Casa, D. J., Stearns, R. L., Lopez, R. M., Ganio, M. S. McDermott, B. P., Yamamoto, L. M., Yeargin, S. W., Roti, M. W., & Armstrong, L. E. (2008). Influence of hydration at various intensities during trail running in the heat. ACSM 55th Annual Meeting Indianapolis, Presentation Number 832.

This study determined the influence of hydration status at various intensities on physiological responses and performance during trail running in the heat. Competitive, well-trained distance runners (M = 9; F = 8), completed four trials in the heat (~26.5ºC) within two weeks. Experimental conditions were: a) hydrated/race, b) dehydrated/race, c) hydrated/sub-max, d) dehydrated/sub-max. On each occasion, Ss ran three loops (4 km/loop, 12 km total) on trails with a four-minute rest between loops. For the dehydrated/race and dehydrated/sub-maximal conditions, dehydration was achieved by fluid restriction both before and during the run. For race trials, runners were instructed to run as fast as possible, with incentives for best times. In sub-maximal trials, subjects ran at a comfortable pace that was matched each trial by providing feedback about pace four times per loop.

At post-run, the two dehydrated conditions had a greater body mass loss in relation to a three-day baseline versus the hydrated conditions. Ss ran faster for the hydrated-race condition than the dehydrated-race condition but there was no difference between the two sub-maximal runs. Core temperature immediately post-run was greater for the dehydrated versus the hydrated race condition. Core temperature was greater for the dehydrated versus the hydrated sub-maximal run post loop 2, post-run, and 10 and 20 minutes post-run. Heart rate was greater for dehydrated versus hydrated conditions in both race and sub-maximal runs. Thirst, thermal sensation, POMS, rating of perceived exertion, lactate, and urine osmolality were greater for dehydrated versus corresponding hydrated trial post-run.

Implication. During the race and sub-maximal trials, enhanced hydration decreased physiological strain and during the race improved performance. These findings refute previous field studies that minimize the importance of hydration.

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