Hyponatremia, defined as a low concentration of sodium in the bloodstream, is more commonly known as "water intoxication," a condition that tends to occur most commonly in athletes competing in endurance sports. Hyponatremia also arises in conjunction with certain forms of kidney disease and medication use.

Hyponatremia is connected to both dehydration and sodium levels within the body during endurance exercise. The body maintains a relatively constant balance in the sodium level present in the blood, in all conditions. The regulation of sodium is carried out in a number of mechanisms, beginning with the nephron, a center forming part of the kidney. The kidney is a part of the excretory system that produces urine, an important aspect of how the body regulates its fluid and sodium levels.

The initial signal to initiate any changes in the sodium level present in the body occur in the hypo-thalamus, the portion of the brain that serves as a signal and command center. The hypothalamus directs the thyroid gland regarding changes that it senses within the fluid system; these changes are acted on by the nephron, in determining whether further sodium will be retained in the organ to maintain sodium balance, or whether sodium will be released.

In circumstances where the entire body is under stress, such as in the performance of an endurance event, dehydration is a common fact. When the body ingests fluids, typically water, without its proper sodium level in place, the proportion of sodium in the system is correspondingly reduced. As sodium is essential in its role as an electrolyte for the transmission of the messages of the central nervous system to the musculoskeletal system, reductions in sodium create malfunctions throughout the body.

The symptoms of hyponatremia are similar to those caused by dehydration: confusion and disorientation, accompanied difficulties in muscle function, and nausea. If untreated, hyponatremia can lead to the onset of a coma, and death. It is relatively common for some competitors in extreme endurance competitions such as the Hawaii Ironman event, to simultaneously experience both dehydration and hyponatremia: the athletes reach the end of the second stage, the bicycle portion, experiencing a feeling of thirst and the sensation of a bloated stomach that makes the competitors feel nauseous.

The chemical composition of perspiration and the cardiovascular system assist in the understanding of sodium imbalance in endurance sport. Perspiration contains between 2.0 and 3.5 g of salt per quart (liter) when released from the body. A common rate of perspiration among endurance athletes is 1 qt (1 l) or greater per hour of competition; in an event such as the Ironman, where an average skilled competitor will take 12 hours to finish the race, an athlete will lose over 12 quarts/liters of fluids through perspiration, a sodium loss totaling between 24 and 42 grams.

As with athletic hydration strategies, the consumption of appropriate amounts of sodium in the days leading up to an event is essential. It is impossible to effectively consume sufficient sodium by way of sports drinks to address a sodium shortfall during the event; most sports drinks would require the consumption of as much as two quarts/liters of their fluid to permit the ingestion of 1 gram of salt. A target for athletes during Ironman-type sports is to ingest 1 gram of sodium per hour of competition.

An alternative cause of hyponatremia is the ingestion of excessive amounts of fluids. In the first great popular running boom of the late 1970s and early 1980s, the common wisdom regarding hydration tactics was to consume as much water or fluids as possible during the course of a marathon or endurance event. When the amount of fluid ingested exceeded the amounts lost to perspiration (a relatively common occurrence in cooler temperatures), the sodium levels in the body became proportionately less. As it is the proportion of sodium to the fluids in the body that determines how water levels will be regulated, and not the absolute amount of sodium that dictates proper system function, the risk of hyponatremia was greatly increased.

As pronounced as the physical symptoms of hyponatremia may appear, the condition is capable of a ready cure. If salt is ingested, in the form of a salt and water mixture, a sport drink with sodium added, or through a food such as soup with salt, the sodium balance will begin to readjust to its healthy levels within minutes. If treated quickly, hyponatremia will not cause any long-term negative effect, not will it damage any of the internal organs involved in the maintenance of the sodium and water balance, such as the kidneys.

SEE ALSO Hydration strategy in distance running; Sodium and sodium deficits; Thermoregulatory system.