Disentangling the effects of fluid retention and low muscle mass on bioelectrical impedance-derived indices: A stratified analysis based on body water status.

BACKGROUND & AIMS: Bioelectrical impedance analysis (BIA) is widely used to assess skeletal muscle mass and cellular health; however, interpretation of BIA-derived indices is often confounded by alterations in fluid distribution. In particular, the extracellular water to total body water ratio (ECW/TBW) is influenced by both fluid retention and low muscle mass, thereby complicating the clinical evaluation of muscle-related indices. This study aimed to disentangle the effects of low muscle mass and fluid retention on BIA-derived indices by stratifying patients according to ECW/TBW and total body water indexed to height squared (TBW/height2).

METHODS: This cross-sectional study included 101 patients with stable chronic heart failure who underwent BIA. Participants were stratified into four groups based on ECW/TBW (<0.400 or ≥0.400) and TBW/height2 (below or above the median). Analysis of covariance (ANCOVA) was used to compare skeletal muscle mass index (SMI) and phase angle (PhA) across groups, adjusting for peak work rate (peak WR). Extended models additionally adjusted for sex, age, and N-terminal pro-B-type natriuretic peptide (NT-proBNP).

RESULTS: After adjustment for peak WR, significant group differences in SMI were observed. Compared with the group with low ECW/TBW and low TBW/height2, SMI was significantly higher in groups with elevated ECW/TBW, with the highest values observed in those with both elevated ECW/TBW and higher TBW/height2. Bonferroni-corrected pairwise comparisons confirmed significant differences among all groups, indicating distinct contributions of fluid retention and muscle mass to BIA-derived SMI. For PhA, groups with higher ECW/TBW exhibited significantly lower adjusted PhA compared with those with lower ECW/TBW, regardless of TBW level. These findings remained consistent in extended ANCOVA models adjusting for sex, age, and NT-proBNP. No significant interactions between group and peak WR were observed for either SMI or PhA.

CONCLUSIONS: Stratification using both ECW/TBW and TBW/height2 allows discrimination between the effects of fluid retention and low muscle mass on BIA-derived indices. Higher SMI values and lower PhA values were observed in the presence of either fluid retention or muscle mass reduction, supporting the need for a multidimensional interpretation of BIA in these patients.