No effect of 5% hypohydration on running economy of competitive runners at 23°C

Purpose: Although running economy (RE) is recognized as an integral component of successful endurance performance and is affected by numerous factors, little is known about the influence of body water loss on RE. This investigation examined the effects of hypohydration (HY) on RE and associated physiological responses. Methods: Ten highly trained collegiate distance runners (mean ± SD; age, 20 ± 3 yr; height, 178.5 ± 6.3 cm; body mass, 66.7 ± 5.4 kg; V̇O_2max, 66.5 ± 4.1 mL·kg^-1·min^-1) participated in four experiments on separate days, twice in a euhydrated (EU) and twice in a HY state (-5.5 and -5.7% body mass loss achieved during 24 h). At each hydration level, subjects performed one 10-min treadmill run per day (23°C environment), at either 70% V̇O_2max (EU 70% or HY 70%) or 85% V̇O _2max (EU 85% or HY 85%) in a randomized, repeated-measures design. Cardiopulmonary, metabolic, thermal, hormonal, and perceptual variables were measured. Results: No between-treatment differences existed for RE (EU 70%, 46.3 ± 3.2; HY 70%, 47.2 ± 3.8; EU 85%, 58.6 ± 2.8; HY 85%, 58.9 ± 4.1 mL·kg^-1·min^-1), postexercise plasma lactate concentration (EU 70%, 1.9 ± 0.6; HY 70%, 1.8 ± 0.6; EU 85%, 6.5 ± 3.5; HY 85%, 6.4 ± 3.5 mmol·L^-1), or rating of perceived exertion. HY resulted in a greater (P < 0.05 to 0.001) heart rate (HR), rectal temperature, and plasma norepinephrine concentration (NE), concurrent with reduced cardiac output, stroke volume, and respiratory exchange ratio. Conclusion: HY did not alter the RE or lactate accumulation of endurance athletes during 10 min of exercise at 70 and 85% V̇O_2max. These findings indicate that HY had no effect on RE, but that it increased physiological strain in a 23°C environment.