Within-day variability of whole-body and segmental bioelectrical impedance in a standing position

To assess the variation in whole-body and segmental bioelectrical impedance measured in the standing position over the course of a day.

Sixteen healthy men aged 29.6 +/- 3.1 years participated in this study.

Impedance between the hands (Z H-H), the feet (Z F-F), and the both hands and feet (Z H-F) was measured using a bioelectrical impedance analysis (BIA) data acquisition system at 500 microA and 50 kHz. Body weight, Z H-H, Z F-F and Z H-F were measured every 3 h from 0900 to 1800.

Body weight was slightly, but significantly, changed at 1200 (-0.4 +/- 0.4 kg, P < 0.05), 1500 (+0.4 +/- 0.3 kg, P < 0.05) and 1800 (-0.2 +/- 0.2 kg, P < 0.05) compared to their respective previous measurement values. Z H-H significantly increased (P < 0.05) and Z F-F significantly decreased (P < 0.05) over the course of a day. The mean changes in impedance from 0900 to 1800 were +27.2 Omega (Z H-H) and -31.8 Omega (Z F-F), respectively. There were no changes in Z H-H at 1500 and in Z F-F at 1800 compared to their respective previous measurement values. The Z H-F value did not change significantly between 0900 and 1800.

Although changes in standing whole-body impedance during the day are small, arm and leg segmental impedance measurements change significantly. Therefore, it is necessary to measure the impedance at regularly scheduled times when evaluating whole-body %fat or body water content using the segmental BIA method.