Age- and sex-standardised lean and fat indices derived from bioelectrical impedance analysis for ages 7-11 years: functional associations with cardio-respiratory fitness and grip strength.

Indices for lean and fat mass adjusted for height derived from bioelectrical impedance for children aged 7 years have been published previously and their usefulness in the clinical assessment of undernutrition has been demonstrated. However, there is a need for norms that cover a wider age range and to explore their functional significance. The aim of the present study is to derive lean and fat indices for children aged 7-11 years and investigate associations with objective measures of cardio-respiratory fitness and grip strength. Subjects were 9574 children participating in the Avon Longitudinal Study of Parents and Children (ALSPAC). Bioelectrical impedance analysis (BIA) data collected longitudinally between ages 7 and 11 were used to derive lean and fat indices using the method of standardised residuals. Cardio-respiratory fitness (CRF) (9 years) and grip strength (11 years) were also measured. Correlation coefficients and 95 % CI were calculated to assess the strength of association between lean index, fat index and CRF and grip strength. Equations for calculating lean and fat indices in children aged 7-11 years relative to the ALSPAC population are presented. Lean index was linearly associated with CRF (rboys 0.20 (95 % CI 0.15, 0.25), rgirls 0.26 (95 % CI 0.22, 0.30)) and grip strength (rboys 0.29 (95 % CI 0.26, 0.32), rgirls 0.26 (95 % CI 0.23, 0.29)). BMI showed slightly weaker associations, while fat index was unrelated to either CRF or grip strength. Lean indices relate to muscle function and fitness while fat index does not.