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

Linda P Hunt ¹, Anna Ford ¹, Matthew A Sabin ¹, Elizabeth C Crowne ² and Julian PH Shield ^1*

¹ University of Bristol, United Kingdom
² Bristol Royal Hospital for Children, United Kingdom

^* To whom correspondence should be addressed. E-mail: j.p.h.shield{at}bristol.ac.uk.

Accepted 10 January 2007

Abstract

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 aged 12.8 (6.9 to 18.9) years, mean BMI SDS +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 Standard Deviation Score (SDS) (BMI-z score) using British 1990 Growth Reference Data. Contemporous adiposity estimated by fatness measured by Tanita Bioimpedance segmental body composition analyser (BC- 418MA).

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 to 0.6 is required to be relatively certain of definite percentage fat reduction.

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

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