Bioelectrical impedance changes during the menstrual cycle

Numerous physiological factors that may influence estimates of body composition based on bioelectrical impedance analysis (BIA) have been recognized. The two studies to date investigating changes in bioelectrical impedance during the menstrual cycle have produced conflicting results. Male control subjects were included in this study in an attempt to discern variations in bioelectrical impedance associated with the menstrual cycle in women from those due to other physiological variables affecting both sexes. Body weight and bioelectrical impedance (tetrapolar Holtain Analyzer; 50 kHz, 800 μA) were measured two to five times per week for one menstrual cycle (21–34 days) on 42 women (6 taking an oral contraceptive, OC), aged 19.0–34.4 years, and for 22–32 days on 28 men (18.9–24.1 years). Body weight in the non-OC women decreased significantly between menses and the late follicular phase, and increased significantly between the late follicular phase and the premenstruum (paired t test, P < 0.05). These changes were not correlated with matching bioelectrical impedance changes (linear regression, P > 0.05). Bioelectrical impedance in the non-OC women, and body weight and bioelectrical impedance in the OC women and the men, did not differ significantly over the measurement period. Likewise, estimated lean body mass (LBM) and total body water (TBW) derived from BIA remained stable. The intraobserver test-retest reliability (Smeas) of bioelectrical impedance was 12.3 ohm (2.8% of mean bioelectrical impedance) which produced as much as a 10–12% error in estimating LBM and TBW. Measurement error was reduced to 4 ohm in a subgroup where electrode positions were marked with indelible ink. These results indicate that body composition assessments based on BIA measured at 50 kHz are not affected by the menstrual cycle. Am. J. Hum. Biol. 9:155–161, 1997.