Abstract
Objective
It is commonly advised that individuals should avoid losing >2% of their body weight during exercise because dehydration can ultimately lead to hyperthermia, which will impair performance. However, body weight losses of ≥3% appear to be well tolerated in those participating in prolonged endurance exercise. This study sought to determine the relationship between hydration status and thermoregulation of runners participating in a hot ultramarathon and to assess whether a weight loss of >2% body weight leads to elevations in core temperature.
Methods
Thirty runners of the 2014 161-kmWestern States Endurance Run agreed to take part in the study. Body weight and core temperature were measured at the start, at 3 locations during the race, and at the finish.
Results
Of the 30 starting participants, 20 finished the race. Ambient temperature at data collection sites ranged from 17°C to 39°C, and the highest core temperature recorded was 39.4°C. Core temperature did not significantly differ during the race (P = .77) and was not correlated with body weight change (P = .19) or finish time (P = .11). However, body weight change was directly associated with finish time (r = .60, P = .006), such that the fastest runners had the greatest loss in body weight (∼3.5–4.0%). Neither core temperature nor percent body weight change differed between race finishers and non-finishers (P = .63 and .17, respectively).
Conclusions
While participating in endurance exercise up to 30 hours in hot conditions, runners can tolerate weight losses of >3% body weight without significant rises in core temperature. Therefore, since body weight losses of >3% were not associated with a rise in core temperature, an emphasis on fluid replacement for weight losses of this magnitude during prolonged exercise is not justified as a preventative measure for heat-related illnesses.
Supported by the Western States Endurance Run Foundation.