Likewise, when you sweat heavily for an extended period, you’re not losing just plain water. You’re also losing a significant amount of sodium, which needs to be maintained within a certain range to avoid potentially serious problems like exercise-induced hyponatremia (a serum sodium concentration <130 mEq/L), which can also occur if you drink too much plain water, in a short time period. It isn’t very easy to give yourself hyponatremia: An otherwise healthy person would have to combine hours of heavy sweating with drinking significantly more water than they’re actually losing during the exercise and not replacing the lost sodium. You can avoid this by making sure your water intake during and immediately after exercise is equal to the amount you lose (see above about weighing yourself).
For particularly long endurance workouts OR exercise sessions in hot and humid weather, one may need more than the Tolerable Upper Intake Level for sodium (2,300 milligrams daily). That's why commercial sports drinks are often recommended for athletes taking part in such endurance events. Make sure the sports drink contains about 120–170 milligrams of sodium per 8-ounce serving. If you are looking for a less expensive option for the commercial sports drink, try this simple recipe. You may also need to replace lost sodium by consuming high-sodium foods and beverages following the event. High sodium food choices could include: canned soups, beans and vegetables; boxed side dishes like macaroni and cheese, potato mixes, or rice blends; Oriental style entrees with soy sauce; pizza; jarred spaghetti sauce; salted nuts; and regular cheese.
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Wedro, Benjamin C. "Heat Exhaustion," accessed July 2011. MedicineNet.com.