Skin temperature and skin blood flow affect bioelectric impedance study of female fat-free mass

This study examined the effects of skin temperature (Ts) and skin blood flow (SKBF) on bioelectrical impedance (BIA) measurements of body composition in healthy young females. The Lukaski (FFM(LUK)) and Guo (FFM(GUO)) BIA equations for measuring female fat-free mass (FFM) were used. All subjects (N = 20, age = 18-22 yr) underwent the following measurements under three ambient temperatures (T(amb)): Ts and SKBF for the calf, thigh, biceps, and chest; oral temperature (T(OR)); and the BIA measures of resistance (R) and reactance (Xc). The three T(amb) were 17, 25, and 35 degrees C which were considered as cold (CT), neutral (NT), and hot (HT) conditions, respectively. Their underwater weighing (UWW), lung residual volume, and skinfold thickness were measured in the NT. Data were analyzed using repeated measures of ANOVA and Tukey post-hoc test. We observed that in the CT mean SKBF and Ts decreased (P < 0.05) and R and Xc increased (P < 0.05), compared with those in both NT and HT. However, in the HT both SKBF and Ts increased and R deceased, but Xc remained unchanged relative to the NT. In these subjects, a net change in Ts of 17 degrees C resulted in a net change in the BIA measure of R of 46 ohms or 2.5 ohms per degree C. These changes affected the estimate of FFM(LUK) between CT, NT, and HT, but not the estimate of FFM(GUO) Regarding the two BIA equations for estimating FFM, the Guo equation underestimated FFM(UWW) (P < 0.05) in the CT, NT, and HT, and the Lukaski equation underestimated FFM(UWW) (P < 0.05) only in the CT, compared with that in the UWW technique. Ambient temperature affects Ts and SKBF which in turn influence the BIA measures of R and Xc, especially in the cold ambient temperature; the Guo BIA equation consistently underestimated FFM of young nonobese Chinese women in all temperatures; and the Lukaski equation closely approximates the FFM in the neutral and hot conditions compared with the FFM(UWW).