EFFECTS OF HYDRATION CHANGES ON BIOELECTRICAL-IMPEDANCE IN ENDURANCE-TRAINED INDIVIDUALS

Purpose: The purpose of this study was to determine how differences in hydration states and ion content of hydrating fluids affected bioelec trical impedance (BI) and hydrostatic weighing (HW) measurements. Meth ods: Fifteen athletic subjects aged 19-56 yr were recruited. Relative body fat (%), fat-weight (FW), and fat-free weight (FFW) were assessed using BI and HW under normal conditions (N), hypohydration (HPO), reh ydration (RHY), and superhydration (SHY) states. During the RHY and SH Y trial periods, subjects were hydrated with either distilled water or an electrolyte solution (ELS). HPO and SHY levels were set at 38 of e ach person's normally hydrated body weight. Results: Comparison betwee n the distilled water and the ELS trials indicated that hydration solu tion had no effect on BI or HW. Thus, the results presented are the tr ial means of both hydration solutions combined. Both BI and HW were sh own to be highly test-retest reliable (r-values: 0.96 and 0.99, respec tively). The effects of exercise induced HPO followed by RHY on body c omposition values indicated that HW was very stable across measurement periods while BI was not. From N to the HPO state, BI %BF declined fr om 14.4 +/- 5.3% to 12.3 +/- 5.3%, respectively. After RHY, BIA %BF in creased to 15.5 +/- 5.8%. Similar findings occurred when subjects were superhydrated (N-BI = 13.2 +/- 5.3%; SHY-BI = 15.4 +/- 5.66). With a comparison of the intercepts and slopes of KW and BIA for the N and SH Y states, it was clear hydration status significantly affected the int ercepts (HW: 0.37 vs. BI: 1.85) and not the slopes (HW: 1.00 vs BI: 0. 99). As a result, a majority of all fluid changes were interpreted as FW by BI. During HPO, 82% of the weight loss was considered FW while d uring RHY or SHY, 128% and 85% of the water weight regain/gain was con sidered FW. Conclusion: These results indicate that BI is not a valid technique in athletes, especially when wanting to determine body compo sition effects of training/detraining. This study indicates that even small fluid changes such as those that occur with endurance training m ay be interpreted incorrectly as changes in an athlete's body fat cont ent.