Extracurricular physical activity participation modifies the association between high TV watching and low bone mass.

OBJECTIVES: To examine whether different sedentary behaviours are associated with the risk of low bone mineral content in adolescents, and if so, whether extracurricular physical-sporting activity influences this association. MATERIALS AND METHODS: A total of 277 adolescents from Zaragoza (168 females and 109 males) aged 13.0-18.5 yr within frame work of the multicentre AVENA study participated in this study. Bone mineral content (BMC), lean mass, and fat mass were measured with DXA. Physical activity and sedentary independent variables: participation in extracurricular physical-sporting activity (PA), h/d of television watching, playing video/computer games during school days and on weekend days and doing homework/studying. They all were assessed by questionnaire. The main outcome was low BMC, as defined by BMC Z-score for age and sex < percentile 10. Logistic regression was used to test the interaction and association of PA and sedentary variables with low BMC, after controlling for confounders like height, maturational status or lean mass. RESULTS: Among the sedentary variables studied, only television watching > or =3 h/d was associated with an increased risk for low BMC in males (OR, 95% CI: 7.01, 1.73 to 28.40), after controlling for sexual maturation. When PA was in the models, television watching was not any longer associated with low BMC, while PA was so (OR, 95% CI: 0.23, 0.09 to 0.55). Involvement in such activity reduced the risk of low bone mass by 76% (P<0.01) independently of body mass, height and fat mass, but not of the lean mass. CONCLUSION: Watching television for 3 or more h/d seems to be associated with an increased risk for low BMC in male adolescents. However, this association is mediated by participation in PA, suggesting that negative consequences of excessive television watching on adolescent bone health could be counteracted by sport participation. Longitudinal data and randomized controlled trials will confirm or contrast our findings.

Body fat measurement in adolescents: comparison of skinfold thickness equations with dual-energy X-ray absorptiometry.

OBJECTIVE: To compare the most commonly used equations to predict body fatness from skinfold thickness, in male and female adolescents, with dual-energy X-ray absorptiometry (DXA) as a reference method of fatness measurement. DESIGN: Cross-sectional nutrition survey. SETTING: General adolescent population from Zaragoza (Spain). SUBJECTS AND METHODS: A total of 238 Caucasian adolescents (167 females and 113 males), aged 13.0-17.9 y, were recruited from 15 school groups in 11 public and private schools. The percentage fat mass (%FM) was calculated by using skinfold-thickness equations. Predicted %FM was compared with the reference %FM values, measured by DXA. The lack of agreement between methods was assessed by calculating the bias and its 95% limits of agreement. RESULTS: Most equations did not demonstrate good agreement compared with DXA. However, in male adolescents, Slaughter et al equations showed relative biases that were not dependent on body fatness and the limits of agreement were narrower than those obtained from the rest of equations. In females, Brook's equation showed nonsignificant differences against DXA and the narrowest 95% limits of agreement. Only biases from Brook and Slaughter et al equations were not dependent on body fatness in female adolescents. CONCLUSIONS: Accuracy of most of the skinfold-thickness equations for assessment of %FM in adolescents was poor at the individual level. Nevertheless, to predict %FM when a relative index of fatness is required in field or clinical studies, Slaughter et al equations may be used in adolescents from both sexes and the Brook equation in female adolescents.

Body composition in adolescents: measurements and metabolic aspects.

INTRODUCTION: Adolescence is a decisive period in human life in which important body composition changes occur. Increase of total body mass and its relative distribution are mainly related to gender and pubertal development. OBJECTIVE: This review explores the specific measurements that may be used in this age group to assess excess body fat and to define obesity and overweight. RESULTS: Identification of subjects at risk for adiposity requires simple anthropometric cutoffs for the screening of overweight and obesity. In this context, BMI criterion is the most frequently used but, in spite of its high sensitivity and specificity, an important number of adolescents classified as overweight or obese do not have really high adiposity (32.1% of females and 42% of males). Excess total body fat and intra-abdominal visceral fat are related to metabolic abnormalities that increase the risk of cardiovascular diseases. Waist circumference seems to be the best simple anthropometric predictor for the screening of the metabolic syndrome in children and adolescents. CONCLUSIONS: Early identification of adolescents at risk for adiposity and its related metabolic complications requires reliable, simple and specific measures of excess body fat for this age group.

Exploración de la composición corporal en niños y adolescentes mediante absorciometría dual de rayos X (DEXA) / Body composition in children and adolescents by dual energy X-ray absorptiometry (DEXA)

Actualmente existen numerosos métodos de referencia para cuantificar la composición corporal en el niño y el adolescente. Uno de los que más interés ha suscitado durante los últimos años es la absorciometría dual de rayos X (DEXA). Las ventajas principales del DEXA son: la posibilidad de determinar tres compartimentos corporales (masa mineral ósea, masa grasa y masa no grasa sin hueso) con precisión y exactitud, la capacidad para cuantificar la densidad mineral ósea y el empleo de una metodología no invasiva y segura que utiliza tiempos de medida cortos. En el presente trabajo se revisan detalladamente los fundamentos metodológicos, aplicaciones, peculiaridades e inconvenientes del DEXA en la edad pediátrica (AU)