Trends of Blood Pressure Distributions in a Northeast Rural Japanese Population

In Japan recently there has been a rapid change in living and eating patterns along with accelerated urbanizations in rural communities. This provides an unique opportunity to observe a critical "natural experiment" of change in both risk factors and disease. Data on blood pressures and time trends were obtained in population-based surveys conducted in rural Japan ; the first survey from 1963-66, the second from 1972-75, and the third from 1980-83. This report describes the population trends over 20 years for blood pressure distributions and its related factors. Between the 1960s and the 1970s, the prevalence of a higher blood pressure level declined although no remarkable shift of the blood pressure distribution was observed. However, between the 1970s and the 1980s, there was a significant downward shift of the blood pressure distribution. The decline in blood pressure between the 1960s and 1970s was primarily related to a fall in the prevalence of elevated blood pressure, attributable in turn to the greater use of anti-hypertensive medications and improved blood pressure control. In contrast, the large downward shift in the whole blood pressure destributions between the 1970s and the 1980s was attributable partly to improved hypertension control and apparently also to changes in environmental factors of associated behaviors that affected the whole distribution. The correlation between blood pressure and relative weight index was weak in the 1960s but became stronger in the 1970s and the 1980s. The significant blood pressure trend in a whole population are consistent with the reported decline in stroke incidence in this population. J Epidemiol, 1993; 3 : 63-70.


ORIGINAL CONTRIBUTION
epidemiological methods of blood pressure measure-ment5). Measurements of several subjects with different blood pressure levels were practiced on each arm simultaneously with separate sphygmomanometer, with a Y-connection to two stethoscopes, or, alternately, on the right arm in random order by a trainee and then an experienced observer. The training was repeated until systematic observer error was minimized. This procedure was repeated among observers for each survey period to reduce observer differences.
Relative body weight was calculated from Minowa's standards based on medians of body weight distribution by height and sex for 5,086 healthy Japanese in 1960 to 19626). Regression equaltion of standard weights was 0.002235 x height(cm)2-0.1206 x height(cm)+15.92 for men and 0.003578x height(cm)2-0.5653 x height(cm)+52.81 for women. Minowa's relative weight index (RWI) was defined as percent deviation of actual weight from standard weight. For example, a man with 160 cm in height and 65 kg in weight shows 20% in RWI and 25 in body mass index caldulated as weight(kg)/height(m)2.

Statistical analyses
T-tests were used for continuous variables and chisquare tests for comparisons of prevalences. All pvalues are two-tailed. Table 1. The number of subjects and participants, and participation rates by age and sex, in 1963-1966, 1972-1975 and 1980-1983

RESULTS
Trends of blood pressure by age and sex Figures 1 to 3 show the systolic blood pressure distributions by age and sex during the three survey periods. Between the 1960s and the 1970s, there was no distinct shift in distribution, but the prevalence of systolic blood pressure greater than or equal to 180 mmHg declined for all age-sex groups. Remarkable downward shifts of the distribution were seen in all age-sex categories between the 1970s and the 1980s, with the larger distribution shifts among the older age groups.
A similar trend was observed in the distribution of diastolic blood pressure (not shown). Between the 1960s and 1970s, the distribution of diastolic blood pressure did not shift remarkably for any age-sex group, although the prevalence of diastolic blood pressure greater than or equal to 100 mmHg declined. Between the 1970s and the 1980s, blood pressure distributions declined for all age-sex groups.
The prevalence of hypertension by age and sex is shown in Table 2, defined as the percent of participants systolic blood pressure * 160 mmHg and/or diastolic blood pressure * 95 mmHg, regardless of antihypertensive medication use. The prevalence declined significantly between the 1960s and the 1980s for all age-sex groups, with the greater decrease occurring between the 1970s and the 1980s. Table 3 depicts the frequency of antihypertensive medication use, defined as : (a) the percent of all hypertensives taking antihypertensive medication (hypertensives defined as those with systolic blood pressure * 160 mmHg, or diastolic blood pressure * 95 mmHg, and/or taking antihypertensive medication), and (b) the percent of persons with antihypertensive medication among all survey participants. Frequency (a) was less than 23.0% in the 1960s and increased significantly for every age-sex group between the 1960s and the 1980s. Frequency (b) was less than 9.0% in the 1960s and also rose significantly for all age-sex categories between the 1960s and the 1980s. The increased frequency (b) was more remarkable in the first decade than in the second. Table 4 shows the prevalence of normotension according to past or present antihypertensive medica-   1963-1966 , 1972-1975, and 1980-1983. tion use. Past or present antihypertensive medication use were considered as an indicator of hypertension control. Normotension was defined as systolic blood pressure < 140 mmHg and diastolic blood pressure <90 mmHg, regardless of antihypertensive medication use. The prevalence of normotension with past or present medication use increased significantly for every age-sex group between the 1960s and the 1980s . The proportion of normotensives without medication did not change between the 1960s and 1970s but increase significantly for all age-sex groups between the 1970s and the 1980s.
Trends of prevalence of overweight and associations between blood pressures and relative weight index (R WI) Prevalence of overweight (RWI * 20%) and the proportion of hypertensive persons among the nonoverweight and among the overweight was shown at three survey periods (Table 5). Hypertensive persons are defined as those with systolic blood pressurez160 mmHg and/or diastolic blood pressure * 95 mmHg, and/or taking antihypertensive medications . Prevalence of overweight increased between the 1960s and the 1980s in all age-sex groups except for men aged 50-59 and 60-69. The increase in the prevalence occurred primarily between the 1960s and the 1970s .
At each survey, the proportion of hypertensive persons was higher among the overweight for all age-sex groups except for men ages 40-49. For men ages 40-49, the proportion of hypertensive persons did not differ between the overweight and the non-overweight in the 1960s, but increased significantly among the Table 2.
The percent frequency of antihypertensive medication use by age and sex, in 1963-1966, 1972-1975 and 1980-1983. Frequency (a) : the percent of persons taking antihypertensive medication among hypertensive persons (systolic blood pressure > = 160 mmHg, and/or diastolic blood pressure > =95 mmHg, and/or antihypertensive medication use).

DISCUSSION
Substantial changes in blood pressure distributions were observed for a rural Japanese between the 1960s and the 1980s by our population-based study with a high response rate.
It was noteworthy that trends in blood pressure distributions between the 1960s and the 1970s were different from those in the 1970s and the 1980s. Between the 1960s and the 1970s, the prevalence of systolic blood pressure * 180 mmHg or that of diastolic blood pressure z 110 mmHg declined, although no remarkable shift of the blood pressure Table 5. Prevalence of overweight and the proportion of hypertensive persons by relative weight index (RWI), by age and sex, in 1963-1966, 1972-1975 and 1980-1983. Differences from the 1963-1966 values: *p<0.05, **p<0.01, ***p<0.001 Differences from the 1972-1975 values: +p<0.05, ++p<001 +++p<0.001 Differences from the values with RWI<20%: ++p<0.05, ++++p<0.01, ++++++p<0,01 These data lead us to infer that the significant decline in population blood pressure levels between the 1970s and the 1980s was due not only to improvements in hypertension control with medications but also to changes in nonpharmacological factors. We speculate that overall improvement in nutrition, specifically better protein and fat intake, and lower salt intake, the lessening of hard manual work 3,4,7,8) may have contributed to the blood pressure decline.
Before the 1960s, people in Akita rural communities took a traditional Japanese diet : a large amount of rice (400 g or more per day) with salty miso soups and salt-preserved pickles9). Dietary protein sources were limited to rice, fish and beans. They did not have access to meat and dairy foods.
Salt-preserved fish and vegetables were not only for preservation itself but also for appetizer of plain rice. According to our nutrition studies for men aged 40-59 in Ikawa town, the percent of animal protein calories increased from 5.8% in the 1960s and 7.1% in the 1980s. The percent of animal fat calories doubled from 4.5% in the 1960s to 9.6% in the 1970s. Intake of salt declined from 20 g to 14 g. Percent calories of carbohydrate also declined from 67% to 57%3). A remarkable increase in animal protein and fat was due to increased consumption of meat and dairy foods, which was associated with socioeconomic development such as increased cash income, improved food transport and refrigeration, and in part with our community-based campaigns Japanese, there was a higher prevalence of physical inactivity in urban residents and rural clerical workers than rural farmers and manual workers23). From now on, however, attention should be paid to the prevention of overweight and elevated serum total cholesterol, for primary prevention of hypertension and coronary heart disease.