People lose water and salts from their bodies at different rates during exercise, he said. Heat and humidity also play a role in the rate of this loss. Calculating fluid loss is as simple as weighing yourself before and after exercise and comparing that number to the amount of fluid you consumed throughout.
"All serious distance athletes should find out what their rate of fluid loss is and individualize their fluid intake prior to a distance event," Dr. Levine said. "It's also good to accept some mild dehydration during a long race. There are plenty of Web sites available now that show how to customize your fluid intake."
He also added that taking along salty snacks to eat during the race is a good way of combating hyponatremia. Generally, athletes of all types are instructed prior to activities that water consumption is necessary to prevent illness from heat and to maintain performance levels.
It is also clear, however, that fixed global recommendations for fluid replacement may not be optimal for individual athletes of different body types and with varying degrees of training and heat acclimatization.