RESUMEN
OBJECTIVE: To examine the associations among several body mass index (BMI) metrics (z-scores, percent of the 95th percentile (%BMIp95) and BMI minus 95th percentile (ΔBMIp95) as calculated in the growth charts from the Centers for Disease Control and Prevention (CDC). It is known that the widely used BMI z-scores (BMIz) and percentiles calculated from the growth charts can differ substantially from those that directly observed in the data for BMIs above the 97th percentile (z = 1.88). STUDY DESIGN: Cross-sectional analyses of 8.7 million 2- to 4-year-old children who were examined from 2008 through 2011 in the CDC's Pediatric Nutrition Surveillance System. RESULTS: Because of the transformation used to calculate z-scores, the theoretical maximum BMIz varied by >3-fold across ages. This results in the conversion of very high BMIs into a narrow range of z-scores that varied by sex and age. Among children with severe obesity, levels of BMIz were only moderately correlated (r ~ 0.5) with %BMIp95 and ΔBMIp95. Among these children with severe obesity, BMIz levels could differ by more than 1 SD among children who had very similar levels of BMI, %BMIp95 and ΔBMIp95 due to differences in age or sex. CONCLUSIONS: The effective upper limit of BMIz values calculated from the CDC growth charts, which varies by sex and age, strongly influences the calculation of z-scores for children with severe obesity. Expressing these very high BMIs relative to the CDC 95th percentile, either as a difference or percentage, would be preferable to using BMI-for-age, particularly when assessing the effectiveness of interventions.
Asunto(s)
Índice de Masa Corporal , Sobrepeso/diagnóstico , Obesidad Infantil/diagnóstico , Preescolar , Estudios Transversales , Femenino , Gráficos de Crecimiento , Humanos , Masculino , Encuestas Nutricionales , Sobrepeso/epidemiología , Obesidad Infantil/epidemiologíaRESUMEN
OBJECTIVE: To examine the accuracies of body mass index (BMI) and skinfold thicknesses in classifying the body fatness of 7365 8- to 19-year-old subjects in a national sample. STUDY DESIGN: We used percent body fat determined by dual-energy x-ray absorptiometry (PBFDXA) between 1999 and 2004. Categories of PBFDXA and the skinfold sum (triceps plus subscapular) were constructed so that that numbers of children in each category were similar to the number in each of 5 BMI categories based on the Centers for Disease Control and Prevention growth charts. RESULTS: Approximately 75% of the children and adolescents who had a BMI-for-age ≥ 95th percentile (considered obese) had elevated body fatness, but PBFDXA levels were more variable at lower BMIs. For example, only 41% of the boys who had a BMI < 25th percentile, had a similarly low PBFDXA. The use of the skinfold sum, rather than BMI, slightly improved the identification of elevated levels of body fatness among boys (P = .03), but not among girls (P > .10). A low sum of the triceps and subscapular skinfold thicknesses was a better indicator of low PBFDXA than was a low BMI, but differences were smaller among children with greater levels of body fatness. Among girls who had a PBFDXA above the median, for example, BMI and the skinfold sum were correlated similarly (r = 0.77-0.79) with body fatness. CONCLUSIONS: Both BMI and skinfold thicknesses are fairly accurate in identifying children who have excess body fatness. In contrast, if the goal is to identify children who have low body fatness, skinfold thicknesses would be preferred.
Asunto(s)
Absorciometría de Fotón , Tejido Adiposo/anatomía & histología , Índice de Masa Corporal , Grosor de los Pliegues Cutáneos , Adolescente , Niño , Precisión de la Medición Dimensional , Femenino , Humanos , Masculino , Adulto JovenRESUMEN
OBJECTIVE: To assess associations among youth television (TV) viewing and parental TV viewing, parental knowledge of the American Academy of Pediatrics (AAP) recommendations to limit children's TV viewing time to < or = 2 hours per day (knowledge), and parental limits on the frequency of children's TV viewing (rules). STUDY DESIGN: Adult participants in the 2002 Styles surveys answered questions about their weekly TV viewing, knowledge, and rules. TV viewing time of children of the adult participants (520 boys and 525 girls) age 10 to 18 years was also collected. Associations between hours of child TV viewing and parental TV viewing, parental knowledge, and parental rules were quantified using linear regression techniques. RESULTS: Variables included in multivariate regression models accounted for 8% to 18% of the variance in TV viewing among boys and girls. Parent TV viewing was significantly associated with TV viewing in 10- to 12-year-old and 16- to 18-year-old boys and girls. Knowledge was not associated with TV viewing in boys and girls in this sample. Rules were associated with TV viewing in boys and girls of all ages. CONCLUSIONS: Our findings suggest that parental TV viewing and rules limiting their child's TV time may play an important role in children's weekly TV viewing time.