Summer is here! Temperatures are rising and training will soon be ramping up as the fall marathon season is right around the corner. An important part of getting to the starting line healthy will be to manage the balance between sweat loss and fluid intake. Sounds easy, right?  Run. Get thirsty. Drink. Repeat. If this is you, it might be time to rethink the way you drink.

Using thirst as an indicator of hydration is ineffective. Research has shown that runners typically underestimate the amount of sweat loss and fluid intake during training/racing leading to dehydration of up to 2-5% of body weight.  

In a recent study, a group of 18 experienced marathon runners competed in a 10-mile race where fluids were provided at mile markers 2,4,6, and 8. Runners could drink fluids as much as desired but were not required to drink.  

Upon completion of the event, runners were asked questions on perceived sweat loss and perceived fluid intake.  While the runners were good at estimating the amount of fluid intake during the race, sweat loss was underestimated by an average of 42.5%.  In addition, an average of only 30.5% of sweat loss per runner was replenished by fluids. The lack of an accurate perception of sweat loss lead to 2% body weight loss from dehydration.

Perceived thirst was also measured and only accounted for 20% of the variation of fluid intake.  These findings suggest that thirst was a poor predictor of drinking behavior and it did not guarantee proper intake of fluids.

Instead, researchers suggest measuring body weight changes before and after runs to determine an accurate fluid replacement plan.

Passe D, Horn M, Stofan J, Horswill C, Murray R. Voluntary dehydration in runners despite favorable conditions for fluid intake. Int J Sport Nutr Exerc Metab. 2007;17(3):284-295.

Adam Saloom, SPT