Hyponatraemia in marathon runners

Exercise-associated hyponatraemia is a biochemical disorder that can occur during prolonged endurance events such as marathons, trail races or ultra-endurance events. It corresponds to an excessive dilution of sodium in the blood, usually caused by drinking too much water during exercise — hyponatraemia is defined as a blood sodium concentration below normal (generally < 135 mmol/L).

In runners, this can cause symptoms ranging from simply feeling unwell to serious neurological problems. Although relatively rare, marathon runners’ hyponatraemia is now well known to sports physicians and is the subject of specific hydration guidelines.

Sodium is an essential electrolyte that maintains the body’s fluid balance, nerve transmission and muscle contraction. During a marathon, two phenomena can cause hyponatraemia: excessive water intake and sodium loss through sweating. When the water ingested exceeds the kidneys’ capacity to eliminate it, the blood becomes diluted and the sodium concentration falls.

The neurological symptoms observed are linked to cerebral oedema caused by this dilution of sodium.

Hyponatraemia in marathon runners remains rare, but it is regularly seen at major endurance races. The runners most at risk are often first-time marathon runners, slower runners and participants in very long events (ultra-trail).

Long races increase the risk for several reasons: the prolonged duration of exercise, numerous aid stations, fear of dehydration, or hydration advice that is sometimes misinterpreted. Some runners drink more than their actual thirst dictates, which promotes dilution of blood sodium.

Risk factors

Drinking at every aid station without feeling thirsty, drinking only water, exercise lasting more than 4 hours, low body weight, heavy sweating, cool weather conditions (less sensation of thirst). Certain medications, particularly anti-inflammatories, can also promote this disorder.

Symptoms can appear gradually during or after the race. Common symptoms are headache, nausea, vomiting, a feeling of swelling, or marked fatigue.

Possible neurological signs

Confusion, agitation, balance problems, seizures, loss of consciousness. These signs reflect cerebral oedema caused by sodium dilution and require immediate medical care.

Drink according to thirst

The most widely recommended strategy today is to drink according to thirst rather than following a fixed hydration volume. This approach limits the risk of overhydration.

Use drinks containing electrolytes

Sports drinks containing sodium help to offset some of the losses due to sweating; they are particularly useful during long races.

Test your hydration strategy in training

As with nutrition, hydration must be tested before competition: drink volumes, type of drink, frequency of intake.