Proper hydration is imperative for athletes striving for peak performance and safety, however, the effectiveness of various fluid replacement strategies in the field setting is unknown. The purpose of this study was to investigate how two hydration protocols affect physiological responses and performance during a 20-km trail running race. A randomized, counter-balanced, crossover design was utilized in a field setting (Mean ± SD: WBGT 28.3 ± 1.9°C). Well-trained male (n=8) and female (n=5) runners (39 ± 14y; 175 ± 9 cm; 67.5 ± 11.1 kg; 13.4 ± 4.6% BF) completed two 20-km trail races (5 x 4-km loop) with different water hydration protocols: (1) ad libitum (AL) consumption and individualized rehydration (IR). Data were analyzed using repeated measures ANOVA. Paired t-tests were used for pre-post measures.
MAIN OUTCOME VARIABLES:
Time, heart rate (HR), gastrointestinal temperature (TGI), fluid consumed, percent body mass loss (BML), and urine osmolality (Uosm). Race times between groups were similar. There was a significant condition x time interaction (P= 0.048) for HR, but TGI was similar between conditions. Subjects drank 30 ± 14% of their water losses in AL and 64 ± 16% of their losses in IR (P<0.001). AL experienced greater BML (-2.6 ± 0.5%) compared to IR (-1.3 ± 0.5%; P<0.001). Pre- to post-race Uosm differences were different between AL (-273 ± 146 mOsm) and IR (-145 ± 215 mOsm, P= 0.032). IR drank twice as much fluid than AL during the 20-km race, leading to impaired hydration in AL (>2%BML). Ad libitum drinking resulted in 1.3% greater BML over the 20-km race, which resulted in no thermoregulatory or performance differences from IR.