Abbreviations ACE: angiotensin converting enzyme; CVD: cardiovascular disease; HDL: high density lipoprotein; LDL: low density lipoprotein; MLR: multivariate logistic regression; WHO: World Health Organisation.
Cardiovascular diseases (CVDs) are the leading causes of death in Sweden , as in most Western countries. Hypertension is one of the four most important predictors of CVDs , it is the most important factor predisposing to stroke and is present in more than 70% of all cases . In 2000, the estimated total number of adults worldwide with hypertension was 932 million . Large-scale population-based investigations are still sparse, and the true prevalence in the population is therefore not fully known .
The prevalence of hypertension varies not only within populations of different countries , but also in different ethnic populations [7,8], and CVD has been shown to be more frequent in certain groups of immigrants in Sweden [9,10]. Obesity, physical inactivity, smoking, salt intake and genetic factors have been associated with hypertension . Such factors are likely to vary in different groups of immigrants.
It has also been reported previously that blood pressure in migrating people from the Tokelau atoll is elevated in New Zealand due to complex physiological, environmental and psychosocial factors . An association was found between hypertension and migrants being integrated in the new society; those migrants who kept their traditions were not affected as much by hypertension. In contrast, a study of endothelial function in identical cotwins in which one migrated from Finland to Sweden suggested a better health in the twin that migrated to Sweden .
Immigration in Sweden has increased during the last few decades, and immigrants now account for 11.5% of the Swedish population, compared to 7.5% in 1980 and 6.7% in 1970 (Statistics Sweden). A large proportion (41%) of the immigrants are of non-European origin, with the most frequent countries of origin being Iran, Iraq, Turkey and Chile (Statistics Sweden). Many of these individuals have migrated for political reasons. Among 60-year-old immigrants, most are of European origin, with the largest group being Finnish. The prevalence of hypertension among immigrants has been measured in previous smaller studies of mostly younger individuals [10,14]. The highest prevalence of hypertension was found in the Finnish-born.
The primary aim of this study was to assess the prevalence of hypertension in different groups of 60-year-old immigrants in Sweden in comparison to Swedish-born men and women. A secondary aim was to determine if the previously reported high prevalence of hypertension in immigrants from Finland could be detected in a large population-based sample or if it could be attributed to metabolic, socio-economic and lifestyle factors or both.
A study of 60-year-old men and women in Stockholm County
Between August 1997 and March 1999, every third person living in Stockholm County who was born between 1 July 1937 and 30 June 1938 was invited to participate in a thorough health screening study. The sample was drawn randomly from a database of the population in Sweden. The participants underwent a physical examination including height, weight and waist measurements. Fasting blood samples were taken for analysis, and a comprehensive questionnaire with validated questions was completed. Diabetes was defined as self-reported or fasting plasma glucose of at least 7.0 mmol/l [15,16].
The study was approved by the ethics committee at Karolinska Institutet, and all participants gave their written consent. All participants achieved preventive individualized cardiovascular lifestyle advice, and newly diagnosed participants were referred to primary care or hospitals.
A total of 5460 participants, 2779 men and 2681 women, were invited, and 4228 individuals (77%) participated, 2036 men (73%) and 2192 women (82%). The response rate among all immigrants (n = 787) was somewhat lower at 68%, but the exact figure for specific immigrant groups were not available. However, in another Swedish study, participation rate was equally high in Finnish-born as in Swedish-born individuals . The immigrants constituted 19.2% of the total participating population of 60-year-old men and women. This corresponds to the percentage of immigrants among 60-year-olds in Stockholm County in December 1998, that is, 21.5% (Finnish 8.3%). For 4104 participants (of whom 8.0% were Finnish), 1971 men and 2133 women, the country of origin and data regarding hypertension were known (Table 1), and those participants were included in this study. The immigrants were divided into groups based on country of origin or cultural and geographical region. The countries of origin of all immigrants have been published elsewhere .
Blood pressure was measured twice, after 5 min of rest with the participant in the sitting position (HEM 711; Omron Healthcare, Bannockburn, Illinois, USA), and the mean of the two measurements was calculated. A wider cuff was used if the participant's upper arm circumference was above 32 cm. New hypertension was defined as systolic or diastolic blood pressure or both of at least 140/90 mmHg measured on one occasion. Known hypertension was defined as self-reported hypertension or intake of antihypertensive drugs in the questionnaire. All hypertensions were defined as either new or known.
Socio-economic and lifestyle factors
Civil status was defined as married/living with a partner (yes/no). Living conditions were defined as living in an apartment (yes/no) (compared to living in a house). An employment variable was defined according to working status (yes, full or part-time/no). The education level was reported as lower or no education/compulsory school, secondary/12-year school and university or college. Smoking habits were coded as current daily smoker, former smoker or never been smoker. Physical activity in leisure time during the past year was grouped into categories as follows: inactive, light activity at least 2 h per week, moderate activity one to two times per week and intensive activity at least three times per week. Categories 1 and 2 were classified as ‘inactive’ and categories 3 and 4 were classified as ‘active’.
The alcohol intake questionnaire had questions concerning present intake of beer, wine and spirits . Average daily intake of alcohol was calculated and four categories of drinking groups were defined as follows: no (0 g alcohol/day), low (0.1–10 g/day), moderate (10.1–30 g/day) and high (>30 g/day) alcohol intake.
Calculations of statistical significance (P values) were performed by t-tests and χ2 analysis for all variables in all immigrant groups separately, compared to Swedish-born (Tables 1 and 2). All tests were two tailed, and the significance level was set at a P value of less than 0.01 due to multiple testing, except for the multivariate logistic regression (MLR) in which it was set at a P value of less than 0.05. Significant factors in Tables 1 and 2 were carried forward to logistic regression. Covariate factors and their association to hypertension were studied by both univariate and MLR (only factors significantly associated hypertension either univariate or MLR were included), as presented in Table 4. The P value for the Pearson's χ2 test of fit for the MLR model was 0.69.
Post-hoc we calculated the power to detect a difference in hypertension prevalence between Finnish-born and Swedish-born participants; it was for men at 0.95 and for women it was at 0.99. To detect a difference between non-European and Swedish-born participants, it was 0.32 for men and 0.15 for women.
Calculations were performed using the computer programs STATA, 9.2 and Confidence Interval Analysis (CIA; Statistics with confidence, 2nd edition).
Tables 1 and 2 show metabolic, lifestyle and socio-economic characteristics of groups of immigrants and Swedish-born men and women, respectively. Statistically significant findings were detected in nearly all variables with a few sex and immigrant differences.
Prevalence of hypertension
Figure 1 shows total prevalence of hypertension in men (62%) and women (45%).
Table 3 shows prevalence of hypertension in groups of immigrants and Swedish-born individuals. The prevalence of hypertension was high, as 61% of men and 44% of women from Sweden were hypertensive. The prevalence of hypertension was highest among immigrants from Finland, comprising 77% of men and 62% of women, and lowest in non-European immigrants, comprising 51% of men and 36% of women.
Results from logistic regression
Table 4 shows the logistic regression (both univariate and multivariate) of important factors and groups of immigrants as odds ratios (OR). Factors associated with hypertension are being born in Finland, diabetes, waist circumference more than 88 cm, ex-smokers and high alcohol intake. Factors negatively associated with hypertension are regular physical activity, college/university and being employed.
After adjustments for important factors (MLR), hypertension was associated with being born in Finland, diabetes, waist circumferences above 88 cm and high alcoholic intake. Factors negatively associated to hypertension were female sex, being born outside Europe, college/university, daily smoking and regular physical activity.
The main finding in this study was the high prevalence of hypertension in general (Fig. 1) and especially among Finnish men and women. The finding of a higher prevalence in Finnish immigrants remained after adjustment for many important factors not adjusted for in previous studies. In immigrants of non-European origin, the prevalence of hypertension was found to be lower than for Swedish-born individuals, after adjustment for other factors.
The prevalence of hypertension in our study is at the same level as in another Swedish study of postmenopausal women . In the World Health Organisation (Monitoring trends and determinants in CVD) (WHO-MONICA) sample from Sweden, prevalence of hypertension in the ages 25–64 was in total 45%, with one-third receiving treatment .
The migration from Finland to Sweden is of importance as it is to a country with a lower burden of hypertension and CVD [5,20]. In contrast, migration to Western societies has been associated with a higher burden of hypertension in a metaanalysis . A high prevalence among Finnish immigrants was reported in a previous smaller study that was not population based  and in one study including only 59 immigrants from Finland . However, the hypertension prevalence in these studies was not adjusted for covariate factors. The prevalence of hypertension in Finland is high in comparison to other European countries . Screenings in Finland for polymorphisms in genes coding for angiotensin-converting enzyme (ACE) and nitric oxide synthase have failed to show associations with hypertension [22,23]. In contrast to these negative findings, a common gene polymorphism in a Finnish population was associated with endothelial dysfunction and decreased arterial vasodilatation . Preventive actions for control of hypertension have, however, been shown to be effective in Finland on both a community and a family level [25,26]. If a genetic or physiologic explanation for the high prevalence of hypertension in Finnish immigrants were to be found, it would be of value not only in itself, but also could help to explain the mechanisms of hypertension and why different people respond differently to antihypertensive therapy in general. It is known that people of African origin have a salt-sensitive hypertension and low levels of renin, and this explains why they often respond well to diuretics and less well to ACE inhibitors .
The low prevalence of hypertension found in immigrants of non-European origin was significant (P < 0.001) after adjustments for covariate factors. This finding is also in accordance with the findings of the WHO-MONICA project showing that the prevalence of hypertension is high in European countries compared with other parts of the world [5,6].
Strengths of the study were first that it was representative and population based, with investigation at the same time of one-third of all 60-year-old men and women in Stockholm County. There was a high total response rate of 77%, and the immigrant response rate, 68%, was much higher than in a similar immigrant study from Norway . The good quality of population registers in Sweden, allowing us to reach the whole population and thus ensuring good representation, also adds certainty to our results.
Some limitations should be mentioned. The number of foreign-born participants was too small to allow analysis other than in large groups such as immigrants of non-European origin. This is a problem when considering the differences between different groups of immigrants, for example, in cultural background, language, reason for migration, duration of stay in Sweden and age when migrating. Thus, though the general conclusions cannot automatically be attributed to all immigrants, they can be attributed to the special mix of subgroups that were included in this study. As European populations in general have a high prevalence of hypertension, this could to some extent justify our grouping together of immigrants of non-European origin. The study was not primarily designed to compare immigrants and Swedish-born participants but the high percentage of immigrants (19%) in Stockholm County made this possible. Furthermore, blood pressure was only measured on one occasion, which may overestimate the prevalence. Such an overestimation should, however, be the same in all studied groups, and single-occasion blood pressure measurements are common practice in epidemiological studies [7,10,29]. Unfortunately, we did not have data on intake of salt and macronutrients, factors that may have an impact on blood pressure . With respect to the high prevalence of hypertension found in Finnish immigrants, there may be a risk of residual confounding by factors we were not able to adjust for.
In conclusion, about every other 60-year-old person in Sweden has high blood pressure, which emphasizes the need for primary preventive actions to be taken. Finnish immigrants are in need of extra attention, as they have an even higher prevalence of hypertension. A further search for the genetic and environmental factors causing hypertension in Finnish individuals is warranted.
The authors gratefully acknowledge Therese Karlsson (Statistics Sweden for data on immigration), Merja Heinonen and Gunnel Gråbergs (for their skilful assistance).
The study was supported by grants from the Stockholm County Council, Karolinska Institutet, the Swedish Heart and Lung Foundation, the Swedish Council for Working Life and Social Research, the Swedish Research Council (Longitudinal Research and K2005-27X-14278-04A), AstraZeneca, Pfizer and Unilever.
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Keywords:© 2008 Lippincott Williams & Wilkins, Inc.
ethnic background; genetics; hypertension; population-based studies; prevalence