Whether hiking in intense heat or running marathons, one has to be concerned about hyponatremia. Symptomatic hyponatremia is diagnosed as low sodium, and is caused by loss of sodium due to drinking large volumes of water.

In reviewing the literature, there have been many cases of low sodium with excessive fluid intake, especially after prolonged exertion and excessive heat exposure. Pulmonary and cerebral edema is not uncommon in severe cases. Dr. Murray from the U.S. Army directly targets this, citing a 7-year study with soldiers who experienced low sodium after excessive water intake and sweat loss.

There are a variety of guidelines established to decrease your risk for hyponatremia, as well as dehydration. Without going into great detail, a rough guideline to follow when in the wilderness and experiencing excessive heat and exertion: Fluid intake should occur at the rate of approximately one quart of fluid intake per hour, not to exceed 1-1/2 quarts per hour, and not to exceed 12 quarts in a 24-hour period. Of course the amount of heat and humidity, as well as the amount of work being performed within any given hour must be consideration, “work” being defined as running, biking, or hiking. Other factors to consider include acclimatization and conditioning.

Hyponatremia Treatment

Healthcare Provider - Medical Treatment
It is important to differentiate between hyponatremia, heat stroke, and heat exhaustion, as symptoms may present very differently. To simplify, any heat illness would most likely involve an increases core body temperature, typically above 101 degrees Fahrenheit. In hyponatremia, the temperature is usually normal. In heat illness and heat stroke, the treatment of choice is rapidly cooling down of the body by increasing fluid intake. In hyponatremia, fluid must be restricted, with an increase in sodium intake.

References

1. Paul S. Auerbach, Wilderness Medicine
2. Thanks to Dr. Murray for his time spent with Survive Outdoors staff in reviewing hyponatremia.