We've all learned that fluid replacement is critical to replace fluids lost when exercising, but drinking pure water exclusively isn't the safest choice for those who participate in very strenuous or long-duration exercise.
When significant amounts of fluid are lost through high-intensity exercise, replacement with water alone can lead to a chemical imbalance in the body and deficiencies in electrolytes, which are nutrients critical for organ functioning. The electrolytes in our body include sodium, potassium, chloride, calcium and phosphate, but sodium is the substance of most concern when replacing fluids lost through exercising.
Hyponatremia is a condition in which the body's stores of sodium are too low, and this condition can result from drinking extreme amounts of water. Hyponatremia can lead to confusion, lethargy, agitation, seizures, and in extreme cases, even death. Early symptoms are nonspecific and subtle and may include disorientation, nausea, or muscle cramps. The symptoms of hyponatremia may also mimic those of dehydration, so athletes experiencing these symptoms may be given more water to drink, further worsening the condition.
It's important to note that drinking water is a healthy habit, and our bodies absolutely require water to function. Drinking the recommended eight glasses of water per day will not lead to electrolyte imbalances or hyponatremia. Hyponatremia is only a danger when extremely high volumes of fluid are lost and replaced with water - for example, when athletes engage in vigorous activity for a protracted time (such as in marathons or triathlons), or when any strenuous physical activity is carried out in very hot temperatures.
In one study, 62 out of 488 runners in the Boston marathon who gave blood samples at the finish line had abnormally low blood sodium levels. Hyponatremia was more common in the thinnest runners - those with a body mass index (BMI) of about 20 - than in runners of normal weight. Hyponatremia tended not to affect the fastest runners (those who finished the marathon in about two hours) and was more common in those who took four hours or more to finish the race.
Hyponatremia is a medical emergency, and persons with suspected exercise-associated hyponatremia should receive immediate emergency care.
To prevent hyponatremia and electrolyte imbalances, athletes should replace lost body fluid with drinks that contain electrolytes, such as sports drinks.
REFERENCE: Almond CS, Shin AY, Fortescue EB, Mannix RC, Wypij D, Binstadt BA, Duncan CN, Olson DP, Salerno AE, Newburger JW, Greenes DS. Hyponatremia among runners in the Boston Marathon. N Engl J Med. 2005 Apr 14;352(15):1550-6.