Rethinking Biva in Obesity: Systematic Vector Displacement Across the Adiposity Spectrum

To characterize BIVA vector patterns in adults with excess adiposity defined by FMI and examine the implications of obesity-related vector displacement for BIVA interpretation. A retrospective cross-sectional analysis of adults attending a tertiary obesity clinic was conducted. Excess adiposity was defined using sex-specific FMI thresholds based on NHANES criteria. Resistance and reactance were normalized for height (R/H, Xc/H), and BIVA patterns were described overall and stratified by sex and ethnicity. Sex- and ethnicity-specific tolerance ellipses were derived from bivariate normal models and visually compared with Piccoli/NHANES reference ellipses for elevated adiposity. Correlation structure and ellipse geometry were evaluated within each sex–ethnicity stratum. In 3072 adults with FMI-defined excess adiposity, impedance vectors showed a predominantly downward displacement characterized by lower Xc/H and lower phase angle, with variable R/H behavior across sex and ethnicity strata. This directional pattern was observed across the spectrum of FMI severity and remained stable across sex and major ethnic groups. Sex and ethnicity modulated mean vector position and ellipse dispersion but did not alter the overarching adiposity-related displacement. The correlation between R/H and Xc/H (r ≈ 0.63–0.75) was preserved in all strata, indicating intact underlying biophysical relationships. In adults with obesity, displacement of BIVA relative to classical reference ellipses primarily reflects the electrophysiological signature of excess adiposity rather than overt fluid overload or intrinsic cellular failure. These findings support the development of obesity-adjusted BIVA reference frameworks, further refined by sex and ethnicity, to improve the specificity and physiological interpretability of raw impedance measurements in obesity care.