Background – Although obesity is primarily an increase in the fat mass of the body the condition is also associated with changes in body fluid content. In the morbidly obese (BMI > 40 kgm-2 ) the extracellular water compartment
(ECW) is often expanded such that the ECW:ICW (intracellular water) ratio is increased (> 0.8) and may remain so
even after weight reduction.
objective – To determine the size of body fluid compartments in obese, but not morbidly obese,subjects.
Design – Whole body impedance was measured in 92 obese (mean BMI = 34.9 kgm ) but otherwise healthy females using multiple frequency bioimpedance analysis (Bodystat Quadscan 4000). Total body water (TBW), ECW and ICW volumes were determined according to the mixture theory model of body fluid volumes using different published values for fluid resistivity constants and also using the proprietary Bodystat Quadscan software. Fat and fat-free mass measurements were simultaneously obtained using DXA analysis.
Outcomes – Predicted TBW volumes varied slightly depending upon the specific resistivity constants used for calculation but averaged 37.2 L or 40 % of body weight (BW). This compares to a TBW of 50-60 % BW commonly observed in normal weight individuals. Irrespective of the method of calculation, the mean ECW:ICW ratio was 0.62, typical of that found in normal weight subjects.
Conclusions – Despite the subjects being obese, with a body fat content approaching half (45.7 %) of body weight, this study showed no evidence of expansion of ECW compared to normal individuals unlike that previously reported for the morbidly obese.