ORIGINAL ARTICLE

Clinical measures of adiposity and percentage fat loss: which measure most accurately reflects fat loss and what should we aim for?

Linda P Hunt^*, Anna Ford^*, Matthew A Sabin, Elizabeth C Crowne, Julian P H Shield

Department of Endocrinology, Diabetes and Metabolism, Bristol Royal Hospital for Children and University of Bristol, Bristol, UK

Correspondence to:
Dr J Shield
Bristol Royal Hospital for Children, Upper Maudlin St, Bristol BS2 8AE, UK;j.p.h.shield{at}bristol.ac.uk

Objective: To determine which clinical measure of childhood obesity should be monitored to best reflect change in adiposity in a weight management programme and estimate the degree of change needed to be relatively certain of fat reduction.

Subjects: 92 obese children with a mean (range) age of 12.8 (6.9–18.9) years and a mean body mass index standard deviation score (BMI SDS) of +3.38 (+2.27 to +4.47) attending a hospital-based clinic on a regular, 3 monthly basis.

Measurements: Pairs of weight and height measured up to 2.41 years apart used to derive BMI as kg/m², and adjusted for age and gender to give weight and BMI SDS (BMI-z score) using British 1990 Growth Reference Data. Contemporaneous adiposity estimated by fatness measured by a bioimpedance segmental body composition analyser.

Results: Changes in BMI-z scores, compared to BMI, weight and weight SDS, most accurately reflected loss of fat. Reductions of 0.25, 0.5, 0.75, and 1 BMI SDS equate to expected mean falls in total body fat percentage of 2.9%, 5.8%, 8.7% and 11.6%. Approximate 95% prediction intervals indicated that a fall in BMI SDS of at least 0.6 over 6–12 months (or 0.5 over 0–6 months) is consistent with actual fat loss.

Conclusion: Change in BMI-z score best reflects percentage fat loss compared to BMI, weight and weight SDS. The wide variation in likely percentage fat loss for a given BMI SDS reduction means a loss of 0.5–0.6 is required to be relatively certain of definite percentage fat reduction.

Abbreviations: AIC, Akaike information criterion; BMI, body mass index; IOTF, International Obesity Task Force; MCMC, Markov chain Monte Carlo; PI, prediction interval; SDS, standard deviation score

Keywords: BMI-z scores; adiposity; bio-impedance; obesity

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