Combination of DXA and BIS body composition measurements is highly correlated with physical function-an approach to improve muscle mass assessment.

Rationale: Fluid volume estimates may help predict functional status and thereby improve sarcopenia diagnosis.

Main Result: Bioimpedance-derived fluid volume, combined with DXA, improves identification of jump power over traditional measures.

Significance: DXA-measured lean mass should be corrected for fluid distribution in older populations; this may be a surrogate of muscle quality.

Purpose: Sarcopenia, the age-related loss of muscle mass and function, negatively impacts functional status, quality of life, and mortality. We aimed to determine if bioimpedance spectroscopy (BIS)-derived estimates of body water compartments can be used in conjunction with dual-energy X-ray absorptiometry (DXA) measures to aid in the prediction of functional status and thereby, ultimately, improve the diagnosis of sarcopenia.

Methods: Participants (≥ 70 years) had physical and muscle function tests, DXA, and BIS performed. Using a BMI correction method, intracellular water (ICW), extracellular water (ECW), and ECW to ICW (E/I) ratio was estimated from standard BIS measures. Jump power was assessed using jump mechanography.

Results: The traditional measure used to diagnose sarcopenia, DXA-derived appendicular lean mass (ALM) corrected for height (ALM/ht), was the least predictive measure explaining jump power variability (r = 0.31, p < 0.0001). The best measure for explaining jump power was a novel variable combining DXA ALM and BIS-derived E/I ratio (ALM/(E/I); r = 0.70, p < 0.0001). ALM/(E/I) and ICW had the highest correlation with jump power and grip strength, specifically jump power (r = 0.84 and r = 0.80, respectively; p < 0.0001).

Conclusions: The creation of a novel variable (ALM/(E/I)) improved the ability of DXA to predict jump power in an older population. ALM/(E/I) substantially outperformed traditional lean mass measures of sarcopenia and could well be an improved diagnostic approach to predict functional status. DXA-measured ALM should be corrected for fluid distribution, i.e., ALM/(E/I); this correction may be considered a surrogate of muscle quality.