Hyponatremia is a rare, but devasting consequence of hydrating too much without salt replacement. Fortunately, rehydrating right makes hyponatremia easy to prevent and can boost performance.
In 2005, a study published in the New England Journal of Medicine highlighted the prevalence of hyponatremia in marathon runners (Hyponatremia among Runners in the Boston Marathon, 2005. Christopher Almond, et al.). The study found that thirteen percent of Boston marathon finishers had hyponatremia and 0.6 percent of Boston marathon finishers had “critical" hyponatremia. They also found that slower runners had a higher prevalence of hyponatremia. However, at the Boston marathon slow is relative. Weight gain during the race (replenishing fluids faster than they are lost) was also a predictor of hyponatremia. So what is hyponatremia, and why is it harmful and even potentially fatal?
Hyponatremia is when sodium concentration in the blood drops below the normal physiological range. A physiological normal concentration of sodium is 135-145 meq/L, but during hyponatremia the sodium concentration can decrease to 130 meq/L and in severe cases can go below 120 meq/L. In athletes, this occurs when fluids, but not solutes, are replaced as they are lost in sweat. Sweat is hypoosomotic, meaning that it contains salt but at a lower concentration than what is found in the blood. A marathoner who sweats four liters of sweat with 40 meq/L of sodium loses about 160 meq of sodium. If the marathoner replaces the lost fluid with four liters of pure water, his or her sodium concentration will be significantly reduced.
So why is hyponatremia bad for us? The body relies on a fine balance of solutes between the cells and blood. If the solute concentrations are not balanced inside and outside our cells, fluid will flow down its concentration gradient and into or out of the cells in a process called osmosis. Furthermore, salt balance is essential for any organ that relies on salt gradients to maintain an electrical gradient and conduct electrical signals (i.e. the heart and brain, two very important organs!). The primary concern of hyponatremia is cerebral edema from water being sucked from the blood into neurons and other cells in the brain. This occurs because the dilute blood is sending water osmotically into the concentrated cells (acute hyponatremia leads to loss of solute in the blood and extracellular fluid, but not inside the cells). Because the brain cannot expand in the skull, intracranial pressure increases, similar to the effect of a intracerebral tumor or hydrocephalus.
At a hyponatremic serum sodium concentration of 125 to 130 meq/L, nausea and malaise may develop, which can be mistakenly attributed to the rigors of the physical exertion. As the serum sodium concentration drops below 120 meq/L, headache, lethargy, seizures, coma and respiratory arrest may ensue. The resulting encephalitis can lead to permanent brain damage.
The acute nature of hyponatremia seen in strenuous exercise prevents the brain from having a chance to adapt by reducing its intracellular osmolytes. To prevent the acute onset of hyponatremia seen in marathoners and other dehydrating athletic events, it is important to replenish lost sweat with both fluids and solutes. Gatorade or an alternative sports drink will do the trick. Another method is small salt tablets with your water (Based on your weight loss during an athletic event, its possible to predict the amount of salt lost in sweat. Rates vary from person to person, but on average about 500mg of salt is lost per pound of lost body weight due to sweating).
Hyponatremia is rare, but because the consequences are so severe and it is easily prevented it is good to be aware of it. Furthermore, one study found that dehydration is far more prevalent than hyponatremia in ultra-endurance athletes (The prevalence of exercise-associated hyponatremia in 24-hour ultra-mountain bikers, 24-hour ultra-runners and multi-stage ultra-mountain bikers in the Czech Republic, 2014. Daniela Chlibkova, et al.). Thus, it is important to stay hydrated, without overdoing it. As Christopher Almond and colleagues demonstrated, weight gain is a sign of hyponatremia. Weight gain likely means that one is drinking more fluids than were lost in sweat. Thus, they recommend using scales as a quick assessment of hyponatremia. In most cases that is not very practical.
For safe hydration, there are steps to take before a race or during an athletic event. It is important to be hydrated, but drinking too much before does not help. First, humans are not camels and therefore any extra water that the body sees goes straight to the bladder. Second, large quantities of water before a race or athletic event washes out the concentrating gradient in the kidney’s renal medulla. Losing the renal medulla concentration gradient actually inhibits one’s ability to concentrate urine and conserve water when they need it most!
In summary, to prevent hyponatremia and dehydration a comfortable amount of water will suffice beforehand. During and after the race or athletic event, it is important to replace salt lost in the sweat with Gatorade, other sports drinks or simply salt tablets with your water.