Hypertension is a principal cause of mortality and morbidity in China(Reference He, Gu and Wu1), and is also one of the most important cardiovascular risk factors all over the world(Reference Roccella and Bowler2). A recent cross-sectional study in China estimated that 129 million people aged 35–74 years have hypertension(Reference Gu, Reynolds and Wu3). Although the exact causes and mechanisms of hypertension are not well known, it is generally believed that genetic factors(Reference Kamide, Kokubo and Yang4) as well as environmental factors, such as more sodium intake(Reference Ukoh, Ukoh, Okosun and Azubike5, Reference He, Markandu and Mac Gregor6), cigarette smoking(Reference Arquizan, Touze, Moulin, Woimant, Ducrocq and Mas7, Reference Sorensen, Kristensen, Bang, Svendsen, Wiinberg, Buttenschon and Talleruphuus8) and mental work(Reference Pickering9, Reference Steptoe and Cropley10), are involved in determining blood pressure levels and the prevalence of hypertension. In addition, the prevalence of hypertension in different races may also differ(Reference Ordunez, Munoz, Espinosa-Brito, Silva and Cooper11, Reference Rehman, Hutchison, Hendrix, Okonofua and Egan12).
There are fifty-six ethnic groups in China. Han is the largest ethnic group and Zhuang is the largest minority. There are also forty-three ethnic subgroups of the Zhuang. Hei Yi Zhuang dwelling in Napo County (a county near Vietnam) of Guangxi is an isolated subgroup. They worship black colour, and they like to wear black garments and pants. Black colour has become the marking of Hei Yi Zhuang. The population size is 51 655. Because of isolation from the other ethnic groups, the special customs and cultures including their clothing, intra-ethnic marriages, dietary habits and lifestyle are still totally conserved to the present day(Reference Li, Yin, Lin and Yang13). Little is known about the association of diet and lifestyle with blood pressure in this population. We hypothesise that there may be significant differences in the dietary patterns, lifestyle and blood pressure levels between the two ethnic groups. Therefore, the present study was undertaken to detect the association of diet and lifestyle with blood pressure and the prevalence of hypertension in the Guangxi Hei Yi Zhuang and Han populations.
Methods
Populations
A total of 1068 subjects of Hei Yi Zhuang from seven villages in Napo County, Guangxi Zhuang Autonomous Region, China, were surveyed by a stratified randomised cluster sampling. The ages of the subjects ranged from 20 to 84 years, with an average age of 46·75 (sd 15·66) years. There were 487 (45·6 %) males and 581 (54·4 %) females. All of them were peasants. The numbers of subjects between the ages of 20–29, 30–39, 40–49, 50–59, 60–69 and ≥70 years were 156 (14·6 %), 255 (23·8 %), 177 (16·6 %), 196 (18·4 %), 196 (18·4 %) and 88 (8·2 %), respectively. They accounted for 2·1 % of the total Hei Yi Zhuang population. During the same period, a total of 933 subjects of Han Chinese from nine villages in the same county were also surveyed by the same method. The mean age of the subjects was 45·46 (sd 15·76) years (range 20–89 years). There were 414 (44·4 %) men and 519 (55·6 %) women. These subjects were also peasants. The numbers of subjects between the ages of 20–29, 30–39, 40–49, 50–59, 60–69 and ≥70 years were 183 (19·6 %), 230 (24·7 %), 173 (18·5 %), 149 (16·0 %), 117 (12·5 %) and 81 (8·7 %), respectively. All study subjects were essentially healthy and had no evidence of any chronic illness, including hepatic, renal, thyroid, cardiac dysfunction or diabetes mellitus. Some subjects were treated with antihypertensive drugs, such as nifedipine (calcium channel blocker) and/or catopril (angiotensin converting enzyme inhibitor). The study was approved by our institutional ethics committee, and the study subjects gave informed consent to participate in the study.
Epidemiological survey
The survey was carried out using internationally standardised methods(14), following a common protocol. Information on demographics (age, gender, race/ethnicity and residential area), socio-economic status (education level achieved, marital status and annual household income), health-related behaviour (alcohol consumption, cigarette smoking and physical activity) and dietary intake was collected with standardised questionnaires. Dietary information of each subject was obtained using a 24h dietary recall method(Reference Lyu, Yeh, Lichtenstein, Li, Ordovas and Schaefer15). Detailed descriptions of all foods, beverages and supplements consumed during the 24h period before the interview, including the quantity, cooking method and brand names, were recorded by a chief physician. Quantitative estimate of cumulative nutrient intake per day in each food was based on food tables derived from the 2002 Chinese Food Composition Tables(Reference Yang, Wang and Pan16). Overall physical activity was ascertained with the use of a modified version of the Harvard Alumni Physical Activity Questionnaire(Reference Paffenbarger, Wing and Hyde17), which included questions about the number of hours per day (mean of a regular weekday and a regular weekend day) spent sleeping and in sedentary, light, moderate and vigorous activities; the interviewer ensured that the total time added up to 24 h. Current smoking was defined as more than one cigarette per day. Consumption of alcohol included questions about the number of liangs (about 50 g) of rice wine, wine, beer or liquor consumed during the preceding 12 months. The physical examination included blood pressure, height, weight and waist circumference. Blood pressure readings were taken with a standard mercury sphygmomanometer after at least 5 min of rest, while the subject was in a sitting position. The values used in the current analysis are means of three measurements taken by the same investigator at about 5min intervals. Systolic blood pressure (SBP) was determined by the first Korotkoff sound, and diastolic blood pressure (DBP) by the fifth Korotkoff sound. Pulse pressure (PP) was calculated as the difference between SBP and DBP. Mean arterial pressure (MAP) was calculated as DBP plus one-third times PP(Reference Zheng, Sun and Li18). Body weight (to the nearest 100 g on calibrated scales) and height (to the nearest 0·5 cm) were measured while the participants were wearing only light indoor clothing and no shoes. BMI was calculated as weight (kg) divided by height (m) squared. Waist circumference was measured with a non-stretchable measuring tape, at the level of the smallest area of the waist, to the nearest 0·1 cm.
Measurements of lipids and apolipoproteins
Blood samples were drawn from an antecubital vein in all subjects after an overnight fast. The blood was transferred into glass tubes and allowed to clot at room temperature. Immediately following clotting, serum was separated by centrifugation for 15 min at 3000 rpm. The levels of total cholesterol, TAG, HDL cholesterol (HDL-C) and LDL cholesterol (LDL-C) in samples were determined enzymatically using commercially available kits: Tcho-1, TG-LH (RANDOX Laboratories Ltd, Crumlin, UK), Cholestest N HDL and Cholestest LDL (Daiichi Pure Chemicals Co., Ltd, Tokyo, Japan), respectively. Serum apo A1 and apo B levels were measured by an immunoturbidimetric assay (RANDOX Laboratories Ltd). All determinations were performed with an autoanalyser (Type 7170A; Hitachi Ltd, Tokyo, Japan) in the Clinical Science Experiment Center of the First Affiliated Hospital, Guangxi Medical University.
Diagnostic criteria
Hypertension was defined as an average SBP of 140 mmHg or greater and/or an average DBP of 90 mmHg or greater(Reference Hansson, Hedner and Himmelmann19), and/or self-reported pharmacological treatment for hypertension within the 2 weeks prior to the interview. The subjects with only SBP ≥ 140 mmHg and DBP < 90 mmHg were defined as isolated systolic hypertension. Awareness of hypertension was defined as a self-report of any prior diagnosis of hypertension by a health-care professional. Treatment of hypertension was defined as a self-reported use of pharmacological medication for the management of high blood pressure within the 2 weeks preceding the participant’s interview. Control of hypertension was defined as having an average SBP <140 mmHg and an average DBP <90 mmHg in the context of pharmacological treatment of hypertension. The normal values of serum total cholesterol, TAG, HDL-C, LDL-C, apo A1, apo B and the ratio of apo A1 to apo B in our Clinical Science Experiment Center were 3·10–5·17, 0·56–1·70, 0·91–1·81, 1·70–3·20 mmol/l, 1·00–1·76, 0·63–1·14 g/l and 1·00–2·50, respectively. Normal weight, overweight and obesity were defined as a BMI < 24, 24–28 and >28 kg/m2, respectively(20).
Statistical analysis
The data were organised and analysed using EXCEL XP (Microsoft, Seattle, WA, USA) and SPSS for Windows version 11·5 (SPSS Inc., Chicago, IL, USA). Means and standard deviations (sd) as well as frequency distributions of participant characteristics were calculated. The difference of two parameters between Hei Yi Zhuang and Han was tested by Student’s unpaired t-test. One-way ANOVA was performed to assess the differences of three and more parameters. Significant difference was then subjected to multiple comparison using the Newman–Keuls test. The difference of percentage was tested by the χ 2 tests. In order to evaluate the association between the MAP and ethnic group (Han = 0; Hei Yi Zhuang = 1), gender (female = 0; male = 1), age (year), education level (years), physical activity (h/week), BMI (kg/m2 = 1), waist circumference (cm), total cholesterol (mmol/l), TAG (mmol/l), LDL-C (mmol/l), apo B (g/l), alcohol consumption (non-drinkers = 0; <25 g/d = 1; 25–49 g/d = 2; 50–99 g/d = 3; ≥100 g/d = 4), cigarette smoking (non-smokers = 0; <10 cigarettes/d = 1; 10–19 cigarettes/d = 2; 20–39 cigarettes/d = 3; ≥40 cigarettes/d = 4) and the intakes of total energy (kJ/d), total fat (g/d), dietary cholesterol (mg/d), total dietary fibre (g/d) and salt intake (g/d), unconditional logistic regression analysis were also performed in combined population of Hei Yi Zhuang and Han, Hei Yi Zhuang, and Han, respectively. The backward multiple logistic regression method was used to select the risk factors significantly associated with MAP. The total intake of each nutrient was summed over all foods consumed. Matlab5.0 software was used for the quantitative estimate of cumulative nutrient intake per day(Reference von Kamp and Schuster21). A P value of <0·05 was considered significant.
Results
General characteristics between Hei Yi Zhuang and Han
The demographic characteristics, dietary intake and other lifestyle factors between Hei Yi Zhuang and Han are shown in Table 1. The intakes of carbohydrate, total dietary fibre and salt, mean residential altitude, and the percentages of subjects who consumed alcohol in Hei Yi Zhuang were higher than those in Han (P < 0·05−0·001), whereas the education level, body weight, BMI, waist circumference, and the intakes of total energy, total fat, protein and dietary cholesterol in Han were higher than those in Hei Yi Zhuang (P < 0·001 for all). There were no differences in physical activity level, body height, the percentages of subjects who smoked cigarettes and sex distribution between the two ethnic groups (P > 0·05).
Table 1 General characteristics and blood pressure levels between the Hei Yi Zhuang and Han populations
SBP, systolic blood pressure; DBP, diastolic blood pressure; PP, pulse pressure.
Data are presented as means and standard deviations or ‡as numbers of subjects and percentages.
Blood pressure levels and prevalence of hypertension
As shown in Table 1, the levels of SBP and PP in Hei Yi Zhuang were higher than those in Han (P < 0·001 for each), but there was no difference in DBP levels between Hei Yi Zhuang and Han (P > 0·05). The crude prevalence rates of hypertension and isolated systolic hypertension were also higher in Hei Yi Zhuang than in Han (25·19 % v. 17·26 % and 12·45 % v. 3·86 %, P < 0·001 for each, respectively).
Awareness, treatment and control of hypertension
Of the 269 subjects with hypertension in Hei Yi Zhuang, 8·55 % were aware of their high blood pressure, 4·46 % were treated and 1·86 % were controlled, whereas the rates of awareness, treatment and control in Han were 21·12 % (P < 0·001), 15·53 % (P < 0·001) and 10·56 % (P < 0·001), respectively (Table 1).
Association of demographic and lifestyle factors with blood pressure
The association of sex, age, BMI, alcohol consumption and cigarette smoking with blood pressure levels between Hei Yi Zhuang and Han is shown in Table 2. SBP, DBP and MAP in both Hei Yi Zhuang and Han were higher in men than in women (P < 0·001 for all), but there was no significant difference in PP between both sexes. SBP, DBP, PP and MAP increased progressively with age in both ethnic groups (P < 0·01–0·001). SBP levels in both ethnic groups were higher in drinkers than in non-drinkers (P < 0·01 for each). The levels of DBP and MAP in both ethnic groups were higher in drinkers than in non-drinkers, or in smokers than in non-smokers (P < 0·001 for all). In addition, the levels of DBP and MAP in Han were higher in drinkers than in non-drinkers (P < 0·01).
Table 2 Association of demographic characteristics and lifestyle factors with blood pressure levels between the Hei Yi Zhuang and Han populations
SBP, systolic blood pressure; DBP, diastolic blood pressure; PP, pulse pressure; MAP, mean arterial pressure.
Mean values were significantly different from those of male, BMI ≤ 24 kg/m2, non-drinker or non-smoker subgroups of the same ethnic group: *P < 0·05, **P < 0·01, ***P < 0·001.
Mean values were significantly different from those of Hei Yi Zhuang: †P < 0·05, ††P < 0·01, †††P < 0·001.
Association of demographic and lifestyle factors with the prevalence of hypertension
The association of demographic and lifestyle factors with the prevalence of hypertension is shown in Table 3. The prevalence of hypertension in both ethnic groups was higher in males than in females, or in smokers than in non-smokers (P < 0·01 for all). In addition, the prevalence of hypertension in Han was higher in drinkers than in non-drinkers (P < 0·01).
Table 3 Association of demographic characteristics and lifestyle factors with the prevalence of hypertension between the Hei Yi Zhuang and Han populations
SBP, systolic blood pressure; DBP, diastolic blood pressure.
Number of subjects was significantly different from those of male, BMI ≤ 24 kg/m2, non-drinker or non-smoker subgroups of the same ethnic group: *P < 0·05, **P < 0·01, ***P < 0·001.
Number of subjects was significantly different from those of Hei Yi Zhuang: †P < 0·05, ††P < 0·01, †††P < 0·001.
Correlative factors for mean arterial pressure
Table 4 gives the results of multivariate logistic regression analysis. MAP was positively correlated with Hei Yi Zhuang, male, age, physical activity, BMI, waist circumference, total cholesterol, TAG, alcohol consumption, cigarette smoking, and total energy, total fat and salt intakes, and negatively associated with education level and total dietary fibre intake in a combined population of Hei Yi Zhuang and Han (P < 0·05–0·001). MAP was positively correlated with male, age, physical activity, TAG, alcohol consumption, cigarette smoking, and total energy, total fat and salt intakes, and negatively associated with education level and total dietary fibre intake in Hei Yi Zhuang (P < 0·05–0·001), whereas it was positively associated with male, age, physical activity, BMI, waist circumference, total cholesterol, alcohol consumption, cigarette smoking, and total energy, total fat and salt intakes, and negatively associated with education level and total dietary fibre intake in Han (P < 0·05–0·001). No association was observed between MAP and LDL-C, apo B or dietary cholesterol intake (P > 0·05).
Table 4 Correlative factors for mean arterial pressure between the Hei Yi Zhuang and Han populations
Discussion
In the present study, we show that SBP and PP levels are higher in Hei Yi Zhuang than in Han. The prevalence of hypertension and isolated systolic hypertension is also higher in Hei Yi Zhuang than in Han. The disparity in blood pressure and the prevalence of hypertension between the two ethnic groups is multifactorial. The vast majority of Hei Yi Zhuang people reside in the mountainous areas. The mean residential altitude is higher than that in Han. It has been found that as the altitude increases gradually, the reference value of whole blood contrast viscosity also increases gradually, and the correlation is quite obvious(Reference Pei, Zhu, Li, Sui and Gou22, Reference Palareti, Coccheri, Poggi, Tricarico, Magelli and Cavazzuti23). A recent study has shown that chronic hypoxia causes marked activation of the sympathetic nervous system in healthy humans and increased SBP, despite normalisation of the arterial O2 content with acclimatisation(Reference Calbet24). Increased secretion of catecholamine(Reference Calbet24, Reference Ohkuwa, Itoh, Yamamoto, Minami and Yamazaki25), vasopressin(Reference Maresh, Kraemer, Judelson, VanHeest, Trad, Kulikowich, Goetz, Cymerman and Hamilton26) and adrenocorticotropin(Reference Raff, Shinsako, Keil and Dallman27) may promote the development of hypertension by the renin–angiotensin–aldosterone system(Reference Porto, Garcia, Dieuzeide, Gonzalez and Pirola28).
Corn gruel or hoecake is the staple food throughout the year in Hei Yi Zhuang. There are three meals a day on an ordinary day, and four meals on a farming day. Salt is the main flavouring. The intake of salt is very high. Long-term high salt intake is an important risk factor for hypertension(Reference Ukoh, Ukoh, Okosun and Azubike5). Several large randomised trials and meta-analyses of trials have shown that a reduction in salt intake lowers blood pressure in hypertensive individuals(Reference He, Markandu and Mac Gregor6, Reference Whelton, Appel and Espeland29).
The climate in Napo County is like the south subtropics. Evaporation is quick, and the weather is very dry and cold in winter at the mountain top. Cold exposure has been shown to be a risk factor for hypertension(Reference Kim, Jung, Hong, Kim, Jang and Kim30). SBP and DBP were significantly higher in the cold-exposed workers. In logistic regression analysis, cold exposure severity was a significant variable affecting hypertension in cold-exposed workers(Reference Kim, Jung, Hong, Kim, Jang and Kim30).
Hei Yi Zhuang people reside in the infertile mountain area, which is 30% soil with 70% rock. The earnings derive mostly from planting corn and paddy. It is not easy for adolescents to go to junior high school. Thus, the education level is widespread lower, and the knowledge of necessary health care is very poor(Reference Ordunez, Munoz, Espinosa-Brito, Silva and Cooper11, Reference Oliveria, Chen, McCarthy, Davis and Hill31).
The association of sex and age with blood pressure has been extensively studied. Currently, it is believed that the prevalence of hypertension is higher in males than in females, and both blood pressure and the prevalence of hypertension increase with age(Reference Carroll, Ring, Hunt, Ford and Macintyre32, Reference Takizawa, Ura and Saitoh33). The present study also shows that SBP and DBP are higher in men than in women in both ethnic groups. SBP, DBP, PP and MAP in both ethnic groups are all increased progressively with age. MAP is also positively correlated with male gender and age in both populations. This may relate to the overwork and unwholesome lifestyle in men(Reference Xu, Tang and Jin34). Job stress has been shown in prospective studies to have adverse effects on the development of hypertension and coronary artery disease, particularly in men(Reference Pickering9). SBP and DBP during the working day were greater in high job demand participants who were stress reactive than in other groups(Reference Steptoe and Cropley10).
Epidemiological studies have revealed that the prevalence of hypertension was 2- to 6-fold higher in obese than in normal weight populations(Reference Droyvold, Midthjell, Nilsen and Holmen35, Reference Biscevic36). An increase in BMI and a decrease in BMI were significantly associated with increased and decreased SBP and DBP, respectively(Reference Droyvold, Midthjell, Nilsen and Holmen35). In the present study, however, we fail to show an association of BMI and blood pressure in Hei Yi Zhuang. The reason for this discrepancy is unclear. In this study, the subjects with a BMI > 24 kg/m2 were only 11·52 % in Hei Yi Zhuang, suggesting that the association of BMI and blood pressure is weak in this population. In a recent investigation, Biscevic(Reference Biscevic36) has shown that there was a higher correlation of blood pressure with waist circumference, than with BMI. In the present study, we also fail to show the association of MAP and waist circumference in Hei Yi Zhuang. However, our study shows that MAP was positively correlated with TAG in Hei Yi Zhuang, and was positively correlated with total cholesterol in Han(Reference Tran, Komatsu, Nguyen, Nguyen, Yoshimura, Takahashi, Wariishi, Sakai and Yamamoto37). The relationship between hypertension and hyperlipidaemia remains poorly understood. High blood pressure in patients with hyperlipidaemia may be a consequence of vascular endothelial dysfunction(Reference Xu, Tang and Jin34).
Alcohol in larger amounts increases blood pressure and overall mortality(Reference Arquizan, Touze, Moulin, Woimant, Ducrocq and Mas7, Reference Sorensen, Kristensen, Bang, Svendsen, Wiinberg, Buttenschon and Talleruphuus8, Reference Marmot, Elliott, Shipley, Dyer, Ueshima, Beevers, Stamler, Kesteloot, Rose and Stamler38). The mechanisms involved in the pressor effect of a moderate dose of alcohol primarily reflects an increase in cardiac output and heart rate, possibly a consequence of increased sympathetic nerve activity(Reference Grassi, Somers, Renk, Abboud and Mark39). Alcohol also alters cell membranes, allowing more calcium to enter, perhaps by inhibition of intracellular sodium transport(Reference Kojima, Kawano, Abe, Sanai, Yoshida, Imanishi, Ashida, Kimura, Yoshimi and Matsuoka40). In the present study, we also show that SBP, DBP and MAP levels in both ethnic groups were higher in drinkers than in non-drinkers. MAP was also positively correlated with alcohol consumption in both ethnic groups. These results suggest that alcohol consumption is a main risk factor for hypertension in both populations. The effect of different kinds of wine on blood pressure is not well known. In this study, 90 % of the wine consumed by Hei Yi Zhuang was corn wine and rum, in which the alcohol content is lower. On the contrary, a great deal of the wine consumed by Han is rice wine, in which the alcohol content is higher.
Cigarette smoking is a well-established cardiovascular risk factor. Cigarette smoking raises blood pressure, probably through the nicotine-induced release of noradrenaline from adrenergic nerve endings. In addition, smoking causes an acute and marked reduction in radial artery compliance(Reference Giannattasio, Mangoni, Stella, Carugo, Grassi and Mancia41). When smokers quit, a trivial rise in blood pressure may occur, probably reflecting a gain in weight(Reference Terres, Becker and Rosenberg42). Waist girth increased more in men who quit smoking than in other men. An increase in waist girth during follow-up strongly predicted incident hypertension(Reference Niskanen, Laaksonen, Nyyssonen, Punnonen, Valkonen, Fuentes, Tuomainen, Salonen and Salonen43). In our study, we show that DBP and MAP levels in both ethnic groups were higher in smokers than in non-smokers. MAP was also positively correlated with cigarette smoking in both ethnic groups, suggesting that cigarette smoking is another independent risk factor for hypertension in these populations.
The rates of awareness, treatment and control are important indices of hypertensive treatment and prognosis. The rates of awareness, treatment and control in our country were 44·7 %, 28·2 % and 8·1 %, respectively, in subjects aged 35–74 years in 2001(Reference Gu, Reynolds and Wu3, Reference Li, Jiang and Ma44). The results of the present study show that the rates of awareness, treatment and control in Hei Yi Zhuang were significantly lower than those in Han, and were also significantly lower than those in the rural population of China. The reason for this status is likely multifactorial; however, a lack of public awareness and understanding of hypertension and its complications may contribute to the epidemic of uncontrolled hypertension in this population. Increasing public awareness of hypertension using public education and health provider strategies should be a high national health priority.
In conclusion, hypertension is found to be highly prevalent in the Hei Yi Zhuang population, and the rates of awareness, treatment and control are relatively lower. These results underscore the urgent need for developing a high blood pressure education programme to coordinate the effort of detection, prevention and treatment of hypertension in the national minority area of China.
Acknowledgements
Conflict of interest:No conflicts.
Funding support:This study was supported by the National Natural Science Foundation of China (No. 30360038).
Author contributions:Y.R. conceived the study, participated in the design, carried out the epidemiological survey, collected the samples and drafted the manuscript; L.H. performed the statistical analyses and helped to draft the manuscript; W.J., L.W. and Y.D. carried out the epidemiological survey, collected the samples and measured the lipids; P.S., H.J. and L.X. carried out the epidemiological survey and collected the samples. All authors read and approved the final manuscript.
