Change in body fat percentage predicts change in DEXA TBMC but not HIP or spine BMD

Change in bone mineral content (BMC) and bone mineral density (BMD) is an important component of the follow-up to pharmacological, nutritional or exercise interventions to preserve bone mass. Dual energy x-ray absorptiometry (DEXA) is a widely used to measure total body bone mineral content (TBMC) and site specific BMD. The purpose of this study was to predict the impact of change in percent body fat on TBMC, trochanter (TRO), femoral neck (FN) and lumbar spine BMD measured by Norland XR-26 DEXA in United States Naval Academy (USNA) midshipmen (MIDN). 220 female (Ht=1.66±.07m; Wt=60.9±7.2kg; Age=18.5±.9yo) MIDN were measured at entry to the USNA and again one year later (Table I). Values are reported as mean±SD. Table I: Baseline Year 1 Change DEXA Body Fat(%) 31.5±6,1 34.5±5.9 3.1±4.6 TBMC (g) 2649±330 2728±333 3.3±3.5% FN (g/cm²) 1.044±.13 1.052±.12 0.90±4.0% TRO (g/cm²) 0.846±.11 0.843±.10 -0.41±5.1% Spine (g/cm²) 1.12±.13 1.13±.13 1.18±5.1% Change in TBMC was significantly related to change in DEXA percent bodyfat (r²= .57; p<. 001). TRO, FN and Spine BMD were not related to change in DEXA percent bodyfat (r²=.001; p>,80). MIDN did not increase in stature and their exercise habits were unrelated to change in TBMC. We conclude that DEXA TBMC measurements should be interpreted with care because of possible confounding with change in DEXA percent body fat. DEXA BMD at specific sites may provide better clinical information for assessing change in bone after specific medical interventions.