External validation of a prediction model for estimating fat mass in children and adolescents in 19 countries: individual participant data meta-analysis

OBJECTIVE

To evaluate the performance of a UK based prediction model for estimating fat-free mass (and indirectly fat mass) in children and adolescents in non-UK settings.

DESIGN

Individual participant data meta-analysis.

SETTING

19 countries.

PARTICIPANTS

5693 children and adolescents (49.7% boys) aged 4 to 15 years with complete data on the predictors included in the UK based model (weight, height, age, sex, and ethnicity) and on the independently assessed outcome measure (fat-free mass determined by deuterium dilution assessment).

MAIN OUTCOME MEASURES

The outcome of the UK based prediction model was natural log transformed fat-free mass (lnFFM). Predictive performance statistics of R², calibration slope, calibration-in-the-large, and root mean square error were assessed in each of the 19 countries and then pooled through random effects meta-analysis. Calibration plots were also derived for each country, including flexible calibration curves.

RESULTS

The model showed good predictive ability in non-UK populations of children and adolescents, providing R² values of >75% in all countries and >90% in 11 of the 19 countries, and with good calibration (ie, agreement) of observed and predicted values. Root mean square error values (on fat-free mass scale) were <4 kg in 17 of the 19 settings. Pooled values (95% confidence intervals) of R², calibration slope, and calibration-in-the-large were 88.7% (85.9% to 91.4%), 0.98 (0.97 to 1.00), and 0.01 (-0.02 to 0.04), respectively. Heterogeneity was evident in the R² and calibration-in-the-large values across settings, but not in the calibration slope. Model performance did not vary markedly between boys and girls, age, ethnicity, and national income groups. To further improve the accuracy of the predictions, the model equation was recalibrated for the intercept in each setting so that country specific equations are available for future use.

CONCLUSION

The UK based prediction model, which is based on readily available measures, provides predictions of childhood fat-free mass, and hence fat mass, in a range of non-UK settings that explain a large proportion of the variability in observed fat-free mass, and exhibit good calibration performance, especially after recalibration of the intercept for each population. The model demonstrates good generalisability in both low-middle income and high income populations of healthy children and adolescents aged 4-15 years.

Is the environment near home and school associated with physical activity and adiposity of urban preschool children?

Preventing sedentary behavior and adiposity in childhood has become a public health priority. We examined urban social and built environment characteristics as correlates of physical activity and anthropometry among 428 preschool children from low-income families in New York City. We measured the children's height, weight, skinfold thicknesses, physical activity by accelerometer, and covariates. We geocoded home and Head Start center addresses and estimated the following for an area within 0.5 km of those two locations using a detailed geographic database: neighborhood composition, walkability, crime and traffic safety, and aesthetic characteristics. Generalized estimating equations were used to examine the associations of area characteristics with physical activity or adiposity, adjusted for characteristics of the child, mother, and home. Participants were 2-5 years old, 53% female, 83% Hispanic, and 43% either overweight or obese. Of the walkability indicators, land use mix was associated with physical activity (26 more activity counts/minute per standard deviation increase in mixed land use, p = 0.015) and subway stop density was associated with adiposity (1.2 mm smaller sums of skinfold thicknesses sum per standard deviation increase in subway stop density, p = 0.001). The pedestrian-auto injury rate, an indicator of traffic safety problems, was associated with physical activity and adiposity (16 fewer activity counts/minute, p = 0.033, and 1.0 mm greater skinfold thickness per standard deviation increase in pedestrian-auto injuries, p = 0.018). Children living in areas with more street trees were more physically active and those living in areas with more park access had smaller skinfolds. However, many of the tested associations were not statistically significant and some trends were not in the hypothesized direction. Efforts to enhance walkability, safety, and green spaces in the local environment may be relevant to physical activity and adiposity, and therefore to the health of preschool-aged children from low-income families.