Manipulation of Muscle Creatine and Glycogen Changes DXA Estimates of Body Composition.

Standardising a dual x-ray absorptiometry (DXA) protocol is thought to provide a reliable measurement of body composition. PURPOSE We investigated the effects of manipulating muscle glycogen and creatine content independently and additively on DXA estimates of lean mass. METHOD Eighteen well-trained male cyclists undertook a parallel group application of creatine loading (n=9) (20 g/d for 5 d loading; 3 g/d maintenance) or placebo (n=9) with crossover application of glycogen loading (12 v 6 g/kg BM/d for 48 h) as part of a larger study involving a glycogen-depleting exercise protocol. Body composition, total body water, muscle glycogen and creatine content were assessed via DXA, bioelectrical impedance spectroscopy and standard biopsy techniques. Changes in the mean were assessed using the following effect-size scale: >0.2 small, >0.6, moderate, >1.2 large and compared with the threshold for the smallest worthwhile effect of the treatment. RESULTS Glycogen loading, both with and without creatine loading, resulted in substantial increases in estimates of lean body mass (mean ± SD; 3.0 ± 0.7 % and 2.0 ± 0.9 %) and leg lean mass (3.1 ± 1.8 %and 2.6 ± 1.0 %) respectively. A substantial decrease in leg lean mass was observed following the glycogen depleting condition (-1.4 ± 1.6 %). Total body water showed substantial increases following glycogen loading (2.3 ± 2.3 %), creatine loading (1.4 ± 1.9 %) and the combined treatment (2.3 ± 1.1 %). CONCLUSIONS Changes in muscle metabolites and water content alter DXA estimates of lean mass during periods in which minimal change in muscle protein mass is likely. This information needs to be considered in interpreting the results of DXA-derived estimates of body composition in athletes.