Mechanical and physiologic modifiers and bioelectrical impedance spectrum determinants of body composition.

Fat-free mass and total and percentage body fat were determined by dual-energy X-ray absorptiometry (DXA) and total-body and segmental impedance measures were taken at 16 frequencies from 5 to 1300 kHz in a sample of white men and women aged 18-30 y. Plots of total-body and segmental impedance against frequency for each individual indicated that the general shape of these curves was described by the same mathematical function consisting of three components-ai, bi, and ci,-that contain information derived from the individual measurements of impedance summarized across the spectrum of current frequencies. Total-body and segmental multifrequency impedance were significantly correlated with hemoglobin, hematocrit, and serum sodium, potassium, creatinine, and osmolality. Regression models of body composition with total-body or segmental impedance measures at discrete frequencies or the impedance spectrum variables were similar to corresponding findings for impedance models at 50 kHz. The segmental impedance spectrum variables for total and percentage body fat and the ratios of low- to high-frequency impedance from the trunk were significantly associated with total body fatness as measured by DXA.