ABSTRACT
OBJECTIVES: To explore the activitystat hypothesis in primary school children by asking whether more physical activity (PA) in school time is compensated for by less PA at other times. STUDY DESIGN: Observational, repeated measures (four consecutive occasions over a 12-month period). SETTING: South-west England. PARTICIPANTS: A total of 206 children (115 boys, aged 8-10 years) from 3 primary schools (S1, S2 and S3), which recorded large differences in PA during school time. MEASUREMENTS: Total PA (TPA) and its moderate-and-vigorous component were recorded weekly by accelerometry, in school and out of school, and adjusted for local daily rainfall and daylight hours. Habitual PA was assessed by linear mixed-effects modelling on repeated measures. RESULTS: S1 children recorded 64% more in-school PA, but S2 and S3 children compensated with correspondingly more out-of-school PA, so that TPA between the three schools was no different: 35.6 (34.3-36.9), 37.3 (36.0-38.6) and 36.2 (34.9-37.5) Units, respectively (P=0.38). CONCLUSIONS: The PA of children seems to compensate in such a way that more activity at one time is met with less activity at another. The failure of PA programmes to reduce childhood obesity could be attributable to this compensation.
Subject(s)
Energy Intake , Energy Metabolism , Health Behavior , Motor Activity , Obesity/prevention & control , Schools , Body Mass Index , Child , Cohort Studies , England/epidemiology , Exercise , Female , Humans , Leisure Activities , Male , Obesity/epidemiology , Physical Education and TrainingABSTRACT
OBJECTIVE: Several studies suggest that taller children may be wrongly labelled as 'overweight' because body mass index (BMI) is not independent of height (Ht) in childhood, and recommend adjustment to render the index Ht independent. We used objective measures of %body fat and hormonal/metabolic markers of fatness to investigate whether BMI and the corresponding fat mass index (FMI) mislead in childhood, or whether taller children really are fatter. DESIGN: Longitudinal observational study measuring children annually from age 7 to 12 years. SUBJECTS: Two hundred and eighty healthy children (56% boys) from the EarlyBird study. MEASUREMENTS: BMI (body mass (BM)/Ht(2)), FMI (fat mass (FM)/Ht(2)), %body fat ((FM/BM) × 100, where FM was measured by dual-energy X-ray absorptiometry), fasting leptin (a hormonal measure of body fatness) and insulin resistance (a metabolic marker derived from the validated homeostasis model assessment program for insulin resistance--HOMA2-IR) were all analysed in relation to Ht. Alternative Ht-independent indices of BM and FM were compared with BMI and FMI as indicators of true fatness and related health risk. RESULTS: BMI and FMI correlated with Ht at each annual time point (r~0.47 and 0.46, respectively), yet these correlations were similar in strength to those between Ht and %fat (r~0.47), leptin (r~0.41) and insulin resistance (r~0.40). Also, children who grew the most between 7 and 12 years showed greater increases in BMI, FMI, leptin and insulin resistance (tertile 1 vs 3, all p<0.05). BMI and FMI explained ~20% more of the variation in %fat, ~15% more in leptin and ~10% more in insulin resistance than the respective Ht-independent reformulations (BM/Ht(3.5) and FM/Ht(7), both p<0.001). CONCLUSION: Taller children really are fatter than their shorter peers, have higher leptin levels and are more insulin resistant. Attempts to render indices of BM or FM independent of Ht in children seem inappropriate if the object of the index is to convey health risk.