Sports Science Update: Dehydrated? No Sweat.

A new study suggests that the fitter you are, the less hydration matters.

It is well known that dehydration detrimentally affects exercise performance and increases thermoregulatory strain during exercise – at least in the exercise science laboratory. But in the real world, athletes routinely experience moderate levels of dehydration in training and competition without any apparent loss of performance. Could it be that dehydration affects exercise performance less in trained than in untrained individuals?

Researchers from the University of Otago in New Zealand recently addressed this question in an intriguing study. Six trained and six untrained subjects each completed two exercise trials on separate occasions. In both trials they rode stationary bikes for 40 minutes at 70 percent VO2peak and then completed as much work as possible (i.e. covered as much virtual distance as possible) in 40 minutes. But they performed one trial in a dehydrated state and the other in a fully hydrated state. Interestingly, heart rate was significantly higher and drifted upward more in untrained subjects during the first, moderate-intensity portion of the dehydrated trial as compared to the hydrated trial, whereas there was no difference between this portion of the two trials in trained subjects. Performance in the maximum performance portion of the exercise trials was negatively affected by dehydration for both groups, but the effect was twice as great for untrained subjects as for trained subjects.

Based on these results, the authors of the study, which was published in the October 6 edition of the British journal Acta Physiologca, concluded, “Mild hypohydration exacerbated cardiovascular and thermoregulatory strain and tended to impair endurance performance, but aerobic fitness attenuated the physiological effects.”