Importance of Salt: Exercise-Associated Hyponatremia

Sodium is an electrolyte, that regulates the amount of water that’s in and around the cells. A normal blood sodium level is between 135 and 145 mmol/L. Hyponatremia occurs when the blood sodium concentration is abnormally low. Exercise-associated hyponatremia (EAH) results from a dilution of total extracellular sodium concentration due to a loss of sodium via sweat and urine, when fluid is replenished with a sodium-free solution or hypotonic fluid, such as water.

EAH is defined as a plasma sodium concentration of <135 mmol/L during or after endurance and ultra-endurance performance and was first described by Timothy Noakes when observed in ultra-marathoners competing in the Comrades Marathon in the mid-1980s, a 90-km long road-running race, held in Durban, South Africa (Noakes, Goodwin, Rayner, Branken, & Taylor, 1985). It is well-established that a decrease in plasma sodium concentration <135 mmol/L occurs because of drinking too much water — cause the sodium in the body to become diluted. That leads to cells swelling, witch can  cause life-threatening health problems.

Clinically, a mild hyponatremia will lead to no or very unspecific symptoms such as nausea, headache and fatigue. A pronounced hyponatremia (<120 mmol/L) will lead to central nervous symptoms due to cerebral edema, and respiratory failure such as seizures, decreased consciousness or coma and can lead to death when plasma sodium concentration reaches values of <110–115 mmol/L. The severe hyponatremia can occur during or after prolonged physical exertion for 4–6 h or longer (Hew-Butler, et al., 2005) and can be detected up to 24 h after the end of the exercise (Hew-Butler, et al., 2015)

Ultra- Athletes continue to dye from the complications due to Hyponatremia. The medical reports discribe hyponatremia asociated deaths such as the death of a 22-year-old male fitness instructor who finished the London Marathon in the year 2007 an collapsed and died upon arrival due to EAH (Hughes, 2012). Or t he death of a 30-year-old age triathlete who died after the Ironman Frankfurt in the year 2015 due to EAH with hyponatremic encephalopathy  (Krabel, 2015)

Lately published narrative review from Swiss, Greek an Czech scientists presents new findings about the risk of EAH and the aspects of sex, race location, sports discipline, and length of performance. (Knechtle B, 2019) The prevalence of EAH depends on the duration of an endurance performance (i.e., low in marathon running, high to very high in ultra-marathon running), the sports discipline (i.e., rather rare in cycling, more frequent in running and triathlon, and very frequent in swimming), sex (i.e., increased in women with several reported deaths, which could be caused by the lower body mass or, in very rare cases, the so-called Varon–Ayus syndrome with severe hyponatremia), the ambient temperature (i.e., very high in hot temperatures) and the country where competition takes place (i.e., very common in the USA, very little in Europe, practically never in Africa, Asia, and Oceania. Races in Europe seemed to be held under rather moderate conditions whereas races held in the USA were often performed under thermally stressing conditions (i.e., greater heat or greater cold)) (Knechtle B, 2019).

Prevention of EAH in ultra-endurance performance is of critical importance. The best methods to prevent from fluid overload are drinking according to thirst, reducing the availability of fluids along the routes of exercise and monitoring weight changes during exercise (Hew-Butler, Loi, Pani, & Rosner, 2017). Evidence-based advice for optimal hydration practices to amateur and professional athletes should be encouraged. The   Statement   of   the   third  International   Exercise-Associated   Hyponatremia   Consensus Development Conference in 2015 California states strategies to prevent EAH. Avoiding of over-hydration/over-drinking fluids is a priority, while mild dehydration at 3% of body weight is tolerable, without reducing performance in temperatures between−10 to 20◦C. Drinking when thirsty can be an effective and safe strategy for optimal hydration, as opposed to the potentially dangerous advice “drink as much as possible”. An individualized plan according to the needs of the athlete, based on weight changes during training, may be an effective strategy, although considerations should be made in extreme environments (Hew-Butler, et al., 2015).  It is the amount of fluid ingested rather than the amount of sodium ingested during exercise that drives the final blood sodium concentrations. It is important to know, that sodium-containing sports drinks, which are hypotonic, will not prevent EAH in athletes who overdrink during exercise (Hew-Butler, Loi, Pani, & Rosner, 2017).