MECHANICAL AND PHYSIOLOGICAL MODIFIERS AND BIOELECTRICAL-IMPEDANCE SPECTRUM DETERMINANTS OF BODY-COMPOSITION

Fat-free mass and total and percentage body fat were determined by dua l-energy X-ray absorptiometry (DXA) and total-body and segmental imped ance measures were taken at 16 frequencies from 5 to 1300 kHz in a sam ple of white men and women aged 18-30 y. Plots of total-body acid segm ental impedance against frequency for each individual indicated that t he general shape of these curves was described by the same mathematica l function consisting of three components-a(i), b(i), and c(i)-that co ntain information derived from the individual measurements of impedanc e summarized across the spectrum of current frequencies. Total-body an d segmental multifrequency impedance were significantly correlated wit h 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 impedanc e models at 50 kHz. The segmental impedance spectrum variables for tot al and percentage body fat and the ratios of low- to high-frequency im pedance from the trunk were significantly associated with total body f atness as measured by DXA.