BIOELECTRIC IMPEDANCE AND HYDROSTATIC WEIGHING WITH AND WITHOUT HEAD SUBMERSION IN PERSONS WHO ARE MORBIDLY OBESE

Objective To compare hydrostatic weighing with and without head submer sion and bioelectric impedance analysis (BIA) for measurement of body composition of persons who are morbidly obese. Design Body composition was determined using 3 methods: hydrostatic weighing with and without head submersion and BIA. Residual volume for the hydrostatic weighing calculation was determined by body plethysmography. Subjects Subjects were 16 morbidly obese men (142.5 kg mean body weight) and 30 morbidl y obese women (125.9 kg mean body weight) living in the Salt Lake Coun ty, Utah, area. Morbid obesity was defined as 40 kg or more over ideal weight. Statistical analysis One-way, repeated-measures analysis of v ariance was followed by Scheffe post hoc tests; body-fat measurement m ethod served as the repeated variable and percentage of body fat as th e dependent variable. Men and women were analyzed separately. In addit ion, degree of agreement between the 3 methods of determining body com position was determined. A regression equation was used to calculate b ody density for hydrostatic weighing without head submersion. Two new BIA regression equations were developed from the data of the 16 men an d 30 women. Results Values for percentage body fat from hydrostatic we ighing with and without head submersion (41.8% vs 41.7%, respectively) were the same for men but differed for women (52.2% vs 49.4%, respect ively, P<.0001). Values for body fat percentage measured by BLA were s ignificantly lower for men (36.1%) and women (43.1%) (for both, P<.000 1) compared with values from hydrostatic weighing methods. BIA underpr edicted percentage body fat by a mean of 5.7% in men and 9.1% in women compared with the traditional hydrostatic weighing method. Applicatio ns/conclusions BIA tended to underpredict the measurement of percentag e body fat in male and female subjects who were morbidly obese. Hydros tatic weighing without head submersion provides an accurate, acceptabl e, and convenient alternative method for body composition assessment o f the morbidly obese population in comparison with the traditional hyd rostatic weighing method. In population screening or other settings wh ere underwater weighing is impractical, population-specific BIA regres sion equations should be used because general BIA equations lead to co nsistent underprediction of percentage body fat; compared with hydrost atic weighing.