Objective: To directly examine the effect of moderate water deficit on the physiologic responses to various exercise intensities in a warm outdoor setting.
Intervention(s): Participants completed four 12-km runs (consisting of three 4-km loops) in the heat (average wet bulb globe temperature = 26.5°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).
Main Outcome Measure(s): 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°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.