youngest age group, those who had arrived recently had higher socioeconomic position than the long-stay residents. Socioeconomic position was measured using multiple indices which have been used to examine health differences within this group28access to cars (classified as access to two or more cars, one car, or no car), housing tenure (classified as living in owner-occupied, privately rented, local authority rented or other housing), overcrowding (one or fewer people per room, more than one person per room), and occupational social class (classified as professional or intermediate, skilled non-manual, skilled manual, partly skilled or unskilled, or other, which includes those without a stated or an adequately described occupation).
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The change in stroke mortality at older ages possibly reflects factors associated with patterns of health-seeking behaviour in a sub-group that is particularly vulnerable. Older Black Caribbeans, particularly men, report high levels of mistrust for National Health Service doctors and often seek private health care.30,31 There is high usage of natural remediesbush teas, evening primrose oil, and laxativesin this population which may be related to non-compliance with prescribed medication.23,31,32 The determinants of usage were studied in detail by Cappuccio et al.33 They found that migrants of African origin (Caribbeans or West Africans) were twice as likely as South Asians to use alternative remedies and suggested that usage could be related to cultural determinants in the country of origin. It is possible that non-compliance to prescribed medicines may be influenced by religious beliefs and by cultural differences affecting communication with General Practitioners.31 This is no doubt compounded by the fact that migrant Caribbeans are socially and economically disadvantaged34,35 and tend to live in deprived inner city areas36 where provision of care is likely to be under-resourced. An alternative explanation could be that that this older age cohort adapted to western lifestyle with an increase of risk factors (e.g. increase in weight, reduction in physical activity, increase in salt intake) leading to stroke. The latency period for diseases such as cancer and circulatory diseases is long and it is possible that the reported changes in risk behaviour1921 are too recent to have had an impact on the younger age cohorts studied here.
Recently, there has been a surge in research concerning the origins of cardiovascular disease in people of African heritage, building on the approach of Barker37 that growth in early life is important. In a study of Jamaican school children aged 15 years, blood pressure and the propensity for diabetes were inversely related to birthweight and crown-to-heel length.38 In a cohort of American adolescents in Bogulusa (Chicago), differences between African and White Americans in blood pressure were mainly attributed to differential growth trajectories in childhood.39 The findings from migration studies argue against an interpretation based solely on genomic variation. In a study of genetically similar populations of West African descent in rural and urban Cameroon, in the Caribbean and in the UK, there were differences in blood pressure, fasting glucose levels, and obesity between these sites, with those in rural Cameroon at the lower end and those in Manchester at the higher end.16,40 There were also differences in dietary intake and exercise, with Black Caribbeans in Manchester having higher energy foods and a more sedentary lifestyle than people in rural Cameroon.41,42
There are some limitations to this study. About 20% of migrant Caribbeans were assumed to have returned home or re-emigrated elsewhere during follow-up, and were excluded from the study. The socioeconomic circumstances of these emigrants were similar to those of those who remained but their health status is not known. There are some documented life histories of return migrants to the Caribbean,43,44 consisting mainly of those who return on retirement to take advantage of the lower cost of living and to get more from their British pensions.45 If return migration is selective in terms of health, in that the less healthy return, this could mean that a relatively healthier cohort remains in the UK.
Adjusting for differences in socioeconomic position among migrants is problematic mainly because of issues of cultural sensitivity of measures and the disruption of class position following migration.28,34 Difference in socioeconomic position by duration of residence is a potential confounder and a range of measures from the 1971 Census was used in this analysis. In the comparable analyses of South Asians,29 socioeconomic position at both the 1971 and 1981 Censuses (10 years after the start of study) was used; the latter were thought to be a more accurate measure because of the initial underemployment that might have occurred following migration. The use of socioeconomic position in 1981 was not feasible in this analysis of the Caribbeans because of the smaller sample size and hence the smaller number of deaths if the first 10 years of follow-up had been excluded. The role of chance cannot be ruled out as a likely explanation for the different findings for the South Asians given the small number of events. Other potentially confounding factors such as gender (earlier migrants are more likely to be men than women), marital status (single men have higher mortality than married men), and region of residence (earlier migrants may move to more affluent areas with better access to care) could also not be investigated further because of the sample size. The lack of power from the small number of events also precluded any serious consideration of the substantial (but not statistically significant) HR for CHD deaths at ages 4554 and stroke at ages 3544 but is worthy of note for any future similar research.
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KEY MESSAGES
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