Bioelectrical impedance variation in healthy subjects during 12 h in the supine position

Background and aim: Bioelectrical impedance analysis is used to assess human body composition. Studies have shown that meal ingestion and change of body posture affects bioelectrical impedance, but none has studied bioelectrical impedance variation in supine subjects. The aim was to examine the bioelectrical impedance variation in healthy subjects during 12 h in the supine position. Methods: Bioelectrical impedance was measured 16 times during 12 h in 18 healthy subjects. An identical meal was given at breakfast, lunch, and dinner. Results: Mean (standard deviation) impedance at 50 kHz increased from 558 (87) Ω at study start to 584 (95) Ω at study end (P<0.05). Bioelectrical impedance is reduced after ingestion of the first meal, but not following the meals at 1230 and 1730. Calculated body fat content increased from a baseline mean (SD) of 21.7 (6.1) % body fat to 23.9 (6.7) % body fat at study end (P<0.05). Conclusions: Bioelectrical impedance increased during 12 h in supine subjects. The increase is probably explained by a shift in body fluids from the extremities to thorax during the day and the importance of strict measurement standardisation both in epidemiological studies and clinical practice is underlined.