In bioelectrical impedance spectroscopy (BIS), it is assumed that the current path is only extracellular at the lowest frequencies and that it is both extra- and intracellular at the highest frequencies. We tested validity of BIS assumptions in bodybuilders who have an increased intracellular volume due to hypertrophy of muscle fibres. The study was observational cross-sectional in a study group of thirty professional bodybuilders compared with thirty control subjects. Resistance (R) and reactance (Xc) vector components fitting the Cole's arc with BIS (SFB3 analyser) were compared with components at 50 kHz frequency. The average Cole's arc in bodybuilders was significantly smaller and shifted to the left in the R–Xc plane (both R and Xc values were smaller at any individual frequency). The ratio of Xc at 5 kHz and Xc at the characteristic frequency was 70 % in bodybuilders and 64 % in control subjects, indicating a huge intracellular flow of the electric current at low frequencies in both groups (expected ratio close to 0 if the current path was extracellular). As a consequence of a common path, the correlation coefficient between R values at 50 kHz and at other frequencies (from 0 to infinity) was 0·94 to 1·00. The correlation between total body water estimated with BIS or with R at 50 kHz was 0·98. Hence, there was equivalence between information provided by the vector components R and Xc at 50 kHz and that provided by 496 correlated vectors that were measured with BIS.