Bioelectrical impedance analysis in the assessment of body composition – methodological considerations
Bioelectrical impedance analysis (BIA) has become a widely used method for body composition analysis. Body compartments like fat mass (FM), visceral adipose tissue, fat-free mass or skeletal muscle mass can be estimated and improve the diagnosis of obesity and malnutrition. Impedance raw data, like the phase angle, give additional information on tissue quality and the prognosis of the patient. There are, however, challenges that can impair the assessment of BIA measurements. (i) Body composition differs among ethnicities. We found a higher percentage of FM in Mexicans compared to Caucasians with the same BMI. FM percentage is similar in normal weight Japanese and Mexicans, whereas in overweight Japanese it is similar compared to Caucasians for men and women with the same BMI. These differences may lead to differences in obesity related cardiometabolic risk. We also found ethnic differences in muscle mass distribution and impedance raw data. (ii) In obese subjects, the assessment of body composition by two compartment methods is limited, because assumptions become inaccurate with higher amount of adipose tissue. FM is therefore underestimated by BIA and dual X-ray absorptiometry and it is overestimated by air displacement plethysmography. Corrections for BIA equations were developed. (iii) Impedance raw data are dependent on the measurement configuration used by a BIA device. Phase angle is higher when measured in supine compared to standing position and it is higher when measured on the right side of the body compared to the left side. The placement of the electrodes also has an impact on the phase angle. In conclusion, obesity related cardiometabolic risk may vary among different ethnicities due to differences in FM percentage for subjects with the same BMI. Ethnic differences in body composition and differences in phase angle between different measurement configurations must be considered in the assessment of BIA results. In obesity, BIA equations for FM and FFM should be validated against a multi-compartment model in a reference population that includes obese subjects.