Sweat is comprised mainly of water and sodium and chloride ions. These ions are known as electrolytes. Other electrolytes are also present in small amounts. Studies of electrolyte balance during and after exercise have shown increases in the electrolytes in the blood, but these changes are probably due to water loss and muscle use.
Electrolyte deficiency probably does not occur during marathon running. It is during the recovery period after prolonged sweating when the content of electrolytes in the blood has been shown to be lower. Electrolytes should be replaced following the run. Studies of marathon running have shown that the most important factor is to replace body water lost during the run. Small amounts of glucose taken throughout the run may be helpful as well.
If running in the heat for several consecutive days, try to replace fluids and eat a balanced diet. Add salt to foods and select foods high in potassium such as bananas and citrus fruits. Salt tablets are unnecessary and may be harmful when not taken with adequate water.