Impact of Repeated Acute Exposures to Low and Moderate Exercise-Induced Hypohydration on Physiological and Subjective Responses and Endurance Performance

Abstract

This study aimed to examine whether repeated exposures to low (2%) and moderate (4%) exercise-induced hypohydration may reverse the potentially deleterious effect of hypohydration on endurance performance. Using a randomized crossover protocol, ten volunteers (23 years, V˙O_2max: 54 mL∙kg^-1∙min^-1) completed two 4-week training blocks interspersed by a 5-week washout period. During one block, participants replaced all fluid losses (EUH) while in the other they were fluid restricted (DEH). Participants completed three exercise sessions per week (walking/running, 55% V˙O_2max, 40 °C): (1) 1 h while fluid restricted or drinking ad libitum, (2) until 2 and (3) 4% of body mass has been lost or replaced. During the first and the fourth week of each training block, participants completed a 12 min time-trial immediately after 2% and 4% body mass loss has been reached. Exercise duration and distance completed (14.1 ± 2.7 vs. 6.9 ± 1.5 km) during the fixed-intensity exercise bouts were greater in the 4 compared to the 2% condition (p < 0.01) with no difference between DEH and EUH. During the first week, heart rate, rectal temperature and perceived exertion were higher (p < 0.05) with DEH than EUH, and training did not change these outcomes. Exercise-induced hypohydration of 2% and 4% body mass impaired time-trial performance in a practical manner both at the start and end of the training block. In conclusion, exercise-induced hypohydration of 2% and 4% body mass impairs 12 min walking/running time-trial, and repeated exposures to these hypohydration levels cannot reverse the impairment in performance.

Keywords: exercise-induced dehydration; habituation; hypohydration; performance; repeated.

Figure 1

Experimental design of this study. FAM = familiarization session; Base = baseline measurements; 1 h TT = 1 h exercise + 12 min time-trial; 1 h = 1 h exercise bout only; 2% TT = exercise until a loss of 2% body mass has been lost or replaced + 12 min time-trial; 4% TT = exercise until a loss of 4% body mass has been lost or replaced + 12 min time-trial; EUH = euhydrated; DEH = dehydrated.

Figure 2

Exercise-induced body mass losses at the end of the 12 min time-trial (A) and changes in plasma osmolality (B) and plasma volume (C) measured at the end of the fixed-intensity exercise periods while replacing or not 2 or 4% body mass losses, pre- and post-training. Δ = difference. Values are the means ± SD. EUH = euhydrated; DEH = dehydrated; Cond = condition (2 vs. 4%); Hyd = hydration (EUH vs. DEH); Training = pre vs. post. Only the significant effects are reported.

Figure 3

Differences between DEH and EUH during the fixed-intensity exercise periods for heart rate (A), rectal temperature (B), perceived exertion (C) and perceived thirst (D) through exercise time, while replacing or not 2 or 4% body mass losses, pre- and post-training. Δ = difference. Values are the means ± SD. EUH = euhydrated; DEH = dehydrated; Cond = condition (2 vs. 4%); Training = pre vs. post; Time = % of the fixed-intensity exercise completed. Only the significant effects are reported. * p < 0.05 2 vs. 4%.

Figure 4

Distance completed during the 12 min time-trial (A) and percent performance difference (B) while replacing or not 2 or 4% body mass losses, pre- and post-training. Δ = difference. Values are the means ± SD. EUH = euhydrated; DEH = dehydrated; Cond = condition (2 vs. 4%); Hyd = hydration (EUH vs. DEH); Training = pre vs. post. Only the significant effects are reported.