Strenuous exercise in the heat increases sweating significantly and failure to consume enough water to replace what is sweated out can have serious consequences.

But knowing how much sweat is really being produced may not be intuitive.

Researchers had 39 runners in their 30’s and 40’s complete a 1 hour outdoor run on a challenging course. After the run each was asked to fill race-aid-station cups with as much water as they thought they had sweat out during the run and body weight was measured before and after to determine actual sweat loss.

Runners underestimated volume of sweat lost by about 25% to as much as 75%.

Runners who had measured weight loss during runs in the month prior to the test were no more accurate than those who hadn’t done so.

This suggests that runners may be making serious errors about how much fluid they need to drink during and after runs in hot weather and that pictorial advice might be useful.