So how can you strike a balance between dangerous salt loss from overhydration and dehydration? Almond and colleagues note that individuals vary widely in their need for water and in the rate at which they lose water. The researchers suggest that runners weigh themselves before and after practice races. If you weigh more after a race than before the race, you drank too much. Adjust your fluid intake accordingly, preferably with sodium-containing fluids that replace salt lost from sweating.
Of course, weather conditions play a major role. It's a good idea to train and to test for appropriate fluid intake in the same weather as you expect for your race.
Levine and Thompson point to the advice of USA Track and Field: Use thirst as your guide to fluid replacement.
But the American College of Sports Medicine (ACSM) has a different point of view.
They say that thirst is often a poor indicator of our body's fluid needs.
"Water in particular quenches the sensation of thirst before body fluid replacement is achieved, so thirst should not be the only determinant of how much fluid is consumed under such conditions," says the ACSM.