Changes in bioelectrical impedance during fasting.

Six obese females had daily measurements of bioelectrical impedance during a 2-wk fast and measurement of total body water (TBW) by deuterium oxide dilution before and at the end of the fast. The three related measures of resistance (R), reactance (Xc), and impedance (Z) were found to follow very similar patterns throughout the fast, increasing for the first 7 d and remaining stable for the second 7 d. Weight loss averaged 10 kg and R increased whereas height squared divided by resistance (ht2/R) decreased during the fast with opposite changes during refeeding. Changes in resistance were significantly greater during the first week of fasting than during the second. With the exception of the subject who was the most obese, changes in ht2/R correlated well with changes in TBW (r = 0.94). It is concluded that bioelectrical impedance measurements accurately reflect changes in TBW during a 2-wk fast.