Sweat losses vary greatly from one person to another, from one to three quarts of water per hour. Although it is not essential to replace every drop of sweat lost, excessive loss can result in a medical emergency. Some marathon runners attempt to store up water the day before the event by drinking great amounts of water. However, the body just passes this extra water into the urine. This also can increase the loss of some important electrolytes like the sodium found in salt.
Most runners lose large amounts of salt in their sweat.
Typically, runners have enough salt in their bodies to get through a marathon. Those at greatest risk of hyponatremia take longer than four or five hours to finish the marathon. Low blood sodium causes disorientation, muscle cramps, seizures, even death. Consequently, during the days prior to a marathon the diet should include additional salt. During the marathon, both water and a sports beverage are offered to participants.