Hyponatremia

A failure to consume sufficient sodium when fluid and sodium losses are high can lead to hyponatremia. The word hyponatremia literally means low (hypo) sodium (Na) in the blood (emia). This condition commonly results from the production of a large volume of sweat, which contains both sodium and water, and the consumption of a replacement fluid that has an inadequate concentration of sodium. The sodium dilution that results leads to a reduced blood volume, which is the cause of the symptoms of hyponatremia. The sodium concentration of sweat varies greatly between individual athletes but typically falls within the range of 2.25 to 3.4 grams of sodium per liter of sweat. Given the volume of sweat that can be lost during a race (may exceed well over 1 liter per hour), it is conceivable that an athlete could lose more than 40 grams of sodium over the course of an Ironman competition. If the fluid replacement the athlete chooses has no sodium or is too low in sodium, hyponatremia can result.

It has been suggested that commonly consumed NSAIDs (aspirin, ibuprofen) and any substances that induce a diuretic effect may alter kidney function in a way that exacerbates the risk of hyponatremia during long-duration events.¹¹ Hyponatremia can be caused by many medications, and athletes should check with their doctors regarding the medications they use. Given the very real risks associated with hyponatremia, all reasonable steps should be taken to avoid this condition.

Hyponatremia becomes physiologically important when free water shifts from the blood to the intracellular space. Cellular edema, although well tolerated by many tissues, is not well tolerated by the brain. Therefore, the serious symptoms of hyponatremia are related mainly to cerebral edema. Hyponatremia symptoms include nausea, cramping, slurred speech, disorientation, and general confusion. If allowed to progress, athletes may experience coma and death. Put simply, hyponatremia is a potentially fatal condition that can be avoided if athletes consume only sodium-containing beverages and avoid plain water during a long-duration event.

Athletes experiencing any of the initial symptoms of hyponatremia, such as muscle cramping, may find that consumption of salty foods and sodium-containing sports beverages satisfactorily resolves the symptoms. However, slurred speech, disorientation, and confusion are serious symptoms that require immediate medical attention.⁷⁸