Purpose of review: Several factors limit the use of bioelectrical impedance analysis as a valid predictor of the amount of body fat in morbidly obese individuals. The purpose of this review is to examine the theory and assumptions that may limit the use of bioelectrical impedance analysis in such individuals.
Recent findings: There is currently insufficient validation of bioelectrical impedance analysis equations in obese individuals with body mass indices greater than 34 kg/m. Several factors limit the application of bioelectrical impedance analysis in morbidly obese individuals. Obese individuals have a relatively high amount of extracellular water and total body water, which may overestimate fat-free mass and underestimate fat mass. Central body fat will generally overestimate the percentage of fat-free mass and underestimate the percentage of fat mass in overweight and obese adults with the use of prediction formulas developed in normal weight individuals.
Summary: A relatively increased amount of total body water and a relative increase in extracellular water will result in an underestimation of the percentage of body fat and an overestimation of fat-free mass in the morbid obesity state. A different body build disposition (mainly in those with severe abdominal obesity) will result in an overestimation of the percentage of body fat. New equations are needed to validate bioelectrical impedance analysis in morbidly obese patients.