1 Department of Hygiene and Preventive Medicine, School of Medicine, Iwate Medical University, Morioka, Japan.
2 Department of Health Science, Shiga University of Medical Science, Otsu, Japan.
3 International Center for Health and Society, University College London, London, United Kingdom.
4 Department of Epidemiology and Public Health, Imperial College School of Medicine, London, United Kingdom.
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ABSTRACT |
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ethnic groups; heart diseases; ischemia; mortality; myocardial ischemia; racial stocks; time factors
Abbreviations: ICD-8, International Classification of Diseases, Eighth Revision; ICD-9, International Classification of Diseases, Ninth Revision; IHD, ischemic heart disease.
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INTRODUCTION |
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Declining trends in IHD mortality in Japan might reflect differing trends among persons living in urban settings in comparison with the rest of Japan. We previously reported different trends in dietary intake of fats and cholesterol, and serum total cholesterol levels, for rural and urban populations (8). We found that serum total cholesterol levels in rural populations were lower than in urban populations in the 1960s; although these levels have been increasing more rapidly than those in urban populations, they remain lower. In addition, serum cholesterol levels are higher in younger generations than in older generations, reflecting different dietary habits, a finding in contrast to observations in the United Kingdom and United States (5
).
We theorized that changes in IHD mortality differ among generations and between urban and rural populations. In the present study, we compared trends of premature mortality due to IHD among populations in Tokyo and Osaka prefectures, the most urbanized area of Japan, with those in the rest of Japan from 1969 to 1992.
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MATERIALS AND METHODS |
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We selected the populations of Osaka prefecture (8.7 million in 1990) and Tokyo prefecture (11.8 million in 1990) as the most urbanized in Japan; in 1990, these prefectures contained 17 percent of the entire population in 1.1 percent of the area of Japan. The rest of Japan (population 100 million in 1990) was considered the reference population. Age- and sex-specific populations for the prefectures (5-year age groups) were available for 1965, 1970, 1975, 1980, 1985, and 1990 from the national census, which has been conducted every 5 years and covers more than 99 percent of the population (9).
Age- and sex-specific populations for each year from 1969 to 1990 were calculated by taking a weighted mean of these populations from the national census over 2 consecutive years (9). Estimated populations were used for 1991 and 1992 (10
, 11
). The average age-specific mortality rate for 2 consecutive years was calculated by dividing the number of deaths that occurred over 2 years by the sum of the populations in the age group. To compare the generational difference clearly, we adopted the 3069-year age band since nonspecific causes of death tend to be used for the elderly. We calculated age-adjusted mortality rates for the groups aged 3049, 5059, and 6069 years as well as for those persons aged 3069 years. The standard population of Japan that was used was based on the 1985 national census (9
). Mortality rates for Tokyo and Osaka prefectures and for the rest of Japan were compared between 19691970 and 19771978 (period I) and between 19791980 and 19911992 (period II) to examine changes in the trends of the mortality rates by year. By using the Mantel-Haenszel method, we calculated the age-adjusted mortality ratio and its variance to compare rates between generations and periods.
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RESULTS |
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Age-adjusted heart disease and IHD mortality trends in Tokyo and Osaka prefectures and the rest of Japan
Changes in the age-adjusted (age 3069 years) mortality rates of heart disease and IHD in Tokyo and Osaka prefectures and the rest of Japan in period I (19691978, ICD-8) and period II (19831992, ICD-9) are summarized in table 1. For men, the percentage decline per year in age-adjusted IHD mortality in period I in Tokyo and Osaka prefectures (2.46 percent) was similar to that in the rest of Japan (2.74 percent). In contrast, in period II, there was an accelerated decline in IHD mortality in the rest of Japan (3.50 percent), while the decline was smaller in Tokyo and Osaka prefectures (1.65 percent). Nonspecific heart disease mortality per 100,000 in Tokyo and Osaka prefectures decreased slightly from 41.2 to 37.4 in period I and from 40.6 to 39.4 in period II, while the rates for the rest of Japan changed from 44.4 to 42.5 in period I and from 52.4 to 54.4 in period II. The observed increase from 19691970 to 19911992 in nonspecific heart disease mortality in the rest of Japan was due chiefly to changes in ICD-8 and ICD-9 diagnostic criteria.
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Generational difference in IHD mortality in Tokyo and Osaka prefectures and the rest of Japan
Changes in IHD mortality in Tokyo and Osaka prefectures and the rest of Japan from 1969 to 1992 for the groups aged 3049, 5059, and 6069 years are shown in figure 2 and table 2. While IHD mortality trends were similar across all age groups in the rest of Japan, the declines among younger age groups were smaller than those among older age groups of men in Tokyo and Osaka prefectures in both period I and period II (p < 0.05). The ratio of the per-year percentage change in period II to that in period I ranged from 0.083 for persons aged 3049 years to 0.72 for those in the group aged 6069 years. In the latter group, the decline in IHD mortality was significantly smaller in period II than in period I (p < 0.001). However, in the rest of Japan, the declines were significantly larger in period II than in period I for all age groups of men (p < 0.001). The same phenomenon was observed for women in all age groups in Tokyo and Osaka prefectures. The ratio of the per-year percentage change in period II to that in period I ranged from 0.21 for the group aged 3049 years to 0.79 for those aged 5059 years. In the groups aged 6069 and 5059 years, the decline in IHD mortality was significantly smaller in period II than in period I (p < 0.001). In the rest of Japan, the declining trends remained similar in the groups aged 3049 and 5059 years, and a significant larger decline in period II was observed for the group aged 6069 years (p < 0.001).
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DISCUSSION |
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Comparability of heart disease and IHD mortality trends
The underlying causes of mortality in Japan were coded by using ICD-8 from 1969 to 1978 and ICD-9 from 1979 onward. The IHD criteria changed little during these periods (2). As shown in figure 1, there appeared to be no discontinuity between 1969 and 1978 and after 19791992 regarding IHD mortality. However, central coding for heart disease was different from ICD-8 to ICD-9 (12
). Thus, to avoid systematic bias caused by the coding, we compared changes in heart disease and IHD mortality within two periods by using the same versions of this publication (i.e., 19691978 and 19791992).
The decline in mortality from specific causes might have been due to changes in diagnostic criteria. Recent advances in medical care in Japan may have changed these criteria; in the past, nonspecific terms such as heart failure rather than myocardial infarction tended to be recorded on death certificates (2).
For men, continuous declines in IHD mortality were observed in both populations. Nonspecific heart disease mortality remained unchanged in Tokyo and Osaka prefectures. Although the rate in the rest of Japan increased (per 100,000, from 44.4 in 19691970 to 54.4 in 19911992), this increase was due mainly to the differences in the diagnostic characteristics between ICD-8 (per-100,000 rate, 42.5 in 19771978) and ICD-9 (per-100,000 rate, 52.4 in 19791980). Thus, in both populations, the shift from nonspecific causes to specific causes of death was not observed over time for men. For women, the decline in heart disease mortality was much higher than that for IHD, and the shift might have occurred among women (figure 1). Considering the shift from nonspecific heart disease to IHD, the declining trends in IHD mortality observed for women might have been underestimated. However, these trends were similar in Tokyo and Osaka prefectures and the rest of Japan during period I and period II. Thus, it appears that observed differences in IHD trends between Tokyo and Osaka prefectures and the rest of Japan were real for both men and women.
Recently, Baba et al. compared causes of death listed on death certificates with those ascertained by reviewing the medical charts (13). Corrected IHD mortality rates were 11 percent higher than the rates obtained by using the original coding, but there was no systematic bias over time in the diagnoses between 1983 and 1990. Although these authors compared limited records in the specific region, this finding may also support the finding that the IHD trends for which underlying causes of deaths were used may be real.
There also may be regional differences in the diagnostic criteria, since large hospitals are located mainly in Tokyo and Osaka prefectures. This fact may cause systematic bias in cause-specific mortality rates. For both sexes, ratios of IHD mortality to heart disease mortality have been higher in Tokyo and Osaka prefectures than the rest of Japan (i.e., a nonspecific diagnostic label for IHD, such as heart failure, tends to be used in the rest of Japan). To compare the trends, we used a mortality ratio of 19771978 to 19691970 for Tokyo and Osaka prefectures and of 19911992 to 19791980 for the rest of Japan, instead of comparing the mortality rate directly.
We also compared the trends of IHD mortality between the generations in both populations for both sexes, and we observed a significant difference only in Tokyo and Osaka prefectures. Changes in IHD mortality were clearer for the older than the younger generations. It is unlikely that a shift in the diagnostic criteria occurred for the younger generations in the same population for both sexes. On the contrary, similar declines in IHD mortality were observed in all generations for both sexes in the rest of Japan.
Trends of cardiovascular risk factors in Japan
We reported an increase in cholesterol levels among the Japanese population on the basis of nationwide surveys (5). We also reported a regional difference in the trends of serum total cholesterol levels between rural areas and Tokyo and Osaka prefectures (8
). In these prefectures, serum total cholesterol levels were considerably higher than those in rural populations in the1960s, and there were considerable differences in dietary habits (e.g., the quantity of meat consumed per capita per day was 19.5 g in the rural population and 51.3 g in urban areas (8
)).
Recently, differences in cholesterol levels between the rural and urban areas have been reduced as food consumption habits in the rural population have been westernized (8). However, cholesterol levels are still lower in rural than in urban populations. For example, Keys' dietary factor (14
) is still lower in the rural than in the urban population, although the difference has narrowed because of an increase in the rural population (figure 3).
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Generational and regional differences in the IHD mortality trends
In the present study, Tokyo and Osaka prefectures were selected as the most urbanized areas of Japan, with 17 percent of the entire population, and we compared these prefectures with the rest of Japan. Since the rest of Japan includes some large cities with a population of more than 1 million (six to eight cities during the period) and we included these cities in the rest of Japan, the observed difference in the trends might have been diluted.
For men, 19691970 age-adjusted IHD mortality was broadly similar between Tokyo and Osaka prefectures and the rest of Japan (figure 1). IHD mortality in Tokyo and Osaka prefectures decreased in relation to the rest of Japan in period I. However, in period II, the decline accelerated in the rest of Japan (p < 0.001), while it slowed in Tokyo and Osaka prefectures. A slowdown in the declining trends was observed especially for younger age groups. For women, essentially similar differences were observed. That is, IHD mortality in Tokyo and Osaka prefectures showed a decrease similar to that in the rest of Japan in period I. However, it slowed in Tokyo and Osaka prefectures in period II, while the decline did not decrease in the rest of Japan. These data show that the recent decline in IHD mortality in Japan is not attributable to a decline in Tokyo and Osaka prefectures but mainly to that in the rest of Japan. Konishi et al. reported that, in a rural population with low cholesterol and high blood pressure, patients who died of acute myocardial infarction had a lower degree of stenosis and higher hyaline degeneration of the coronary artery than did patients in an urban population (20). Taking into account the trends of cholesterol and blood pressure levels, these authors concluded that the major risk factor for IHD death in a rural population was high blood pressure (20
).
In our study, differences in mortality trends were observed among age groups of men in Tokyo and Osaka prefectures. In the youngest age group (3049 years), the decline in mortality over a 20-year period was much less than that in older age groups. In the rest of Japan, declining trends were observed equally for all age groups. For period I and period II, age-specific IHD mortality showed a lower percentage change in the group aged 4049 years than in the groups aged 5059 and 6069 years (p < 0.001). This difference in the percentage change in IHD mortality among generations could be explained partially by the birth cohort effect. A major portion of the groups aged 3049 years (1.05 percent) and 5059 years (2.05 percent) in period I became aged 5059 (1.33 percent) and 6069 (2.08 percent) years in period II (table 2). In Tokyo and Osaka prefectures, although different declining trends were observed among different generations, these trends seem to have remained similar over time for the same birth cohorts. These data also support the conclusion that generational and regional differences in the IHD mortality trends are real.
For women, the time trends of IHD mortality in Tokyo and Osaka prefectures showed changes similar to those for men in Tokyo and Osaka prefectures; that is, the decline was larger in period I than in period II, while declining trends in the rest of Japan did not decrease. A generational difference also was observed in Tokyo and Osaka prefectures; the decline was smallest in the group aged 3049 years (p < 0.001).
The regional and generational differences in IHD mortality trends are consistent with trends of dietary habits and cholesterol levels (8). The regional differences in declining trends of blood pressure also agree with these different mortality trends (15
). These data indicate that the middle-aged population in Tokyo and Osaka prefectures, especially men, may have a higher risk for IHD death than the population in the rest of Japan. This finding concurs with a report that IHD incidence among workers recently increased in urban areas (21
). We predict that changes in trends of IHD mortality in Japan initially will be observed in urban populations and may predict the future burden of IHD mortality in Japan. Extensive studies to elucidate the incidence of IHD should be conducted among urban populations.
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ACKNOWLEDGMENTS |
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NOTES |
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REFERENCES |
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