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Electrolytes, which are substances that conduct electricity when dissolved in water, are essential for healthy bodily function. Most importantly, they help keep the body hydrated, especially after sweating.*
As body temperatures rise during bouts of physical activity, the body will attempt to cool itself by increased sweat secretion and thereby removing heat from the body. 580 kcal of heat is removed per 1 kg of evaporated sweat. In warmer temperatures and/or during prolonged high-intensity exercise sweat losses can be considerable.
Besides fluid, sweat also contains the electrolytes sodium, potassium, calcium, magnesium and chloride. See table 2 for concentrations of electrolytes in sweat. Sodium is the most widely lost sweat electrolyte with a concentration average of ~35 mEq•L-1 . Sodium is typically included in sports drink formulations not only because it is lost via sweat but also because it helps to stimulate thirst and retain water. Additionally, the IOM suggests sports drinks contain ~20-30 meq•L-1 sodium, ~2-5 meq•L-1 potassium and ~5-10% carbohydrate.
Dehydration can cause serious health issues such as heat exhaustion, heat stroke, reduced autonomic cardiac stability, reduced cerebral blood flow and rhabdomyolysis. Hyponatremia can result in athletes that do not replace sodium that is lost in sweat. This is a concern in athletes involved in high intensity, ultra-endurance events that only consume water. Symptoms of hyponatremia include headache, vomiting, swollen hands and feet, restlessness, undue fatigue, confusion and disorientation. Hyponatremia can result in dilutional encephalopathy and pulmonary edema. During severe cases of extremely low plasma sodium levels coma, seizure, respiratory arrest and death can occur. Hyponatremia is occasionally seen in football and tennis players who overdrink water to prevent heat cramps.
Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability
Baker L., Sports Med, 2017
American College of Sports Medicine position stand. Exercise and fluid replacement.
Sawka, M.N., L.M. Burke, E.R. Eichner, R.J. Maughan, S.J. Montain, and N.S. Stachenfeld., Med. Sci. Sports Exerc., 2007