Body composition analysis by bioelectrical impedance: effect of skin temperature

Bioelectrical impedance analysis (BIA) was used to estimate body water and composition under both cool (14.4 degrees C, dry bulb) and warm (35.0 degrees C) ambient conditions in eight healthy adult men. The prediction equation provided with the commercially available instrument (RJL Systems) was used with the BIA measurements to estimate body composition. Skin temperature increased from 24.1 +/- 1.81 degrees C in the cool condition to 33.4 +/- 1.36 degrees C in the warm condition. (Mean increase was 9.3 +/- 1.75 degrees C, t = 15.05, P less than 0.01). The corresponding BIA resistances were 461 +/- 48 omega and 426 +/- 47 omega, respectively. (Mean reduction was 35.0 +/- 9.8 omega, t = 10.13, P less than 0.01). This resulted in a significant increase in predicted total body water (cool 47.4 +/- 5.5 l vs. warm 49.9 +/- 5.6 l, t = 3.88, P less than 0.01). Consequently, predicted fat mass was significantly lower in the warm than in the cool condition (8.8 +/- 3.2 kg vs. 11.0 +/- 3.7 kg; mean difference 2.23 +/- 0.69 kg, t = 9.22, P less than 0.01). These findings indicate that varying skin temperature by altering ambient temperature significantly changes resistance measurements and the estimation of total body water and percent fat by BIA. The observed changes in resistance are consistent with an apparent expansion of conductor volume in the warm environment and a reduction in the cooler condition. In this regard, the temperature-induced change in resistance could be due to alterations in cutaneous blood flow and/or compartmental distribution of body water. Thus, BIA measurements should be taken only under well-standardized ambient conditions.