a International Centre for Health and Society, Department of Epidemiology and Public Health, University College London Medical School, 119 Torrington Place, London WC1E 6BT, UK. E-mail: pekka{at}public-health.ucl.ac.uk
b Population Research Unit, Department of Sociology, PO Box 18, FIN-00014, University of Helsinki, Finland.
c Health Care Centre and
d Department of Public Health, Kanazawa Medical University, 11 Daigaku, Uchinada, Ishikawa 9200293, Japan.
e Department of Welfare Promotion and Epidemiology, Toyama Medical and Pharmaceutical University, 2630 Sugitani, Toyama 9300194, Japan.
Abstract
Background To compare socioeconomic differences in behavioural and biological risk factors in Japanese and English 3959-year-old employed men.
Methods We measured systolic and diastolic blood pressure, total and high density lipoprotein (HDL) cholesterol, body mass index (BMI), waist-to-hip ratio and fibrinogen at medical screenings and smoking and alcohol consumption in health questionnaires among Japanese non-manual and manual employees of a steel products company and among English non-manual employees working in Civil Service departments.
Results In England, higher employment grades were advantaged with regard to most risk factors studied. In Japan the picture was different in that higher grades had higher BMI, waist-to-hip-ratio and lower HDL cholesterol. In Japan employment grade differences in these three risk factors are clearly larger among younger men than among older men, while in England age differentials in the grade differences are small. Similar results were obtained for education.
Conclusions Important differences in the social patterning of risk factors were observed in our cohorts of employed Japanese and English men. The contribution that these risk factors make in explaining social differences in health may vary accordingly. Studies that identify the common and unique determinants of socioeconomic health differences in different populations are needed.
Keywords Socioeconomic status, risk factors, comparative study
Accepted 4 October 2000
Although socioeconomic morbidity and mortality differentials have been widely documented in many countries1,2 relatively little research on social inequalities in health in Japan has been carried out to date. The studies that have been published indicate that socioeconomic differences in mortality, morbidity and risk factors are not uniformly smaller in Japan than in other countries.36 The authors are not aware of any comparative research on socioeconomic differences in risk factors that include Japan. The purpose of this paper is to fill in this gap.
Comparative analyses of European societies show that the magnitude of social inequalities in both health and risk factors vary between countries,7,8 and that similar levels of inequalities in total mortality are observed with different specific causes being responsible.9 These results stress the importance of comparative studies in identifying the common and unique determinants of socioeconomic differences in health. Such studies help us to distinguish which among a multitude of factors has the greatest potential to improve health, and to understand the limits of explanations of inequalities in health obtained in particular settings, and thus caution against uncritical extrapolation of results to all countries. Our specific aim is to compare the magnitude of socioeconomic differences in behavioural and biological risk factors in a Japanese and an English cohort of employed middle-aged men.
Methods
Participants
The Japanese data come from a study of non-manual and manual employees of a steel products company in the Toyama prefecture on the western coast of Japan. Of those eligible for the study, 93% participated, and filled in a self-administered health questionnaire and attended screening examination in 19961997. The English cohort, the Whitehall II study,10 is a prospective study of non-manual employees recruited in 19851988, and working in Civil Service departments. For the purposes of this study, we selected men who participated in the baseline of the Japanese study or in the third phase of the English study (19911993) and were economically active and 3959 years old at that time; altogether 2541 Japanese and 5095 English men.
We used grade of employment and education as measures of socioeconomic status. In Japan employment grade was obtained from the company records and in England it was asked as a part of the health questionnaire. On the basis of this information three hierarchical employment grades were defined. In the English cohort the three grades were obtained by collapsing the 12 non-industrial salary-based grade levels used in the Civil Service in the following way: Unified grades 16 (Permanent Secretary to Senior Principal) were combined into the highest group, Unified grade 7 (Principal), senior executive officer, higher executive officer and executive officer formed the second group, and clerical officer and clerical assistant were combined into the bottom group. Other professional and technical staff were assigned by the Civil Service to one of the grade levels on the basis of salary. The Japanese and English grades differed markedly in their salaries; in Japan the lowest employment grade earns about half of that of the highest grade and in England the corresponding proportion is about a quarter. With the 1995 exchange rate the absolute differences in income between the top and bottom grades were about £23 000 and £29 000 in Japan and England, respectively.
Education was obtained from questionnaire information in both cohorts. Three educational categories describing the highest obtained educational certificate were established. The categories are based on years of education needed to obtain a particular educational level: (1) compulsory education, (2) secondary education that has lasted for 12 or 13 years, and (3) higher education that has lasted for more than 13 years. In the English cohort educational information was from the baseline questionnaire in 19851988 and available for 3857 men. Information on education is missing for some men because only the second amended version of the baseline questionnaire enquired about educational qualifications. Missing information is thus by study design and is unlikely to strongly depend on individual characteristics.
Behavioural risk factors were obtained from health questionnaires. Smoking was categorized as: (1) never and ex-smokers, and (2) current smokers. Alcohol consumption was measured in terms of grams per week and categorized as: (1) moderate consumers (consumption below the fourth country-specific quartile of consumption among all those who consume alcohol), (2) abstainers, and (3) heavy consumers (consumption above the fourth country-specific quartile of consumption among all those who consume alcohol). Some differences in question wording in the two cohorts may contribute to differences in the proportion of abstainers.
Information on biological risk factors was obtained from screening examinations. We analysed the following: body mass index (BMI), waist-to-hip ratio, systolic and diastolic blood pressure (mmHg), total plasma cholesterol (mmol/l), plasma high density lipoprotein (HDL) cholesterol (mmol/l) and plasma fibrinogen (mg/dl). In the English cohort resting systolic and diastolic blood pressure were measured with a random zero sphygmomanometer. In the Japanese cohorts a traditional mercury sphygmomanometer was used. Venous blood was taken to determine total cholesterol and HDL cholesterol. In the English cohort total cholesterol was analysed in a centrifugal analyser by enzymic colorimetric methods, and HDL was determined after dextran sulphate-magnesium chloride precipitation of non-HDL cholesterol. In the Japanese cohort both were determined using a Hitachi 7450 auto-analyser (Hitachi, Japan). Plasma fibrinogen concentrations were analysed by the Clauss method in both cohorts.
Statistical methods
We used linear and logistic regression to analyse the data. The age-adjusted socioeconomic differences in risk factors were presented as odds ratios (OR) (logistic regression) and differences in means (linear regression) while taking the highest socioeconomic group as the reference category. In all analyses age was adjusted in single year age groups.
To summarize the data, we estimated the percentage difference between grade I and grade III risk factor levels by multiplying the continuous linear regression coefficient for employment grade by two. Similarly, by taking the exponential of the logistic regression coefficient for continuous grade and squaring it, we obtained an OR between grade I and grade III. This method gives a more stable estimate since data for all three grades are used. Similar calculations were carried out for education. All regression models were carried out in STATA.11 The focus of our analyses is to study socioeconomic differences in behavioural and biological risk factors among Japanese and English men. We do not aim to investigate national differences in the levels of these risk factors.
Results
Proportionally more English employees work in the top grade and have higher educational attainment than Japanese employees (Table 1). In both Japan and England systolic and diastolic blood pressure are higher in the lowest employment grade (Table 2
). In Japan the relationship is U-shaped with lowest blood pressure in grade II. However, the associations are relatively weak. Grade differences in total plasma cholesterol are modest, but in the expected direction.
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Among the Japanese participants smoking is about five times more prevalent than among English participants (Table 3). However, in both countries lower employment grade men smoke more. The relationship between grade and smoking is especially strong among English participants.
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For education we have only presented summary figures for the difference between compulsory and higher education. For many risk factors the differences by education are weaker than by employment grade. In particular, differences in smoking by education are much smaller than by grade in England. In a similar way to grade, higher educated men in Japan have lower HDL and higher BMI and waist-to-hip-ratio than compulsory educated men.
We further analysed grade differences in risk factors in two broad age groups (3949 years and 5059 years). These analyses show that in Japan employment grade differences in risk factors that show advantage for the lower gradesHDL cholesterol, BMI and waist-to-hip ratioare clearly larger among younger men than among older men, while in England age differentials in the employment grade differences are very small (Table 4). The large grade differences at young ages in Japan is mainly due to poor levels of risk factors in the highest grade. In fact, these young high grade men have higher BMI than older high grade men, and have smaller advantage in waist-to-hip ratio and HDL cholesterol compared to their older colleagues than lower class men (results not shown). Grade differences in other risk factors did not vary markedly by age in either country (results not shown).
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The Japanese population has one of the longest life expectancies in the world. In 19901995 the life expectancy at birth was 76.4 years for men and 82.5 years for women.12 Several reasons for the good health record of Japan have been discussed. These include a favourable diet and good nutrition; the Japanese also have lower levels of obesity and total cholesterol than many Western European countries.13,14 Other factors may include proper access to medical care, prosperity and favourable working conditions.15 Low levels of mortality have been achieved at a level of household disposable income inequality that is about average in comparison to other industrialized countries.16,17 However, despite the importance of Japan as a benchmark of long life expectancy, relatively little research on social inequalities in health and risk factors has been carried out to date. This relative scarcity of research evidence also extends to the traditional risk factors, although these are likely to have strongly influenced past trends in stroke and CHD mortality in Japan.18
Our analyses show that in England the lower the employment grade the less favourable the level of HDL cholesterol, BMI, waist-to-hip ratio, plasma fibrinogen and smoking and diastolic and systolic blood pressure were slightly higher in lower grades. The relationship between grade and total plasma cholesterol was weak. The higher employment grades had fewer alcohol abstainers and heavy consumption of alcohol was more common in the higher grades. However, even this disadvantage disappears when a higher cut-point for heavy consumption is used. Corresponding findings have been obtained from nationally representative general population samples in England.19 In Japan the picture was different in some important respects. Higher employment grades had higher BMI, waist-to-hip-ratio and lower (less favourable) HDL cholesterol. Similar results were obtained for education. Future studies need to establish whether social differences in risk factors that we have not been able to analyse in this study (e.g. diet and exercise) exist.
Further analyses of these data show that in Japan employment grade differences in the risk factors that show advantage for the lower gradesHDL cholesterol, BMI and waist-to-hip ratioare clearly larger among younger men than among older men, while in England age differentials in the grade differences are small. The large grade differences at young ages in Japan are mainly due to poor levels of risk factors in the highest grade. In fact, these young high grade men have higher BMI than older high grade men, and have smaller advantage in waist-to-hip ratio and HDL cholesterol in reference to their older colleagues than lower class men. Although we should not neglect the possibility that physical activity at work plays some role in explaining these differences,20 we believe that the age differences in the grade gradient and the relatively poor absolute levels of BMI, waist-to-hip ratio and HDL among young high grade men may be more accurately explained in terms of changes in the Japanese diet. As the Japanese are slowly giving up their traditional diet and are adopting diets that include more animal fats,18,21 young high grade Japanese men may be leading the trend. Large regional differences in diet exist in Japan. Dietary surveys from Toyama's neighbouring prefecture Niigata indicate that diets in this region may still be more traditional than those observed in large urban areas such as Osaka.22 It is thus possible that the socioeconomic patterns we observe for BMI, waist-hip-ratio and HDL in our Japanese cohort are typical for regions where the dietary transition is at an early stage.
International comparisons of health differences may be affected by data comparability problems. With respect to this study the first relates to obtaining comparable information on social status. A specific problem of this study is that the Japanese cohort includes both blue-collar and white-collar employees, but the English cohort includes only white-collar employees. It may thus be possible that the social distance between the Japanese grades is larger than that between the English grades. However, this does not seem to be the case, because differences in both absolute and relative income between the top and bottom status employees are in fact smaller in the Japanese than in the English cohort (Methods). Furthermore, analyses by education differences broadly confirm the pattern and direction of social differences observed for grade. It is thus very unlikely that the variation in the grade differences in risk factors in these two cohorts, particularly in risk factors (HDL, body mass index and waist-to-hip-ratio) that show different directions of association, is an artefact of measurement.
Another data problem relates to the variability of the questionnaire designs and the laboratory measurement of biological risk factors. However, the differences in the mean levels and prevalence of risk factors that are observed between the Japanese and the English cohort are often in the expected direction. Furthermore, even if the level of risk factors were somewhat over- or under-estimated, only very large discrepancies would lead to errors in the estimation of socioeconomic differences.
Thirdly, generalizations of these findings to the respective national populations should be conducted with some caution. Both cohorts consist of employed subjects, a healthy sub-set of the general population. Analyses of the economically active are known to underestimate social class differences in mortality and morbidity,23 but little is known about the size of this bias with respect to risk factors. In addition, both cohorts are recruited in work places and regions that may not be representative of the respective countries more generally.
In summary, these results underscore the importance of comparative studies in identifying the common and unique determinants of socioeconomic differences in health in different populations. We have shown that socioeconomic differences in some of the proximate behavioural or biological determinants of health vary between our cohorts of Japanese and English employees, while others show similar socioeconomic patterns. These results indicate that in different social contexts and historical settings the level and direction of change of social inequalities in health are likely to be driven by different proximate determinants. Future studies need to better establish social differences in various domains of health among Japanese men and also women, and seek to improve our understanding of the contribution of various behavioural and biological risk factors as well as more distal up-stream social structural, household and psychosocial determinants that may impact on these health differences.
KEY MESSAGES
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Acknowledgments
The Whitehall II study has been supported by grants from the Medical Research Council, British Heart Foundation, Health and Safety Executive, National Heart Lung and Blood Institute (HL36310), National Institute on Aging (AG13196), Agency for Health Care Policy Research (HS06516), The New England Medical Centre: Division of Health Improvement, Institute for Work and Health, Toronto, and the John D and Catherine T MacArthur Foundation Research Networks on Successful Midlife Development and Socio-economic Status and Health. PM has a fellowship (70631) from the Academy of Finland and a grant from the Signe and Ane Gyllenberg Foundation. MM is supported by a MRC Research Professorship. The Japanese cohort was supported by a grant from Kanazawa Medical University (C96-11). We also thank all participating civil service departments and their welfare, personnel, and establishment officers; the Occupational Health and Safety Agency; the Council of Civil Service Unions; all participating civil servants in the Whitehall II study; and all members of the Whitehall II study team.
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