Background
A stand-on bioelectrical impedance analysis (BIA) system is a safe, non-invasive, relatively inexpensive method of measuring body composition. However, little is known about how the body composition parameters measured by BIA relate to those determined by dual-energy x-ray absorptiometry (DXA) in an obese, adolescent population.

Objectives
To compare body composition measures from the Tanita stand-on BIA (MC-180MA), determined by a) in-built equations and b) published and derived equations using raw data (resistance (R) and reactance (Xc)), with body composition parameters measured by DXA in obese adolescents.

Methods
66 obese adolescents (age: 10-18y, BMI : 22-48kg/m2) had body composition measured by both BIA (Tanita MC-180MA) and DXA (Lunar Prodigy). Raw data obtained from the BIA, R and Xc at frequencies of 5, 50, 250 and 500 kHz, were used in published bioimpedance spectroscopy¹ and BIA equations². Raw data was also used to generate a new prediction equation for FFM using a double cross-validation method.

Results
Compared to DXA, the BIA in-built equations overestimated fat-free mass (FFM) by 4.3kg [95% limits of agreement -13.9 to 5.3] and underestimated % body fat (%BF) by 10.8% [-11.1 to 35.3]. There were no significant differences in the estimates of body composition parameters determined by published/derived equations using raw data and DXA; the mean differences in FFM and %BF were 0.4-0.9kg and 0.5-0.8%, respectively. However, similar to the BIA in-built equations, all estimates had wide limits of agreement of approximately ±7kg for FFM and ±10% for %BF.

Conclusion
The Tanita BIA (MC-180MA) in-built equations cannot be recommended as a measure of body composition in obese adolescents. However, equations using the raw data can be used to measure body composition in groups, but there is the potential for significant error if used for an individual.