DIFFERENCE OF BODY FAT MEASURED BY MIOELECTRICAL IMPEDANCE AND DUAL ENERGY X-RAY ABSORPTIOMETRY AND THE UNDERLYING FACTORS IN OBESE ADOLESCENTS

M. Kartasurya, J. Eisenkölbl, K. Widhalm

Department of Paediatrics, University of Vienna, Austria

BACKGROUND. Body composition, especially the fat free mass is very important in obesity studies. BIA is a convenient, safe and cheap method for assessing body composition. It measures the body composition not directly, but the body fat mass can be calculated out of the total body water with a formula. As the use of BIA in obese population is not recommende, maybe some adjustments have to be made for measuring the body fat mass accurately. At the moment DXA is the gold standard in measuring body fat mass because it has the best precision. If we know the difference, some adjustments in the formula could be made.

OBJECTIVE. The study shows the difference between body fat mass percentage measured by BIA and DXA method and looks for the influencing factors.

Design/methods. 30 children and adolescence from obese and metabolic disease of outpatient clinic, Department of Paediatrics, University of Vienna, have been included in the study. Bioelectrical Impedance Analyzer BIA 2000-M with Nutri4 software program measured and calculated the body fat percentage of the children. The obese children had to come in the fasting state. They had to lay down on a bed and 4 electrodes were placed on the hand and foot of the dominant side of the body. The resistance and reactance were feeded to the Nutri4 software program together with the weight and the height of the patients. DXA and BIA measurements have been done at the same day if possible.

RESULTS. 14 boys and 16 girls between 6 and 18 years (mean age: 12.3 and 13.16 years) have been measured. The mean difference between the body fat mass percentage measured by BIA and DXA was 4.89 with a standard deviation of 4.00. The BIA value was almost always lower than the DXA value for about 12%. The lower and upper limit of the difference in 95% confidence interval was –6.38 and–3.40. By paired t-test, these results were significantly different (p < 0.001). The correlation between the two measurements is 0.826. The mean percentage of body fat mass measured by BIA was 34.86% ± 7.08% and by DXA 39.75% — 5.63%. The differences were not changed by age and body fat percentage but by sex.

CONCLUSIONS. The results of the study showed that the measurements of body fat percentage were significantly different. BIA measurements were lower than DXA measurements, therefore adjustments on the formula used for calculating the total fat mass in obese children and adolescence are necessary.