Diet-Related Factors, Educational Levels and Blood Pressure in a Chinese Population Sample: Findings from the Japan-China Cooperative Research Project

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1 559 Original Article Diet-Related Factors, Educational Levels and Blood Pressure in a Chinese Population Sample: Findings from the Japan-China Cooperative Research Project Yukio YAMORI 1, Longjian LIU 1,5,6, Lihong MU 2, He ZHAO 3, Youqun PEN 3, Zhiqiong HU 2, Sachiko KUGA 1, Hiroko NEGISHI 1, Katsumi IKEDA 1,4 on behalf of the Japan-China Cooperative Study Group: Chongqing Project As part of the Japan-China Cooperative Research Project of the WHO-Cardiovascular Disease and Alimentary Comparison Study, a cross-sectional study was carried out to investigate risk factors for high blood pressure (BP) in male adults in Chongqing, China. Subjects with hypertension (HT) were defined as those if they had systolic BP (SBP) 140 mmhg or diastolic BP (DBP) 90 mmhg or if they were receiving anti-hypertensive drug therapy. Subjects were also categorized into three groups according to their level of education, i.e., low- ( 6 years), intermediate- (7 9 years), or high- ( 10 years) level education. The results were as follows. (a) 20.3% of subjects had HT, 16.7% had hypercholesterolemia (serum total cholesterol 220 mg/dl), and 23.4% were overweight (body mass index 25 kg/m 2 ). (b) After adjustment for age, SBP and DBP showed a significant positive association with body mass index, urinary sodium (Na) excretion, and total cholesterol (TC) to high-density lipoprotein (HDL) cholesterol ratio (TC/HDL). SBP and DBP tended to be negatively associated with 24 h urinary potassium (K) and magnesium (Mg) excretion. (c) Subjects with the highest educational level had the lowest prevalence of HT (11.6%), followed by those with the low (22.6%) and the intermediate (25.0%) educational levels ( p 0.05). (d) Logistic regression analysis indicated that the relative risks (95%CI) of being overweight, high TC/HDL ratio, high Na excretion and lower educational level ( 10 years) for risk of HT were 5.39 ( ), 1.73 ( ), 1.30 ( ), and 2.56 ( ) respectively. (e) Subjects with the highest educational level had significantly lower Na, significantly lower Na/K ratio excretion, and significantly higher K and Mg excretion values than those with intermediate or low educational levels. In conclusion, BP was strongly associated with BMI, salt intake and other diet-related factors in the study sample. The results emphasize that education plays an important role in public health for the control of high BP in the Chinese population. (Hypertens Res 2002; 25: ) Key Words: diet habits, education, blood pressure, Chinese From 1 World Health Organization Collaborating Center for Research on Primary Prevention of Cardiovascular Diseases, Kyoto, Japan, 2 Department of Epidemiology, Chongqing University of Medical Sciences, Chongqing, China, 3 Chongqing Jian She Hospital, Chongqing, China, 4 School of Human Environmental Sciences, Mukogawa Women s University, Nisinomiya, Japan, 5 Geriatric Research Education and Clinical Center, Donald W. Reynolds Department of Geriatrics, University of Arkansas for Medical Sciences, Little Rock, USA, and 6 Department of Epidemiology, College of Public Health, University of Arkansas for Medical Sciences, Little Rock, USA. This study was financially supported by a grant from the Japan-China Medical Association (Tokyo, Japan). Address for Reprints: Longjian Liu, M.D., Ph.D., University of Arkansas for Medical Science, Geriatric Research Education and Clinical Center, VA Hospital / NLR Campus, 3J, Building 170; 2200 Fort Roots, North Little Rock, AR 72114, USA. liulongjian@yahoo.com Received February 7, 2002; Accepted in revised form April 9, 2002.

2 560 Hypertens Res Vol. 25, No. 4 (2002) Introduction High blood pressure is one of the most important risk factors for stroke and coronary heart disease. These two diseases have become leading causes of death in both developed and developing countries, including China. Because the Chinese population accounts for more than one quarter of the total world population, it is very important to have effective monitoring systems to investigate risk factors for hypertension, coronary heart disease, stroke and other major diseases. Since 1985, we have being working on the Japan-China Cooperative Research Project, as part of the World Health Organization (WHO) Cardiovascular Diseases and Alimentary Comparison (CARDIAC) Study, in 12 cooperative centers in China (1 3). Several important findings were reported in our previous studies (2, 3, 5 7). Accordingly, in the year 2000 we extended our study and set up a new cooperative center in Chongqing, China. In this paper, we report the preliminary results from the new center. Subjects and Methods A total of 177 men aged years old were randomly selected from the Daping District of Chongqing, China by using a cross-sectional study design and cluster randomization method. All participants were Han people, because the majority of Chongqing residents are Han. In the Chongqing center, our study focused on male subjects, because one of our objectives was to examine oxidative DNA damage in relation to blood pressure and lifestyles, including smoking and alcohol consumption. In Chongqing and in China in general there is a much lower prevalence of cigarette smoking and alcohol consumption among females than among males. Therefore, females were not included in the study. All participants were invited to Jian She Hospital (a teaching-practice hospital of Chongqing University of Medical Sciences) to have a physical examination and blood tests between October 16 and 27 of During the field survey, interviews were conducted using a modified questionnaire from the WHO-CARDIAC Study (1). The main items included in the questionnaire were designed to gather information on demographic variables, smoking, alcohol consumption, medical history of diseases, dietary habits and physical activity. A 9 ml blood sample was collected from 8:00 AM to 10:00 AM after participants had fasted 8 12 hours. A 24 h urine sample was collected after the collection procedure had been carefully explained to each participant by a trained investigator. All subjects who participated in the study were on a volunteer basis. They were informed of their right to refuse participation in the study or to withdraw from the study at any time without giving an explanation. The Chongqing University of Medical Sciences and Chongqing Jian She Hospital officially approved the selection of samples and the survey procedure, based on their respective regulations for human research. All collected serum blood samples and urine samples were sent to the WHO Collaborating Center for Primary Prevention of Cardiovascular Diseases, Graduate School of Human and Environmental Studies, Kyoto University, Japan. The laboratory of this center has been well standardized since the start of the WHO-CARDIAC Study in Serum total cholesterol (TC), high density lipoprotein cholesterol (HDL), triglycerides (TG), total protein (TP), urinary creatinine, sodium (Na), potassium (K) and magnesium (Mg) excretion values were measured using an Express Plus Operator machine (Ciba Corning Diagnostics Corp. Westlake Village, USA) (1). In statistical analysis, the body mass index (BMI) was calculated as weight (kg) divided by the square of height (m). Overweight subjects were defined as those with BMI 25 kg/m 2. Hypertensive subjects were defined as those with systolic BP (SBP) 140 mmhg or diastolic BP (DBP) 90 mmhg or those who were receiving anti-hypertensive drug therapy. Subjects were also categorized into three groups according to their level of education, i.e., low- ( 6 years), intermediate- (7 9 years), or high- ( 10 years) level education. Student s t-test, analysis of variance and χ 2 tests were used to examine differences in mean BP, lipid profiles, urinary factors and the prevalence of hypertension among different groups (such as by educational levels, and by those with and without hypertension). Pearson correlation analysis was used to examine associations of BP with age, BMI, sodium excretion, and other factors. Finally, to summarize the strongest determinants of hypertension, a stepwise logistic regression model was used. All data analyses were conducted using SPSS software (SPSS Inc., Chicago, USA) (4). Results A total of 177 subjects participated in the study and completed the questionnaire survey (session one). Among them, 138 had blood tests (participant rate: 80.0%) (session two). And finally, 118 completed urine sample collection (66.7%) (session three). No significant differences were found among the three sessions in terms of age, education and occupational status. The main characteristics of the participants are shown in Table 1. The mean (SD) age, educational level, systolic and diastolic BP (SBP and DBP) were 51 (2.5) years old, 8.2 (2.9) years, (19.5) mmhg and 79.9 (13.3) mmhg, respectively. The percentages of hypertensive, hypercholesterolemic, and overweight patients were 20.3%, 16.7%, and 23.4%. Mean urinary Na, K, and Mg excretion values were (58.5) mmol/day, (10.86) mmol/day and (16.31) mg/day. Table 2 shows that SBP was significantly associated with BMI (Pearson correlation coefficient: 0.33, p 0.01), Na (0.27, p 0.01), total cholesterol (0.17, p 0.05), TC/HDL ratio (0.17, p 0.04) and TG (0.18, p 0.05). DBP was significantly associated with BMI (0.41, p 0.01), Na (0.25,

3 Yamori et al: Diet, Education and BP in Chinese 561 Table 1. Participant Characteristics Mean or % Continuous variables, mean (SD) Age (years) 51 (2.5) Education (years) 8.2 (2.9) Body mass index (BMI) (kg/m 2 ) 23 (3.1) Systolic blood pressure (SBP) (mmhg) 120 (19.5) Diastolic blood pressure (DBP) (mmhg) 79.9 (13.3) Total cholesterol (TC) (mm/dl) (35.0) High density lipoprotein (HDL) (mg/dl) 52.5 (13.6) Triglycerides (TG) (g/dl) 1.9 (1.4) Total protein (TP) (g/dl) 7.5 (0.5) 24-Hour urinary excretion Sodium, Na (mmol/day) (58.5) Potassium, K (mmol/day) 30.9 (10.9) Magnesium, Mg (mg/day) 51.1 (16.3) Categorical variables (%) Educational level (years) Occupational class Workers 84.8 Professionals 15.2 BMI ( 25 kg/m 2 ) (%) 23.4 Hypercholesterolemia (%) 16.7 Hypertension (%) 20.3 Smoking 66.1 Never smoking 23.2 Ex-smoking 10.7 Current smoking 66.1 Alcohol consumption None 36.2 Yes 63.8 Table 2. Correlation Coefficients of Blood Pressure with Body Mass Index and Other Diet-Related Factors SBP p DBP p BMI (kg/m 2 ) Na (mmol/day) K (mmol/day) Mg (mg/day) TC (mg/dl) HDL (mg/dl) TC/HDL ratio TG (g/dl) TP (g/dl) BMI: body mass index, Na: sodium, K: potassium, Mg: magnesium, TC: total cholesterol, HDL: high density lipoprotein cholesterol, TG: triglycerides, TP: total protein. Table 3. Logistic Regression Analysis: Relative Risks (RR) of Hypertension Associated with Less Education and Other Factors RR (95%CI) p Education ( 10 years) 2.56 ( ) 0.01 BMI ( 25 kg/m 2 ) 5.39 ( ) 0.01 Na/K ratio 1.30 ( ) 0.02 TC/HDL ratio 1.73 ( ) 0.01 The logistic regression analysis was performed using a stepwise procedure, taking hypertension (0, 1 for none and yes) as a dependent variable, and age, occupation, smoking, alcohol consumption, education, BMI, Na/K ratio, Mg, TC/HDL ratio, TP, and TG as independent variables. (see text). p 0.01), total cholesterol (0.25, p 0.01), TC/HDL ratio (0.25, p 0.01), TG (0.22, p 0.05), and TP (0.23, p 0.01). Serum HDL and urinary K and Mg excretion were negatively associated with both SBP and DBP, but these associations did not reach the level of statistical significance ( p 0.05). To examine the strongest determinants of hypertension, Table 3 shows the results of stepwise logistic regression analysis, taking hypertension (0, 1 for none and yes) as the dependent variable, and age (years), occupation (0, 1 for workers and professionals), smoking (0, 1 for none and yes), alcohol consumption (0, 1 for none and yes), overweight (0,1 for none and yes), low or intermediate-level education (0, 1 Fig. 1. Prevalence of hypertension by educational levels for education 10 years, and education 10 years), Na/K ratio, Mg (mg/day), TC/HDL ratio and TP (g/dl) as the independent variables. The final model showed that subjects with less education, subjects who were overweight, and subjects

4 562 Hypertens Res Vol. 25, No. 4 (2002) Table 4. Mean Diet-Related Factors by Education Levels Educational levels (years) p Mean (SD) Mean (SD) Mean (SD) Na (mmol/day) (56.0) (51.5) 94.0 (39.7) K (mmol/day) 26.2 (8.0) 31.3 (10.9) 40.3 (10.3) Na/K ratio 7.1 (2.7) 5.1 (2.5) 2.5 (1.3) Mg (mg/day) 47.8 (15.4) 48.5 (14.0) 61.3 (17.8) 0.02 TC (mg/dl) (30.2) (38.3) (38.2) 0.39 HDL (mg/dl) 52.5 (12.7) 51.6 (13.7) 54.0 (15.2) 0.75 TC/HDL ratio 3.6 (1.1) 3.9 (1.0) 3.6 (1.0) 0.45 TG (g/dl) 2.0 (1.6) 1.9 (1.3) 1.7 (1.1) 0.65 TP (g/dl) 7.5 (0.5) 7.5 (0.5) 7.4 (0.6) 0.46 BMI: body mass index, Na: sodium, K: potassium, Mg: magnesium, TC: total cholesterol, HDL: high density lipoprotein cholesterol, TG: triglycerides, TP: total protein. with higher Na/K and TC/HDL ratios were at a significantly higher risk of hypertension. The relative risks (95%CI) of hypertension associated with low or intermediate-level education ( 10 years), overweight (BMI 25 kg/m 2 ), increased Na/K ratio and increased TC/HDL ratio were 2.56 ( ), 5.39 ( ), 1.30 ( ) and 1.73 ( ), respectively. Figure 1 shows that subjects with the highest educational level had the lowest prevalence of hypertension. Table 4 shows that subjects with the highest educational level had significantly lower Na levels, significantly lower Na/K ratios, and significantly higher Mg and K excretion levels. Discussion The present study showed that blood pressure was significantly associated with several diet-related factors, and that these factors, in turn, were strongly associated with the educational levels of participants. The results indicate that improvement in education may play an important role in controlling high blood pressure at the population level. Blood Pressure and Diet-Related Factors A number of studies, including the present one, have shown a positive association between BMI and BP. Perhaps the most interesting finding in the present study was that the study population had a lower mean BMI than populations in the West. This result indicates that an increase in BMI is one of the strongest determinants of high blood pressure, even in populations of relatively lean individuals. Secondly, as compared with our previous studies carried out in 10 Chinese cities (2, 3, 5), the mean BMI increased markedly, from 21.5 kg/m 2 in 1985 to 23.0 kg/m 2 in A similar trend of increasing BMI was reported in our comparison studies among population samples in Taiwan, Guangzhou and Guying (6). The present results emphasized again that an increasing mean BMI in the Chinese population should be of serious concern, since increased BMI is strongly associated with high blood pressure and other diseases, including coronary heart disease, diabetes, etc. (7, 8). High salt intake is associated with risks of high blood pressure, stroke and stomach cancer mortality rates. In this regard, the results of the present study are consistent with previous findings. A significantly positive association was observed between salt intake (as assessed by 24 h urinary sodium excretion) and blood pressure. Our previous findings in 25 population samples demonstrated a significantly positive association between salt intake and stroke mortality rates (7). Higher salt intake in the Chinese population is suggested to be one of the most important risk factors for their higher stroke mortality rate, as compared to populations in the West (7). The salt-bp association may indicate one possible mechanism by which higher salt intake increases risk of stroke. An increase in total cholesterol level in the Japanese population has been observed since the early 1980s (9). However, the trend of changes in total cholesterol levels in the Chinese is not very clear. Compared with our previous findings in the Chinese (using the same study design, but a different city) (5), the mean TC level increased from mg/dl in 1985 to mg/dl in An increase in TC level is suggested to be a marker of an increase in fat food intake, or a socalled Westernized lifestyle. Several studies, including the Framingham Study (8) and our own WHO-CARDIAC Study (7), have observed that increased total cholesterol is a risk factor for coronary heart disease. In terms of public health implications, the most important findings in the present study are that several unhealthy dietary habits were observed in subjects with lower educational levels. For example, subjects with lower education had a higher salt intake, higher sodium-to-potassium ratio, and lower K and Mg intakes. Most studies have confirmed the protective effects of increased K and Mg on lowering blood

5 Yamori et al: Diet, Education and BP in Chinese 563 pressure (10). An inverse association of BP with K and Mg was observed in the present study, although these associations did not reach the level of statistical significance. Taken together, the main findings of this study support the hypothesis that lifestyle is associated with risk of cardiovascular disease (11 15). Finally, three main limitations of the present study should be noted. First, the sample size was somewhat small. This may be one reason for the non-significant associations of BP with K and Mg, etc. Secondly, because this was a cross-sectional study, the findings cannot be used to establish a conclusive cause-and-effect relationship between dietary factors and BP. Thirdly, although we had originally intended to examine the association between oxidative DNA damage and BP, at this stage we are unable to report these results because some of the urine samples were not properly frozen during the long distance trip from Chongqing (in southeast China) to Kyoto, Japan. Fortunately, we did obtain data on dietary intake via questionnaire, and this data should yield further insights into the relation between food intake and BP including anti-oxidative food intake and BP in our subsequent reports. In conclusion, BP was strongly associated with BMI, salt intake and other diet-related factors in the present sample. Subjects with higher educational levels had a lower risk of HT and lower HT risk profiles. These results emphasize that education plays an important role in public health for controlling high BP in the Chinese population. Acknowledgements We express our sincere appreciation for the invaluable cooperation of all participants, and for the support of the Chongqing University of Medical Sciences (CQUMS) and Chongqing Jian She Hospital, Chongqing, China. Special thanks go to Prof. Yatian Qing (Director, Scientific Research Division of CQUMS) and Prof. Runhua Wang (Director, Faculty of Preventive Medicine, CQUMS). We also acknowledge the great contributions of Doctors Qin Miao, Xiaojun Tang, Zhaohui Zhong and Hong Xu (Department of Epidemiology, CQUMS), Changjun Yan and Jiaqin Yan (Department of Occupational Health and Laboratory Department of Chongqing Jian She Hospital, Chongqing, China) for their invaluable assistance in the field survey. We also would like to thank Prof. Dennis H. Sullivan (Executive Vice Chairman, Donald W. Reynolds Department of Geriatrics, University of Arkansas for Medical Sciences, Little Rock, USA) for his helpful suggestions for this manuscript. 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