Variations in truncal body circumferences affect fat mass quantification with bioimpedance analysis

To test the hypothesis that variations in trunk circumferences influence the accuracy of bioimpedance analysis (BIA) for assessment of percent fat mass (%FM).

%FM was predicted with BIA, and compared with air-displacement plethysmography (ADP) in a small sample of 35 overweight (OW), 21 normal weight and 8 underweight volunteers. Waist and hip circumferences were assessed, and 15 of the OW subjects were measured before and after weight reduction.

BIA and ADP provided similar cross-sectional estimates of group mean %FM (28.9±10.0 and 31.3±13.0%, respectively). However, within individuals, there were large between-method differences (Diff(BIA-ADP)) ranging from -13 to +13 %FM. Furthermore, we found a systematic bias of BIA related to the degree of adiposity. Consequently, %FM and fat mass loss during weight reduction in OW were underestimated with BIA when compared with ADP. Waist and hip circumferences were inversely associated with resistance (R) and reactance (P<0.01), and with Diff(BIA-ADP) (P<0.001). In women, the variability in hip circumference explained 76%, and in men, the variability in waist circumference explained 59% of Diff(BIA-ADP).

Resistance changes associated with variations in trunk circumferences decrease resistance, and therefore impair the accuracy of BIA to assess %FM.