¹Institute of Internal Medicine, Division of Nephrology, University of Padova

²Endocrine Unit, San Giuseppe IRCCS Hospital, Piancavallo (Verbania)

³Chair of Endocrinology II, University of Milano, Italy

^aCorrespondence: Professor Antonio Piccoli, Istituto di Medicina Interna, Via Giustiniani, 2, I-35128 Padova, Italy. E-mail: apiccoli@ux1.unipd.it

OBJECTIVE: Conventional body composition methods may produce biased quantification of fat and fat-free mass in obese subjects, due to possible violation of the assumption of constant (73%) tissue hydration. We used an assumption-free, graphical method for interpreting body weight variation in obesity using bioelectrical measurements.

DESIGN: 540 obese subjects with body mass index (BMI) >31 kg/m² without apparent edema were compared to 726 healthy subjects with BMI <31 kg/m² and to 50 renal patients with apparent edema. A subgroup of 48 obese subjects were evaluated again after weight loss (8.6 kg, 3 BMI units) following one-month energy restriction (5 MJ/d, 1200 kcal/d). 32 obese uremic patients were evaluated before and after a dialysis session (3.2 kg fluid removed). Direct measurements obtained from standard 50 kHz frequency bioelectrical impedance analyzer were used as impedance vectors in the Resistance-Reactance Graph.

RESULTS: 1) Impedance vectors of obese subjects could be discriminated from those of edematous patients with 91% correct allocation; 2) A significant lengthening of vectors was observed after fluid loss of 3 kg in obese subjects; but 3) A body weight loss of about 9 kg after energy restriction was associated with no vector displacement.

CONCLUSION: A different impedance vector pattern was associated with body weight loss in obesity due to fluid removal (vector lengthening) versus an energy-restricted diet (no vector displacement).

bioelectrical impedance; weight loss; energy restriction; hemodialysis; obesity; edema