Physical Activity and Sedentary Behavior s Association With Body Weight in Korean Adolescents

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1 International Journal of Sport Nutrition and Exercise Metabolism, 2013, 23, Human Kinetics, Inc. ORIGINAL RESEARCH Physical Activity and Sedentary Behavior s Association With Body Weight in Korean Adolescents Wi-Young So The purpose of this study was to examine whether physical activity (PA) and sedentary behavior were related to weight status in Korean adolescents. A total of 72,399 students in Grades 7 12 participated in the 5th Korea Youth Risk Behavior Web-Based Survey (KYRBWS-V) project in Body mass index, PA, and sedentary behavior were assessed by the KYRBWS-V. The study results show that boys had higher PA and less sedentary behavior than did girls (p <.01). The amounts of vigorous PA (p <.001), moderate PA (p <.001), moderate PA for at least 60 min (p <.001), exercise for muscle strength (p <.001), walking on a weekday (p <.001), and walking on a weekend (p =.012) were less in boys who were more obese than the other boys. The time spent sitting on a weekday (p <.001) and weekend (p <.001) was higher in more obese individuals in boys. Exercise for muscle strength (p =.011) was less in girls who were more obese. Time spent sitting on a weekday (p =.005) and weekend (p <.001) was higher in more obese individuals in girls. However, vigorous PA (p <.001), moderate PA (p <.001), moderate PA for at least 60 min (p =.003), and walking on a weekday (p <.001) were higher for the more obese girls. PA and sedentary behavior could be independent factors that reduce or prevent obesity in Korean adolescents. Keywords: Korea Youth Risk Behavior Web-Based Survey, weight status Obesity and excess weight are becoming serious problems worldwide. According to the World Health Organization (WHO), more than 1.4 billion adults age 20 years and older are already overweight, and more than 200 million men and nearly 300 million women are obese. Moreover, more than 40 million children worldwide less than 5 years old were overweight in 2010 (World Health Organization, 2011b). Furthermore, obesity in adolescents is becoming increasingly common (Foti & Lowry, 2010), whereas physical activity in adolescents is declining (Adams, 2006). Obesity is the result of drinking, physical inactivity, sedentary behavior, and unhealthy eating habits. It is a lifestyle disease and can be prevented by increasing physical activity, decreasing sedentary behavior, and controlling dietary intake, which can be useful strategies in terms of cost effectiveness (Melin & Rössner, 2003). Adolescence is the formative period for learned health The author is with the Dept. of Human Movement Science, Seoul Women s University, Seoul, Korea. behaviors, and these health behaviors can be continued for a lifetime (Telama et al., 2005). According to some reports, in 80% of cases, obesity in adolescence can be linked to adult obesity. Therefore, adolescence is one of the most important periods for developing health behaviors to prevent obesity (Daniels et al., 2005; Kvaavik, Tell, & Klepp, 2003). The global effort to calculate a health index by examining the present conditions and problematic health behaviors of adolescents to create adolescent health policy and health-promotion plans is increasing. Examples of these are the Youth Risk Behavioral Surveillance (YRBS; Eaton et al., 2010) and the Global School-Based Student Health Survey (World Health Organization, 2011a) of the Centers for Disease Control and Prevention (CDC) in America, the Health Behavior in School-Aged Children study (Currie, Samdal, Boyce, & Smith, 2001) of the WHO in Europe, and the Korea Youth Risk Behavior Web-Based Survey (KYRBWS) questionnaire of the Korean Centers for Disease Control and Prevention (2010b). The KYRBWS investigates various health behaviors such as smoking frequency, level of cigarette consumption, frequency of alcohol consumption, the amount of 1

2 2 So alcohol consumed, frequency of severe alcohol intoxication, and eating habits and examines physical activity and sedentary behavior. This information is a useful resource about obesity and could be provided to compare the adolescent health-policy and health-promotion plans on the national and global levels between countries. Many studies have shown that increasing physical activity and decreasing sedentary behavior is associated with decreased obesity worldwide (Eisenmann, Bartee, Smith, Welk, & Fu, 2008; Smith, Phongsavan, Havea, Halavatau, & Chey, 2007). However, only a few studies have examined the relationship between physical activity, sedentary behavior, and obesity in Korea (Shin, Kim, Park, & Song, 2000). Furthermore, although regional studies have been performed on this issue, no nationwide study has ever been performed in Korean adolescents in this regard. Therefore, the purpose of this study was to examine whether physical activity and sedentary behavior were related to weight status in Korean adolescents. The results will be used to explain differences in weight status according to physical activity and sedentary behavior of Korean adolescents and then be used for a comparison among nations and for Korean health-policy development and evaluation. Subject Methods The raw, cross-sectional data of the fifth KYRBWS in 2009 were requested and used for this study (Korean Centers for Disease Control and Prevention, 2010b). The data were the basis for understanding the current level of health-risk behaviors in adolescents and used to calculate an adolescent health index for a health-promotion-project plan and evaluation. The population of this study was Korean middleand high-school students from 800 schools (400 middle and 400 high schools) between September 1, 2009, and November 20, Sampling was conducted by classifying the population into 16 cities and provinces large (population of >1 million) and medium-size cities (population of million) and districts (population of <0.5 million) by using a complex sample design involving stratification, clustering, and multistage sampling (Korean Centers for Disease Control and Prevention, 2010b). Participating students were assigned unique identification numbers by classroom teachers. The students accessed the survey Web page by using their ID numbers and responded to a question about their willingness to participate. Willing participants self-administered the questionnaire anonymously at the school, and those unwilling did not progress further. The KYRBWS was used in a nationally representative study, and because the KYRBWS did not collect private information, ethical approval was not required. The rate of response for the fifth survey was 97.6% (N = 75,066), and students with long absences, learning disabilities, dyslexia, and dysgraphia were excluded. In all, 72,399 students (38,152 boys and 34,247 girls), after excluding abnormal and missing individuals, were included in this study. Details of the data-collection procedure are described by the Korean Centers for Disease Control and Prevention (2010b), and this survey has been shown to be valid and reliable (Bae et al., 2010a, 2010b). The characteristics of subjects are shown in Table 1. Dependent Variables The KYRBWS consisted of eight questions about physical activity based on a Korean health and nutrition survey (Korean Centers for Disease Control and Prevention, 2010a), the American YRBS (Eaton et al., 2010), the Health Behavior in School-Aged Children study of Europe (World Health Organization, 2011a), and the International Physical Activity Questionnaire of the WHO (Craig et al., 2003). The eight questions used to measure physical activity patterns in the sample of adolescents are listed in Table 2. In addition, responses to three questions pertaining to physical activity (Q1, Q2, and Q4) and responses to two questions pertaining to sedentary behavior (Q7 and Q8) were compared in this study. Physical activity was compared with the guidelines of the American College of Sports Medicine (ACSM). Sedentary behavior followed the YRBS guidelines. The ACSM recommends 20 min of vigorous physical activity 3 days/week, 30 min of moderate physical activity at least 5 days/week, and at least 2 days/week of strength training (ACSM, 2009). The YRBS recommends <3 hr/day of sitting, such as watching TV, playing games, and using the Internet (Eaton et al., 2010). Based on these recommendations, in Table 3, the participants were divided into two groups, one that met the requirements and that did not. Independent Variables The subjects height and weight were self-recorded, and their body-mass index (BMI; kg/m 2 ) was calculated using each participant s height and weight. According to the WHO Asia-Pacific standard of obesity, the conditions of people with BMI <18.5, 18.5 to <23, 23 to <25, 25 to <30, and 30 were defined as underweight, healthy weight, overweight, obesity Grade I, and obesity Grade II, respectively (WHO/IASO/IOTF, 2000). Statistical Analysis All the results of this study are presented as means and standard deviations. An independent t test was performed to examine gender differences. One-way ANOVA was used to verify the intergroup differences in physical activity patterns according to the BMI standard (underweight, healthy weight, overweight, and obesity group). Tukey s post hoc test was conducted to specifically confirm the groups that showed differences when a difference between the groups was observed. Physical activity and sedentary behavior were classified as categorical data

3 Physical Activity and Body Weight 3 Table 1 Subject Characteristics Variables Boys (n = 38,152) Girls (n = 34,247) Total (N = 72,399) Age (years), M ± SD ± ± ± 1.75 Height (cm), M ± SD ± ± ± 8.46 Weight (kg), M ± SD ± ± ± Body-mass index (kg/m 2 ), M ± SD ± ± ± 2.95 Weight status, n (%) underweight (body-mass index < 18.5) 9,719 (25.5) 10,012 (29.2) 19,731 (27.3) healthy weight (18.5 body-mass index < 23) 19,685 (51.5) 19,655 (57.4) 39,340 (54.2) overweight (23 body-mass index < 25) 4,382 (11.5) 2,967 (8.7) 7,349 (10.2) obesity Grade I (25 body-mass index < 30) 4,144 (10.9) 1,613 (4.7) 5,757 (8.0) obesity Grade II (body-mass index 30) 222 (0.6) 0 (0.0) 222 (0.3) Family economic status, n (%) high 2,691 (7.1) 1,337 (3.9) 4,028 (5.6) middle high 8,768 (23.0) 6,719 (19.6) 15,487 (21.4) average 17,229 (45.2) 17,259 (50.4) 34,488 (47.6) middle low 6,890 (18.1) 6,825 (19.9) 13,715 (18.9) low 2,574 (6.7) 2,107 (6.2) 4,681 (6.5) City size, n (%) large 20,252 (53.1) 17,708 (51.7) 37,960 (52.4) medium 13,303 (34.9) 12,153 (35.5) 25,456 (35.2) small 4,597 (12.0) 4,386 (12.8) 8,983 (12.4) Grade, n (%) middle first 6,711 (17.6) 5,615 (16.4) 12,326 (17.0) middle second 6,722 (17.6) 5,727 (16.7) 12,449 (17.2) middle third 6,767 (17.7) 5,615 (16.4) 12,382 (17.1) high first 6,626 (17.4) 5,369 (15.7) 11,995 (16.6) high second 5,889 (15.4) 6,102 (17.8) 11,991 (16.6) high third 5,437 (14.3) 5,819 (17.0) 11,256 (15.5) according to whether the ACSM and YRBS guideline was followed (ACSM, 2009; Eaton et al., 2010). Odds ratios and 95% confidence intervals (CI) were calculated using logistic linear regression to examine whether the guidelines were followed according to obesity. Statistical significance was set at p <.05, and all analyses were performed using SPSS version 12.0 software (SPSS, Chicago, IL). Results Gender Differences in Patterns of Physical Activity and Sedentary Behavior Gender differences in the patterns of physical activity and sedentary behavior are shown in Table 2. Compared with the girls, the boys had significantly higher values for all six variables: vigorous physical activity (p <.001), moderate physical activity (p <.001), moderate physical activity for at least 60 min (p <.001), exercise for muscle strength (p <.001), walking on a weekday (p <.001), and walking on a weekend (p <.001). Compared with the boys, the girls had significantly higher values related to sedentary behavior such as sitting on a weekday (p =.002) and sitting on a weekend (p =.008). Differences in Patterns of Physical Activity and Sedentary Behavior According to Weight The differences among patterns of physical activity and sedentary behavior according to weight are shown in Table 4. With increased obesity in the case of boys, vigorous physical activity (p <.001), moderate physical activity (p <.001), moderate physical activity for at least 60 min (p <.001), exercise for muscle strength (p <

4 Table 2 4 Variables Gender Differences in Patterns of Physical Activity and Sedentary Behavior (M ± SD) Boys (n = 38,152) Girls (n = 34,247) t p Q1. In the last 7 days, how many days did you perform vigorous physical activity like heavy lifting, digging, aerobics, or fast bicycling? 3.34 ± ± < No vigorous physical activity 2. Once per week 3. Twice per week 4. Thrice per week 5. 4 times per week 6. Over 5 times per week Q2. In the last 7 days, how many days did you perform moderate physical activity like carrying light loads, bicycling at a regular pace, or doubles tennis? 3.14 ± ± < No moderate physical activity 2. Once per week 3. Twice per week 4. Thrice per week 5. 4 times per week 6. Over 5 times per week Q3. In the last 7 days, how many days did you spend doing moderate physical activity for at least 60 minutes? 3.11 ± ± < No moderate physical activity for at least 60 minutes 2. Once per week 3. Twice per week 4. Thrice per week 5. 4 times per week 6. 5 times per week 7. 6 times per week 8. 7 times per week Q4. In the last 7 days, how many days did you do exercise for muscle strength, like push-ups, sit-ups, or weight lifting? 2.70 ± ± < No exercise for muscle strength 2. Once per week 3. Twice per week 4. Thrice per week 5. 4 times per week 6. Over 5 times per week Q5. In the last 7 days, how much time did you usually spend walking on a weekday? 2.28 ± ± < Under 30 minutes per day minutes to 1 hour per day hours per day 4. Over 2 hours per day

5 Q6. In the last 7 days, how much time did you usually spend walking on a weekend? 2.25 ± ± < Under 30 minutes per day minutes to 1 hour per day hours per day 4. Over 2 hours per day Q7. In the last 7 days, how much time did you spend sitting and watching TV, sitting while on the Internet, or playing a computer game on a weekday? 2.57 ± ± Under 1 hour per day hours per day hours per day hours per day 5. Over 4 hours per day Q8. In the last 7 days, how much time did you spend sitting and watching TV, sitting while on the Internet, or playing a computer game on a weekend? 3.31 ± ± Under 1 hour per day hours per day hours per day hours per day 5. Over 4 hours per day Table 3 Logistic-Regression Analysis for Patterns of Compliance With the Physical Activity and Sedentary Behavior Guidelines According to Weight Status Boys Case β SE OR 95% CI p vigorous physical activity 3 days/week (yes/no) healthy weight 19,685 Ref. underweight 9, <.224 overweight 4, <.012 obesity Grade I 4, <.001 obesity Grade II <.008 moderate physical activity 5 days/week (yes/no) healthy weight 19,685 Ref. underweight 9, <.001 overweight 4, <.823 obesity Grade I 4, <.013 obesity Grade II <.206 exercise for muscle strength 2 days/week (yes/no) healthy weight 19,685 Ref. underweight 9, <.001 overweight 4, <.001 obesity Grade I 4, <.001 obesity Grade II <.001 (continued) 5

6 6 So Table 3 (continued) Girls sitting on a weekday 3 hr/day (yes/no) healthy weight 19,685 Ref. sitting on a weekend 3 hr/day (yes/no) healthy weight 19,685 Ref. Case β SE OR 95% CI p underweight 9, <.001 overweight 4, <.223 obesity Grade I 4, <.001 obesity Grade II <.175 underweight 9, <.001 overweight 4, <.493 obesity Grade I 4, <.001 obesity Grade II <.015 vigorous physical activity 3 days/week (yes/no) healthy weight 19,655 Ref. underweight 10, <.386 overweight 2, <.029 obesity Grade I 1, <.484 moderate physical activity 5 days/week (yes/no) healthy weight 19,655 Ref. underweight 10, <.003 overweight 2, <.042 obesity Grade I 1, <.174 exercise for muscle strength 2 days/week (yes/no) healthy weight 19,655 Ref. underweight 10, <.444 overweight 2, <.080 obesity Grade I 1, <.005 sitting on a weekday 3 hr/day (yes/no) healthy weight 19,655 Ref. underweight 10, <.655 overweight 2, <.158 obesity Grade I 1, <.007 sitting on a weekend 3 hr/day (yes/no) healthy weight 19,655 Ref. underweight 10, <.001 overweight 2, <.021 obesity Grade I 1, <.001 Note. SE = standard error; OR = odds ratio; CI = confidence interval. Body-mass index <18.5, 18.5 to <23, 23 to <25, 25 to <30, and 30 indicated underweight, healthy weight, overweight, obesity Grade I, and obesity Grade II, respectively..001), walking on a weekday (p <.001), and walking on a weekend (p =.012) decreased. Moreover, sitting on a weekday (p <.001) and sitting on a weekend (p <.001) increased as their BMI approached the obesity level. In the case of the girls, exercise for muscle strength (p =.011) decreased as their BMI approached the obesity level. Moreover, sitting on a weekday (p =.005) and sitting on a weekend (p <.001) increased as their BMI approached the obesity level. However, vigorous physical activity (p <.001), moderate physical activity (p <.001), moderate physical activity for at least 60 min (p =.003), and walking on a weekday (p <.001) were higher in girls with higher BMIs. Walking on a weekend did not significantly differ among the groups (p =.474).

7 Table 4 Differences Among Patterns of Physical Activity and Sedentary Behavior According to Weight Status (M ± SD) Healthy weight Underweight Overweight Obesity Grade I Obesity Grade II F p Boys (n = 19,685) (n = 9,719) (n = 4,382) (n = 4,144) (n = 222) Q1. Vigorous physical activity (scored 1 6) 3.38 ± ± ± 1.69* 3.24 ± 1.68*** 3.06 ± 1.67*** <.001 Q2. Moderate physical activity (scored 1 6) 3.17 ± ± 1.65* 3.14 ± ± 1.65** 2.91 ± 1.63*** <.001 Q3. Moderate physical activity for at least 60 min (scored 1 8) 3.16 ± ± 2.11** 3.04 ± 2.08** 3.04 ± 2.09** 2.70 ± 1.87*** <.001 Q4. Exercise for muscle strength (scored 1 6) 2.80 ± ± 1.64*** 2.59 ± 1.68*** 2.45 ± 1.61*** 2.31 ± 1.56*** <.001 Q5. Walking on a weekday (scored 1 4) 2.29 ± ± 0.96* 2.25 ± ± 0.91*** 2.14 ± 0.95*** <.001 Q6. Walking on a weekend (scored 1 4) 2.25 ± ± ± ± ± <.012 Q7. Sitting on a weekday (scored 1 5) 2.52 ± ± 1.32*** 2.53 ± ± 1.34*** 2.59 ± <.001 Q8. Sitting on a weekend (scored 1 5) 3.28 ± ± 1.36*** 3.29 ± ± 1.36*** 3.45 ± 1.41*** <.001 Girls (n = 19,655) (n = 10,012) (n = 2,967) (n = 1,613) Q1. Vigorous physical activity (scored 1 6) 2.21 ± ± ± 1.48*** 2.28 ± <.001 Q2. Moderate physical activity (scored 1 6) 2.37 ± ± ± 1.50** 2.42 ± <.001 Q3. Moderate physical activity for at least 60 min (scored 1 8) 2.02 ± ± ± ± 1.66* <.003 Q4. Exercise for muscle strength (scored 1 6) 1.77 ± ± ± ± 1.14* <.011 Q5. Walking on a weekday (scored 1 4) 2.14 ± ± 0.95*** 2.17 ± ± <.001 Q6. Walking on a weekend (scored 1 4) 2.08 ± ± ± ± <.474 Q7. Sitting on a weekday (scored 1 5) 2.58 ± ± ± ± 1.37* <.005 Q8. Sitting on a weekend (scored 1 5) 3.34 ± ± 1.37** 3.43 ± 1.38** 3.49 ± 1.39*** <.001 Note. Body-mass index <18.5, 18.5 to <23, 23 to <25, 25 to <30, and 30 indicated underweight, healthy weight, overweight, obesity Grade I, and obesity Grade II, respectively. *p <.05, **p <.01, ***p <.001 compared with healthy-weight group (Tukey s post hoc). 7

8 8 So Logistic-Regression Analysis for Patterns of Compliance With the Guidelines for Physical Activity and Sedentary Behavior According to Weight The logistic-regression analysis for patterns of compliance with the physical activity and sedentary behavior guidelines according to weight is shown in Table 3. For boys, the odds ratios (95% CIs) of compliance with the physical activity and sedentary behavior guidelines for vigorous physical activity across weight statuses were ( , p =.012) for overweight individuals, ( , p <.001) for obesity Grade I individuals, and ( , p =.008) for obesity Grade II individuals, compared with individuals in the healthy-weight group (HG). Data for moderate physical activity across weight statuses were ( , p =.001) for underweight individuals and ( , p =.013) for obesity Grade I individuals, compared with individuals in the HG. Data for exercise for muscle strength across weight statuses were ( , p <.001) for underweight individuals, ( , p <.001) for overweight individuals,.680 ( , p <.001) for obesity Grade I individuals, and ( , p =.001) for obesity Grade II individuals, compared with individuals in the HG. Data for sitting on a weekday across weight statuses were ( , p <.001) for underweight individuals and ( , p <.001) for obesity Grade I individuals, compared with individuals in the HG. Data for sitting on a weekend across weight statuses were ( , p <.001) for underweight individuals, ( , p <.001) for obesity Grade I individuals, and ( , p =.015) for obesity Grade II individuals compared with individuals in the HG. For girls, the odds ratios (95% CIs) for compliance with the physical activity and sedentary behavior guidelines for vigorous physical activity across weight statuses were ( , p =.029) for overweight individuals, compared with individuals in the HG. Data for moderate physical activity across weight statuses were ( , p =.003) for underweight individuals and ( , p =.042) for overweight individuals, compared with the HG. Data for exercise for muscle strength across weight statuses were ( , p =.005) for obesity Grade I, compared with individuals in the HG. Data for sitting on a weekday across weight statuses were ( , p =.007) for obesity Grade I individuals, compared with individuals in the HG. Data for sitting on a weekend across weight statuses were ( , p <.001) for underweight individuals, ( , p =.021) for overweight individuals, and ( , p <.001) for obesity Grade I individuals, compared with individuals in the HG. Discussion The WHO reported that the main reason for excess body weight and obesity is increased intake of energy (e.g., high-calorie food) and decreased expenditure of energy (e.g., physical activity; World Health Organization, 2011b). Behavior modifications, diets, exercises, drugs, and operation therapies that reduce obesity are available, but considering the socioeconomic aspect and other side effects, it would be more desirable to recommend increasing physical activity and decreasing sedentary behavior (Thomas & Albert, 2002). This study investigated whether physical activity and sedentary behavior were related to obesity by examining the differences in physical activity, sedentary behaviors, and obesity among Korean adolescents. Boys, compared with girls, showed relatively high values of physical-activity-related variables such as vigorous physical activity, moderate physical activity, moderate physical activity for at least 60 min, exercise for muscle strength, walking on a weekday, and walking on a weekend and showed relatively low values of sedentarylifestyle-related variables such as sitting on a weekday and sitting on a weekend (Table 2). These results indicate that Korean adolescent boys are more active than Korean adolescent girls. This tendency was found in almost all of the previous studies and is thought to be because the cultural and social environments encourage men to be more active than women (Charreire et al., 2011; Eaton et al., 2010; Steele et al., 2010). Higher weight status and lower physical activity in boys (Table 4) show the tendency to lower the possibility (odds ratio) of complying with physical activity recommendations (Table 3). Moreover, we found that physical activity level is a factor that significantly affects obesity in adolescents. Janssen et al. (2005) analyzed the Health Behavior in School-Aged Children Study in 140,000 students from 35 countries and reported that adequate physical activity significantly reduced the risk of overweight in 30 countries. Kimm et al. (2005) reported that regular physical activity reduces or prevents obesity, as found in various studies. Other results suggest that changes in the amount and intensity of physical activity affect BMI and adipose tissue; moreover, a reduction of 10 metabolic equivalents (METs) per week increases BMI by kg/m 2, as determined by a 10-year tracking study (Kimm et al., 2005). The results of these previous studies correspond to the results obtained for Korean boys but differ from those obtained for Korean girls. Higher weight status in girls showed a tendency to be related to increased physical activity (Table 4) and to increase the possibility (odds ratio) of compliance with the physical activity recommendation (Table 3). The reason for this tendency is unknown, but it is thought that other factors such as eating habits or stress levels affect obesity in girls, even though a high weight status induces high energy consumption while physical activity levels are high (Epstein & Wing, 1980; Macias, 2004). In addition, obesity is thought to be one of the greatest stresses for girls during puberty, and physical activity is increased to reduce this stress; however, sex hormones (Pasquali, Vicennati, Gambineri, & Pagotto, 2008), social stress

9 Physical Activity and Body Weight 9 about differences in obesity in girls and boys (Thomas & Albert, 2002), and relatively low level of physical activity compared with boys (Steele et al., 2010) would largely affect obesity in girls. However, for both boys and girls sedentary behavior increased (Table 4) and the possibility (odd ratio) of compliance with the physical activity recommendation decreased as weight increased (Table 3). Highly sedentary behavior is one of the most important factors of obesity in Korean adolescents, and this corresponds to the results of cross-sectional (Janssen et al., 2005), prospective (Dietz & Gortmaker, 1985), and intervention studies (Robinson, 1999) that show that watching television is highly related to obesity in adolescents. Moreover, it should be classified as an independent variable separate from physical activity. Underweight boys showed low physical activity levels and a high level of sedentary behavior similar to the overweight or obese groups. Physical activity can be a factor in stimulating physical growth. Therefore, proper physical activity may not only prevent obesity but also decrease body weight. We hope to further study the relationship between low body weight and physical activity. The limitations of this study are as follows: First, the study was conducted online; all data were self-reported and therefore their accuracy is questionable. The height and weight of the adolescents were not measured directly. Furthermore, the level of obesity that was measured was probably relatively low, because adolescents have a tendency to grow in height and decrease in weight (Bae et al., 2010a). In addition, socioeconomic status was not reported by parents but by adolescents, so it could be inaccurate. Second, this study could not examine cause and effect but could examine the interrelationship because it was cross-sectional study. Third, this study did not investigate dietary habits (energy intake). Therefore, more well-designed studies should be carried out to determine the extent to which dietary habits contribute to obesity. However, this study is different from previous small regional case studies, because it examined adolescents from the entire country (Korea), with a sample population of 72,399. Therefore, it is a representative study dealing with the differences in obesity in Korean adolescents and physical activity and could be used to represent health data of Korean adolescents. Adolescence is one of the most important periods in determining health behavior (Christie &Viner, 2005; Olsson, Fahlén, & Janson, 2008). Therefore, obesity among adolescents should not be regarded as an individual problem but, rather, as a family, school, and community concern. Conclusion Boys showed high physical activity levels and low sedentary behavior compared with girls because of cultural- and social-environment and sexual differences. Boys showed lower levels of physical activity and higher sedentary behavior as their BMI increased, but girls showed higher levels of physical activity and sedentary behavior. Even though the results of the girls were slightly different from those of the boys, physical activity and sedentary behavior could be significant factors that reduce or prevent obesity in Korean adolescents. Acknowledgments This work was supported by a special research grant from Seoul Women s University (2012). References Adams, J. (2006). Trends in physical activity and inactivity amongst US year olds by gender, school grade and race, : Evidence from the Youth Risk Behavior Survey. BMC Public Health, 6, 57. PubMed doi: / American College of Sports Medicine. (2009). ACSM s guidelines for exercise testing and prescription (8th ed.). 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