Development of bioelectrical impedance-derived indices of fat and fat-free mass for assessment of nutritional status in childhood

Abstract

Objectives: (1) To develop a method of manipulating bioelectrical impedance (BIA) that gives indices of lean and fat adjusted for body size, using a large normative cohort of children. (2) To assess the discriminant validity of the method in a group of children likely to have abnormal body composition. Design: Two prospective cohort studies. Setting: Normative data: Avon Longitudinal Study of Parents and Children (ALSPAC), population based cohort; proof of concept study: tertiary feeding clinic and special needs schools. Subjects: Normative data: 7576 children measured aged between 7.25 and 8.25 (mean 7.5) (s.d. ¼ 0.2) years; proof of concept study: 29 children with either major neurodisability or receiving artificial feeding, or both, mean age 7.6 (s.d. ¼ 2) years. Measures: Leg-to-leg (Z T ) and arm-to-leg (Z B ) BIA, weight and height. Total body water (TBW) was estimated from the resistance index (RI ¼ height 2 /Z), and fat-free mass was linearly related to TBW. Fat mass was obtained by subtracting fat-free mass from total weight. Fat-free mass was log-transformed and the reciprocal transform was taken for fat mass to satisfy parametric model assumptions. Lean and fat mass were then adjusted for height and age using multiple linear regression models. The resulting standardized residuals gave the lean index and fat index, respectively. Results: In the normative cohort, the lean index was higher and fat index lower in boys. The lean index rose steeply to the middle of the normal range of body mass index (BMI) and then slowly for higher BMI values, whereas the fat index rose linearly through and above the normal range. In the proof of concept study, the children as a group had low lean indices (mean (s.d.) À1.5 (1.7)) with average fat indices ( þ 0.21 (2.0)) despite relatively low BMI standard deviation scores (À0.60 (2.3)), but for any given BMI, individual children had extremely wide ranges of fat indices. The lean index proved more stable and repeatable than BMI. Conclusions: This clinical method of handling BIA reveals important variations in nutritional status that would not be detected using anthropometry alone. BIA used in this way would allow more accurate assessment of energy sufficiency in children with neurodisability and may provide a more valid identification of children at risk of underweight or obesity in field and clinical settings.

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

“…The essential validity of BIA as a measure has not yet been fully established so we consider in more depth here whether these indices measure something useful and meaningful. In our first paper we were able to show that lean and fat indices varied in the expected manner between children with different body types and across the BMI range (6) . In the present paper we will explore whether variations in lean index correlate with functional measures that can be expected to * Corresponding author: Dr Andrea Sherriff, 9th Floor, University of Glasgow Dental School, 378 Sauchiehall Street, Glasgow G2 3JZ, UK, fax þ44 141 331 2798, email a.sherriff@dental.gla.ac.uk Abbreviations: ALSPAC, Avon Longitudinal Study of Parents and Children; BIA, bioelectrical impedance analysis; bpm, beats per min; CRF, cardio-respiratory fitness.…”

“…The present study, built on our previously published method, developed in this sample at age 7 years (6) . For this analysis a more recent regression equation (11) allowed us to predict total body water (TBW) directly from Tanita foot-to-foot BIA:…”

“…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 produce indices of lean and fat mass which were independent of each other, and adjusted for size, sex and age, and we have recently described our method for calculating these new indices expressed relative to a representative cohort of UK children at a single age (7 years) (6) . The essential validity of BIA as a measure has not yet been fully established so we consider in more depth here whether these indices measure something useful and meaningful.…”

“…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) .…”

“…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) .…”