Influence of hydration on physiological function and performance during trail running in the heat.
BACKGROUND
Authors of most field studies have not observed decrements in physiologic function and performance with increases in dehydration, although authors of well-controlled laboratory studies have consistently reported this relationship. Investigators in these field studies did not control exercise intensity, a known modulator of body core temperature.


OBJECTIVE
To directly examine the effect of moderate water deficit on the physiologic responses to various exercise intensities in a warm outdoor setting.


METHODS
Semirandomized, crossover design.


METHODS
Field setting.


METHODS
Seventeen distance runners (9 men, 8 women; age = 27 +/- 7 years, height = 171 +/- 9 cm, mass = 64.2 +/- 9.0 kg, body fat = 14.6% +/- 5.5%).


METHODS
Participants completed four 12-km runs (consisting of three 4-km loops) in the heat (average wet bulb globe temperature = 26.5 degrees C): (1) a hydrated, race trial (HYR), (2) a dehydrated, race trial (DYR), (3) a hydrated, submaximal trial (HYS), and (4) a dehydrated, submaximal trial (DYS).


METHODS
For DYR and DYS trials, dehydration was measured by body mass loss. In the submaximal trials, participants ran at a moderate pace that was matched by having them speed up or slow down based on pace feedback provided by researchers. Intestinal temperature was recorded using ingestible thermistors, and participants wore heart rate monitors to measure heart rate.


RESULTS
Body mass loss in relation to a 3-day baseline was greater for the DYR (-4.30% +/- 1.25%) and DYS trials (-4.59% +/- 1.32%) than for the HYR (-2.05% +/- 1.09%) and HYS (-2.0% +/- 1.24%) trials postrun (P < .001). Participants ran faster for the HYR (53.15 +/- 6.05 minutes) than for the DYR (55.7 +/- 7.45 minutes; P < .01), but speed was similar for HYS (59.57 +/- 5.31 minutes) and DYS (59.44 +/- 5.44 minutes; P > .05). Intestinal temperature immediately postrun was greater for DYR than for HYR (P < .05), the only significant difference. Intestinal temperature was greater for DYS than for HYS postloop 2, postrun, and at 10 and 20 minutes postrun (all: P < .001). Intestinal temperature and heart rate were 0.22 degrees C and 6 beats/min higher, respectively, for every additional 1% body mass loss during the DYS trial compared with the HYS trial.


CONCLUSIONS
A small decrement in hydration status impaired physiologic function and performance while trail running in the heat.
Casa
Douglas J
DJ
Department of Kinesiology, University of Connecticut, Storrs, CT 06269-1110, USA. Douglas.casa@uconn.edu
Stearns
Rebecca L
RL
Lopez
Rebecca M
RM
Ganio
Matthew S
MS
McDermott
Brendon P
BP
Walker Yeargin
Susan
S
Yamamoto
Linda M
LM
Mazerolle
Stephanie M
SM
Roti
Melissa W
MW
Armstrong
Lawrence E
LE
Maresh
Carl M
CM
D000328:Adult; D000704:Analysis of Variance; D054874:Athletic Performance / Q000502:physiology*; D015992:Body Mass Index; D001831:Body Temperature / Q000502:physiology; D018592:Cross-Over Studies; D003681:Dehydration / Q000503:physiopathology*; D004326:Drinking; D005260:Female; D006358:Hot Temperature; D006801:Humans; D008297:Male; D012420:Running / Q000502:physiology*; D011795:Surveys and Questionnaires