Higher exercise frequency associated with lower blood pressure in Hong Kong adolescents: a population-based study

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1 (2010), 1 6 & 2010 Macmillan Publishers Limited All rights reserved /10 $ ORIGINAL ARTICLE Higher exercise frequency associated with lower blood pressure in Hong Kong adolescents: a population-based study HK So 1, RYT Sung 1,AMLi 1, KC Choi 2, EAS Nelson 1,JYin 1,PCNg 1 and TF Fok 1 1 Department of Paediatrics, The Chinese University of Hong Kong, Hong Kong Special Administrative Region, PR China and 2 Centre for Epidemiology and Biostatistics, The Chinese University of Hong Kong, Hong Kong Special Administrative Region, PR China The aim of this study was to determine the association between blood pressure and the frequency of structured physical training activity in Chinese adolescents. A total of 9558 students aged years underwent anthropometric and blood pressure measurements in a crosssectional growth study. Structured physical training activity was assessed by two simple self-administered questions and parents were asked to complete a questionnaire providing demographic information. Ninety per cent of eligible students participated in the study, of which 94% provided data on physical training frequency for final analysis. Of the boys, 22.6% and of the girls, 14.5% were physically active with extracurricular school exercise at least twice a. Over half of the students did not regularly exercise except during physical education classes at school. Blood pressure had a positive correlation with body mass index (BMI). Both systolic blood pressure and diastolic blood pressure were substantially decreased with increased training frequency (Po0.05). Logistic regression adjusting for age, family history of hypertension, BMI and sleep duration showed that exercising two or more times a had a negative relation with hypertension (odds ratio: 0.63, 95%CI ). In conclusion, structured physical training activity of two or more times a has a beneficial effect on blood pressure in Hong Kong children aged years. advance online publication, 21 January 2010; doi: /jhh Keywords: exercise frequency; blood pressure; adolescents Introduction Hypertension is one of the leading causes of morbidities and mortalities worldwide. It is known that blood pressure (BP) tracks from childhood to adulthood and end organ damages may start in adolescence. 1 5 Therefore, early prevention of hypertension is necessary. Regular exercise has been recommended for preventing and treating high BP in both adults and children. 6 8 The proposed mechanisms for the BP lowering effects of exercise include neurohumoral, vascular and structural adaptations. 9 Decreases in catecholamines and total peripheral resistance, increases in insulin sensitivity and alterations in vasodilators and vasoconstrictors are some of the postulated explanations for the antihypertensive effects of exercise. 10 Previous studies showed that at least 30 min of physical Correspondence: Dr RYT Sung, Department of Paediatrics, The Chinese University of Hong Kong, 6/F Clinical Science Building, Prince of Wales Hospital, Shatin, Hong Kong 852, PR China. yntzsung@cuhk.edu.hk Received 8 July 2009; revised 9 December 2009; accepted 21 December 2009 activities at a frequency of three times per and intensity of 80% of maximal heart rate is required to lower BP. 11 Current CDC guidelines and other experts recommend that children and adolescents should do at least 60 min of physical exercise each day. 11 Although some studies reported that BP is lower in those who participate in physical activity, the results from other studies are inconsistent The relationship of structured physical training activity frequency to BP is even less well documented. 15 The aim of this study was to determine the association between BP and the frequency of structured physical training activity in Chinese adolescents. Methods The study was part of a Hong Kong Growth Survey that aimed to explore the growth pattern characteristics in children and adolescents and to monitor secular trends. Details of this study have been previously described. 19

2 2 Association between BP and exercise frequency Ethics The study was approved by the Joint Chinese University of Hong Kong and New Territories East Cluster Clinical Research Ethics Committee and the Ethics Committee of the Department of Health of the Hong Kong Government. Subjects A list of all schools in Hong Kong was compiled from data held by the Department of Education. One secondary school from each of the 18 districts in Hong Kong was randomly selected and invited to participate in the study. From the selected schools, two classes in each grade were selected in collaboration with the school principal based on timetables and operational needs. All students of the selected classes were invited to join the study. The main reason for nonparticipation was absence (637 students, 6%) from school, 425 students (4%) refused to participate and the reasons were not recorded. In addition to physical examination, the study included a self-administered questionnaire for completion by the students and this was subsequently checked by research staff. A fact sheet explaining the purpose and procedure was given to each student and their parents before measurements were taken at the school. The parents of all participants were invited to complete a questionnaire providing demographic information including family or personal medical history. activity guidelines for adolescents issued from the international consensus conference in An explanation of these two questions with a detailed list of usual exercise training classes in Hong Kong was provided to all participants. Hong Kong schools generally allocate only two 45-min lessons per to PE. 22 Exercise during the PE classes was regarded as a constant within the study sample and therefore not taken into account in this analysis. Prehypertension was defined as systolic blood pressure (SBP) and/or diastolic blood pressure (DBP) greater than or equal to sex-, age- and height-specific 90th percentile or 120 mm Hg (for SBP)/80 mm Hg (for DBP). Hypertension was identified if SBP and/or DBP was greater than or equal to sex-, age- and height-specific 95th percentile or 140 mm Hg (for SBP)/90 mm Hg (for DBP). Statistical analysis Data were presented as mean (s.d.) or frequency (%) as appropriate. Association between frequency of exercise training and BP and hypertension with adjustment for age and potential confounders (body mass index (BMI), family history of hypertension and sleep duration) was assessed using multiple linear regression and logistic regression. All the statistical analyses were performed using SPSS 16.0 (SPSS Inc., Chicago, IL, USA). All statistical tests were two-sided and P-value o0.05 was considered statistically significant. Measurement of anthropometric parameters A team of eight trained research staff visited the selected schools on a prearranged date to collect the anthropometric data. Standing height without shoes was measured using a Harpenden Stadiometer to the nearest 0.1 cm. Body weight and percentage body fat were measured using a portable Tanita body fat monitor/scale (Model BF-522; four contact electrodes with two on each foot). BP and pulse rate were measured oscillometrically using the Datascope Accutorr Plus. BP and hazard ratio were measured at school in the morning, on the right arm, with the children seated and rested for at least 5 min and with the arm supported at heart level. Appropriate sized cuffs were used (bladder width at least 40% of arm circumference, length % of arm circumference). 20 Two BP measurements were taken with a 1-min interval. The average of the two readings was used for analysis. Waist circumference was measured midway between the lowest rib and the superior border of the iliac crest with an inelastic measuring tape to the nearest 0.1 cm. Physical activity was assessed by two questions: (1) Do you currently participate in any regular exercise class other than school physical education (PE) class? (2) If yes, please tick the frequency of training: 1, 2 or 3 and above per. These two questions were designed based on the physical Results Anthropometric measurements and BP were taken from 9558 students aged years (boys ¼ 4722 and girls ¼ 4836). The participation rate was 90% of the study population. Among the participants, 8981 (94%) students (boys ¼ 4430 and girls ¼ 4551) provided data of structured physical training frequency for final analysis. The main reason for nonreporting was that students did not want to reveal any personal information. There were no significant differences in the BP of the children who did not provide valid data on exercise compared with those who did (P ¼ 0.2 for SBP, P ¼ 0.7 for DBP). Of the boys, 10.7% and of the girls, 5.3% were physically active, exercising outside of school at least three times a. Over half of the students did not do any exercise other than PE classes at school. The details of the potential confounders associated with BP are listed in Table 1 according to sex. The comparisons of BP by physical training frequency are shown in Table 2. The rate of hypertension according to age, sex and height was calculated using local references, 23 and was substantially decreased with increased frequency of exercise training (Table 3). BMI was found to be positively associated with hypertension in boys and girls. The logistic regression adjusted for age, family history of

3 Association between BP and exercise frequency Table 1 Characteristics of 8981 Hong Kong Chinese adolescents aged years by sex hypertension, BMI and sleep duration showed that exercise more or equal to two times a had a negative relation with hypertension (odds ratio: 0.63, 95%CI ). Discussion Boy (n ¼ 4430) Girl (n ¼ 4551) Age (years) 14.8 (2.2) 14.9 (2.2) Weight (kg) 54.4 (13.1) 48.2 (9.6) Height (cm) (10.7) (6.5) Waist circumference (cm) 66.8 (8.6) 62.4 (6.6) BMI (kg m 2 ) 20.1 (3.6) 19.6 (3.3) Pulse rate (beats per min) 81.0 (14.0) 85.6 (13.8) Average sleep duration (hours) 8.3 (1.1) 8.2 (1.1) Family history of hypertension c 395 (9.5%) 429 (10.0%) Systolic blood pressure (mm Hg) (12.6) (10.9) Diastolic blood pressure (mm Hg) 67.4 (9.1) 66.8 (8.2) Blood pressure c Normotensive 2524 (57.0%) 3327 (73.1%) Prehypertensive 1488 (33.6%) 866 (19.0%) Hypertensive 418 (9.4%) 357 (7.8%) Frequency of exercise training c None 2601 (58.7%) 3182 (69.9%) Once per 825 (18.6%) 709 (15.6%) Twice per 529 (11.9%) 419 (9.2%) X3 times per 475 (10.7%) 240 (5.3%) Abbreviation: BMI, body mass index. Data were presented as mean (s.d.), except those marked c were frequency (%). Our study showed that in a large representative sample of Hong Kong adolescents, those who had structured physical training activities of two or more times a had significantly lower prevalence of hypertension. Although it is generally believed that physical exercise is beneficial to the body, 22 our finding provides further evidence regarding the relation between frequency of exercise and BP and can be useful for promoting structured exercise in youths. Previous data from a local survey has indicated that Hong Kong children are in general physically less active than their counterparts in the West. 24 This is confirmed by this study that showed over half of the children did not engage in any structured exercise apart from the two 45-min sessions of PE per as part of the standard school curriculum. Low levels of physical activity have been linked to high BMI and elevated BP in childhood and this effect is believed to extend into adulthood. 1 3,13,18,25,26 Childhood and adolescent obesity is a significant problem in Hong Kong. Using the International Obesity Task Force cut-offs, we observed that in our study 16.7% of children were overweight or obese, which was a Table 2 Comparisons of blood pressure by frequency of exercise training Frequency of exercise training (boy, n ¼ 4430) Frequency of exercise training (girl, n ¼ 4551) P-value X3 times per (n ¼ 240) Twice per (n ¼ 419) Once per (n ¼ 709) P-value None (n ¼ 3183) X3 times per (n ¼ 475) Twice per (n ¼ 529) Once per (n ¼ 825) None (n ¼ 2601) (12.6) (12.8) (11.6) (12.0) o0.001 a (10.9) (10.8) (10.5) (9.7) o0.001 a Systolic blood pressure (mm Hg) Diastolic blood 68.2 (9.0) 66.4 (9.3) 66.3 (8.9) 66.0 (9.1) o0.001 a 67.4 (8.2) 66.2 (7.9) 64.6 (8.1) 64.9 (8.0) o0.001 a pressure (mm Hg) Hypertension b 257 (9.9%) 93 (11.3%) 32 (6.0%) 36 (7.6%) c 270 (8.5%) 54 (7.6%) 24 (5.7%) 9 (3.8%) c a One-way ANOVA. b Frequency (%). c Pearson w 2 -test. 3

4 Association between BP and exercise frequency 4 Table 3 Association between frequency of exercise training and blood pressure with adjustment for age, family history of hypertension, obesity index and sleep duration Systolic blood pressure (mm Hg) a Diastolic blood pressure (mm Hg) a Hypertension b Coefficient 95% CI P-value Coefficient 95% CI P-value Odds ratio 95% CI P-value Sex Femala (Ref) Male 4.43 (3.96, 4.90) o (0.26, 0.98) (1.00, 1.37) Age 1.15 (1.03, 1.27) o (0.66, 0.84) o (0.97, 1.05) BMI (kg m 2 ) 1.00 (0.93, 1.07) o (0.36, 0.47) o (1.13, 1.17) o0.001 Average sleep duration (hours) 0.20 ( 0.03, 0.42) ( 0.19, 0.16) (0.96, 1.12) Family history of hypertension No (Ref) Yes 0.93 (0.15, 1.71) (0.23, 1.43) (0.90, 1.46) Frequency of exercise training None (Ref) Once per 0.68 ( 1.33, 0.04) ( 1.15, 0.16) (0.84, 1.27) Twice per 1.45 ( 2.21, 0.68) o ( 1.92, 0.75) o (0.47, 0.85) X3 times per 1.53 ( 2.40, 0.66) ( 2.08, 0.74) o (0.45, 0.86) Abbreviation: BMI, body mass index; Ref, reference group of categorical variable. a Association between frequency of exercise training and blood pressure with adjustment for sex, age, BMI, average sleep duration and family history of hypertension by multiple regression. b Association between frequency of exercise training and hypertension with adjustment for sex, age, BMI, average sleep duration and family history of hypertension by logistic regression. 5.1% increase since It is likely that the escalating number of obese children and adolescents will lead to an increase in the prevalence of hypertension and other cardiovascular diseases in the adult population in the future. 27 Although increasing the participation of adolescents in structured physical activity may not of itself prevent the obesity epidemic, it is likely to make a beneficial contribution. Unfortunately, the demanding lifestyle of young people in Hong Kong, including an overemphasis on academic achievement, may be at the expense of their health and well being. With our data, we hope to persuade educators and parents that even modest increases in physical activity have value and may contribute to developing physical activity patterns that can be sustained into adulthood. Encouraging a lifestyle characterized by regular physical activity gives youngsters the opportunity to reach their full physical potential Concepts and behaviour in adolescent health are not well developed and this age group is one of the most suitable target populations to promote health literacy. 12 If they encounter appropriate concepts of health and health-related behaviour, they are more likely to learn and internalize these concepts about health throughout their teenage and adult lives. This study has a number of limitations. The simple questions used to assess physical activity sessions may be viewed as too crude. Physical activity is a broadly used term, and its heterogeneous nature makes it extremely difficult to characterize and quantify. Some studies try to overcome this problem using mechanical monitors for physical activity assessment and for further demonstration of the dose response effect However, the problem facing development of rigorous techniques for measuring physical activity is the lack of an ideal standard with which to validate the data, thus making it difficult to truly validate any given technique. Therefore, most of the physical activity studies have been based on questionnaires that have the ability to gather activity information in a short time at low cost. 31 The questions used in our study were similar to what have been used before. 13,32 They were not designed to precisely assess physical activity, but rather to show the simple relation between frequency of exercise and rate of hypertension from a public health perspective. Another limitation is nonparticipation of subjects and missing data. There were no differences, however, in the characteristics between those in the current analysis and those who did not provide valid data on physical activity. Findings were also similar if the analyses were restricted to those with complete data on all confounders. In conclusion, this study of a large representative sample of Hong Kong adolescents aged showed a negative association between the frequency of structured physical training activity sessions and BP after adjustment for potential confounding factors. Hong Kong students generally have low levels of physical activity, and most students did not participate in any regular structured exercise sessions apart from their PE lessons.

5 Our study indicates the potential for providing other exercise options within the Hong Kong school system such as increasing the frequency of PE classes to four sessions per or introducing more extracurricular sports activities. What is known about topic K Regular exercise has been recommended for preventing and treating high blood pressure in children. K Results of exercise on blood pressure from previous studies are inconsistent. What this study adds K First to determine the association between blood pressure and the frequency of structured physical training activity in Chinese adolescents. K Structured physical training activity has a beneficial effect on blood pressure in children aged years. Conflict of interest The authors declare no conflict of interest. Acknowledgements We thank the school principals, teachers, parents and students who participated in the study. This research project received financial support from Departmental funds and the Hong Kong Paediatric Society. Contributors: HK coordinated the study, assisted in the supervision of data collection and drafted the article. EAS, RYT and AM prepared the proposal and supervised the study. KC took active part in the statistical work. J undertook recruitment and clinical assessments. PC and TF made substantial contributions to the conception of the study and revising the paper. All authors contributed to the paper s submitted form and have read and approved the final paper. References 1 Berenson GS, Srinivasan SR, Bao W, Newman WP, Tracy RE, Wattigney WA. Association between multiple cardiovascular risk factors and atherosclerosis in children and young adults. The Bogalusa Heart Study. N Engl J Med 1998; 338: Bouziotas C, Koutedakis Y, Nevill A, Ageli E, Tsigilis N, Nikolaou A et al. Greek adolescents, fitness, fatness, fat intake, activity, and coronary heart disease risk. Arch Dis Childhood 2004; 89: Lauer RM, Clarke WR. Childhood risk factors for high adult blood pressure: the Muscatine Study. 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