The earliest development of the principles of salt and water balance, in the 17th century, parallels exactly the beginnings of experimental medicine. Until the 19th century there was no developed concept of dehydration as we know it. There was no therapy for it.

The history can be viewed in 3 parts:

1. Earliest experiments

2. The effects of Cholera, leading to therapeutic insights

3. The 20th century advances through improved theoretical insight and the application of increasingly improved methods of measurement, leading to current insights.

The concept of insensible perspiration first appeared in the works of Erasistratus of Chios (~304-250 BC) and the phenomenon was also identified by Galen. Sanctorius (1561-1636 AD) who was born in Istria (Now Croatia), was the first to measure such loss and in so doing literally founded quantitative experimental medicine, supplying numerical measurements of physiological phenomenon for the first time. The technique he used was to prepare a counter balanced platform on which he (or another subject) could sit while separately weighing the input of food and drink and the output of excretory products.

Sodium is a required element for normal body functions. It is lost in sweat and urine and is replaced in the diet. The body has remarkable ability to maintain sodium and water balance throughout a variety of conditions. But ultra-endurance events challenge this survival mechanism. In hot, humid conditions a large amount of sweat is lost which can disturb sodium and water balance. If adequate hydration and sodium intake don't happen then it can cause dehydration.

Hyponatremia means low concentration of sodium in the blood. It occurs in athletes during long or ultra-distance races. It is caused by loss sweat which is replaced by injected water without salt. This dilutes the sodium in the bloodstream resulting in hyponatremia.

Sweat contains 2.25 - 3.4 grams of salt per liter and the rate of perspiration in a long, hot race can easily average 1 liter per hour. If this loss is not compensated for both water and salt then there is a greater risk of hyponatremia or dehydration.

The symptoms of hyponatremia or dehydration can range from mild to severe and can include nausea, muscle cramps, disorientation, slurred speech, confusion, and inappropriate behavior. As it progresses victims may experience seizures or coma, and death can occur.