Cardiorespiratory Fitness is Strongly Related to the Metabolic Syndrome in Adolescents. Queen s University Kingston, Ontario, Canada

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1 Diabetes Care In Press, published online May 29, 2007 Cardiorespiratory Fitness is Strongly Related to the Metabolic Syndrome in Adolescents Received for publication 16 April 2007 and accepted in revised form 18 May Ian Janssen, PhD 1,2 and Wendy Cramp, BSc 1 1 School of Kinesiology and Health Studies, and 2 Community Health and Epidemiology Queen s University Kingston, Ontario, Canada Brief title: Fitness and the metabolic syndrome in adolescents Funding: Canadian Institutes of Health Research (IJ) Correspondence: Ian Janssen, PhD School of Kinesiology and Health Studies Queen s University 69 Union St. Kingston, Ontario, Canada, K7L 3N6 ian.janssen@queensu.ca Copyright American Diabetes Association, Inc., 2007

2 INTRODUCTION The metabolic syndrome (MetS) is a predecessor of type 2 diabetes and cardiovascular disease. 1 In adults physical activity is important for both preventing and treating MetS. 2 A limited number of studies have examined the impact of physical activity on MetS in youth. 3-7 The results from the few existing studies are conflicting, reporting no associations, weak effects, and strong effects of physical activity. 3-7 These inconsistencies may reflect that most of these studies measured physical activity using self-report questionnaire measures, which have limited validity and reliability in youth. 8,9 While one paper reported that the beneficial effects of physical activity on MetS are limited to girls, 3 published studies of physical activity and MetS in youth have been limited to predominately Caucasian samples and lack ethnic comparisons. Thus, it is unknown if the effects of physical activity on MetS are consistent in different population groups. The purpose of this report is to: 1) examine the relation between objective measures of cardiorespiratory fitness (CRF, the ability to perform endurance activities) and MetS in adolescents, and 2) determine whether the relationship is consistent by gender and ethnicity. RESEARCH DESIGN AND METHODS The study sample consisted of 1561 youth aged years who participated in the National Health and Nutrition Examination Survey (NHANES ), which is a nationally representative crosssectional survey. 10 All participants had a complete fasting ( 12 hours) blood sample as well as measures of waist circumference and CRF. Maximal aerobic capacity ( V O 2max ) was the objective CRF measure obtained in NHANES. V O 2max is a health-related component of physical fitness that relates to the ability of the circulatory and respiratory systems to supply oxygen during sustained physical activity. 11 V O 2max was estimated from heart rate measurements obtained during a multi-stage submaximal test where participants walked or ran on a treadmill to achieve 75-90% of their age-predicted maximal heart rate. 12 Estimated V O 2max was regressed on age within each gender, and the residuals were retained to represent ageadjusted values. The sample was then divided into low, moderate, and high CRF tertiles based on the age-adjusted V O 2max values. In adults, the MetS criteria established by the National Cholesterol Education Program (NCEP) is consistently used in research and clinical settings. 13 According to NCEP, MetS includes having 3 of the following: high waist circumference ( 102 cm in men, 88 cm in women), high plasma triglycerides (>1.69 mmol/l), low HDLcholesterol (<1.04 mmol/l in men, <1.29 mmol/l in women), high fasting plasma glucose ( 5.6 mmol/l), and high blood pressure ( 130/85 mmhg). There is no universally accepted MetS criterion for adolescents. In this study we used age- and gender-specific MetS thresholds for adolescents that were linked to the NCEP adult thresholds using growth curve modeling. 14 The growth curves for each MetS component pass through the adult thresholds at 20 years such that adolescent thresholds are linked to the adult thresholds noted above. Thus, MetS for adolescents was defined as having 3 of the 5 MetS components using the adolescent thresholds. Statistical analyses were performed using STATA Intercooled 7 (StataCorp, 2001) to account for the complex design and weighting of NHANES. The prevalence of MetS was compared across CRF tertiles using χ 2 analyses. The odds of having MetS and its component risk factors were compared across CRF tertiles using logistic regression 2

3 analyses. Age, ethnicity (non-hispanic white, non-hispanic black, Hispanic, and others races including multi-racial), poverty-toincome ratio (based on family income and size, divided into quartiles), smoking (never, former, current), carbohydrate consumption (% calories from carbohydrate, with <40% as low, 40-60% as moderate, >60% as high) and fat consumption (% calories from fat, with <30% as low, 30-40% as moderate, >40% as high) were included as confounders in the logistic regression analyses. RESULTS The prevalence of MetS was 7.6% in the entire sample. The prevalence of MetS decreased across low, moderate, and high CRF tertiles within both males (24.3%, 5.0%, 0.1%) and females (17.3%, 5.9%, 0.9%); and also within the non-hispanic white (30.0%, 6.0%, 0.5%), non-hispanic black (21.3%, 5.0%, 0.5%), and Hispanic (14.4%, 6.5%, 0.9%) ethnic groups. In comparison with the low CRF tertile (referent group, odds ratio = 1.00), the likelihood of having any of the individual MetS components was significantly lower in the high CRF tertile (p<0.05). The odds ratios and associated 95% confidence intervals for MetS components in the high CRF tertile were: 0.02 ( ) for high waist circumference, 0.18 ( ) for low HDL-cholesterol, 0.24 ( ) for high triglycerides, 0.51 ( ) for high plasma glucose, and 0.17 ( ) for high blood pressure. As shown in Table 1, the odds ratios for MetS decreased in a gradient manner when moving from low to moderate to high CRF tertiles. These results were consistent within all gender and ethnic groups (Table 1). Given the small number of subjects (n=72), analysis were not performed within the other race group. CONCLUSIONS CRF was a strong, independent predictor of MetS and its component risk factors in this sample of year olds. Interestingly, the prevalence of MetS in the lowest adolescent CRF tertile was comparable to the overall prevalence of MetS in U.S. adults, 15 while essentially none of the adolescents with a high CRF had the MetS. The results were consistent by gender and ethnicity. The strengths of this study include the large and representative study sample, 10 the use of age-appropriate adolescent MetS criteria that are linked to health-based adult criteria, 14 and the use of an objective CRF measure, which is in largely determined by physical activity participation in recent weeks and months. 11 The use of strong measures may help explain the strength of the relations observed in this study. Most previous studies in this topic have relied on questionnaire measures of physical activity, the imprecision of which may have biased the results towards a null effect. The findings presented in this report provide compelling evidence that moderate and high CRF levels are associated with a significantly lower MetS prevalence in adolescents. Therefore, interventions aimed at combating MetS in this age group may want to focus on improving CRF. 3

4 REFERENCES 1. Ardern CI, Janssen I. Metabolic syndrome and its association with morbidity and mortality. Appl Physiol Nutr Metab. 2007;32: Lakka TA, Laaksonen DE. Physical activity in prevention and treatment of the metabolic syndrome. Appl Physiol Nutr Metab. 2007;32: Ribeiro JC, Guerra S, Oliveira J, et al. Physical activity and biological risk factors clustering in pediatric population. Prev Med. 2004;39: Brage S, Wedderkopp N, Ekelund U, et al. Features of the metabolic syndrome are associated with objectively measured physical activity and fitness in Danish children: the European Youth Heart Study (EYHS). Diabetes Care. 2004;27: Andersen LB, Harro M, Sardinha LB, et al. Physical activity and clustered cardiovascular risk in children: a cross-sectional study (The European Youth Heart Study). Lancet. 2006;368: Ruiz JR, Ortega FB, Rizzo NS, et al. High cardiovascular fitness is associated with low metabolic risk score in children: the European Youth Heart Study. Pediatr Res. 2007;61: Kelishadi R, Razaghi EM, Gouya MM, et al. Association of physical activity and the metabolic syndrome in children and adolescents: CASPIAN Study. Horm Res. 2007;67: Wong SL, Leatherdale ST, Manske SR. Reliability and validity of a school-based physical activity questionnaire. Med Sci Sports Exerc. 2006;38: Bender JM, Brownson RC, Elliott MB, Haire-Joshu DL. Children's physical activity: using accelerometers to validate a parent proxy record. Med Sci Sports Exerc. 2005;37: Centers for Disease Control and Prevention (CDC). National Center for Health Statistics (NCHS). National Health and Nutrition Examination Survey Data. Available at: [ November U.S. Department of Health and Human Services. Physical Activity and Health: A Report of the Surgeon General. Atlanta, GA: Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion; Carnethon MR, Gulati M, Greenland P. Prevalence and cardiovascular disease correlates of low cardiorespiratory fitness in adolescents and adults. JAMA. 2005;294: Expert Panel on Detection Evaluation and Treatment of High Blood Cholesterol in Adults. Executive summary of the third report of the National Cholesterol Education Program (NCEP) Expert Panel on Detection Evaluation and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III). JAMA. 2001;285: Jolliffe CJ, Janssen I. Development of age-specific adolescent metabolic syndrome criteria that are linked to the Adult Treatment Panel III and International Diabetes Federation criteria. J Am Coll Cardiol. 2007;49: Ford ES, Giles WH, Dietz WH. Prevalence of the metabolic syndrome among US adults: findings from the third National Health and Nutrition Examination Survey. JAMA. 2002;287:

5 Table 1. Odds ratios for the metabolic syndrome according to cardiorespiratory fitness. Cardiorespiratory Fitness Tertile Population Group Low Moderate High All subjects (n = 1561) ( )* 0.01 ( )* Males (n = 829) ( )* 0.01 ( )* Females (n = 732) ( )* 0.04 ( )* Non-Hispanic white (n = 416) ( )* 0.01 ( )* Non-Hispanic black (n = 425) ( )* 0.02 ( )* Hispanic (n = 648) ( ) 0.05 ( )* Odds ratios (95% confidence intervals). Odds ratios are adjusted for age, gender, ethnicity, smoking, poverty-to-income ratio, dietary fat intake, and dietary carbohydrate intake. * significantly different from low cardiorespiratory fitness tertile (p<0.05). 5