To function properly, your body needs an assortment of minerals as well as water. Electrolyte minerals in your blood and cells create a constant exchange of fluids, bringing nutrients into your cells and discharging waste products into your bloodstream. When you perspire heavily, you lose large amounts of important electrolytes through your sweat. If you slake your thirst with plain water, you upset your electrolyte balance even more.
Sodium and Chloride
About 66 percent of your total body fluids reside inside cell walls. Sodium and chloride make up most of the electrolytes outside your cells, in your blood. Because sodium carries a small positive electrical charge, it bonds to the oxygen atoms in water molecules. Chloride bonds to water's hydrogen atoms. Charges on higher concentrations of other minerals inside your body's cells attract a greater volume of water. If the mineral balance in both your blood and your cellular fluids stays constant, your body maintains the right fluid balance. As electrolytes leave in sweat and urine, you replace them with minerals in food.
When you lose water through heavy perspiration or normal activities, you don't immediately feel thirst. When the first symptoms of thirst appear, you've already become dehydrated and may have lost as much as two liters of water, according to the Institute of Medicine of the National Academies. Losing water concentrates sodium and chloride levels in your blood, triggering thirst. When you drink water, you feel satisfied before you completely replenish your water reserves. To maintain the right electrolyte balance, your body stores less water. If you drink according to your perceived needs, over the course of a lengthy workout your dehydration will gradually increase.
Intentionally drinking more water than you feel you need could upset your electrolyte balance. Supercharging with water before a workout or drinking excessive amounts during hard training dilutes the sodium and chloride in your blood, causing hyponatremia. Hyponatremia refers to low sodium levels and not an overabundance of water. When blood sodium levels drop below 136 millimoles per liter, you experience muscle cramps, nausea and fatigue, according to the Linus Pauling Institute. Mild hyponatremia makes you disoriented, and you could faint. As sodium levels keep dropping, you risk having seizures and brain damage. Extreme hyponatremia could be fatal.
If you eat the typical modern diet, you ingest more salt than you actually need. Instead of storing salt, as sodium and chloride levels rise in your blood, your kidneys filter out more of these minerals. An adult between 19 and 50 years of age leading an average lifestyle needs only 3.8 g of salt every day -- a little over half a teaspoon. If you exercise strenuously for an hour or more, you lose both electrolytes and water. Drinking a sports drink containing salt, potassium and other important electrolytes maintains your osmotic balance and helps avoid unnecessary fatigue.