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

Abstract

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 (r boys 0·20 (95 % CI 0·15, 0·25), r girls 0·26 (95 % CI 0·22, 0·30)) and grip strength (r boys 0·29 (95 % CI 0·26, 0·32), r girls 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. There has been a recent resurgence of interest in nutritional assessment of children, driven mainly by the advent of the obesity epidemic(1) but there is also an increasing awareness of its importance in the assessment of underweight, with recognition that a low BMI can reflect a low lean mass rather than low fat stores, which has important implications for nutritional management (2,3) . Assessing body composition robustly is labour intensive and expensive, and is particularly difficult in a field or clinical setting. There is thus a need for a relatively simple but reliable measure of body composition. Bioelectrical impedance analysis (BIA) offers a potential solution to this practical problem, but its use in childhood has been limited by a lack of child norms and doubts about its validity. BIA is actually a measure of lean mass, but as both lean and fat mass vary with height, estimates derived from BIA are usually presented as percentage fat ((total mass 2 lean mass)/total mass) as a simple means of adjusting fat mass for size. As a measure of adiposity in adults this has merit, but it has the effect of rendering lean mass invisible.In children, lean mass constitutes around 85 % of body weight, and varies considerably with age and sex (4,5) and in some disease states (3,6) . A child with a low lean mass could thus have a high percentage fat, despite a normal or even low fat mass (7) . We therefore recognised the importance of developing a method of manipulating BIA to pro...

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