Exercise‑associated hyponatremia (EAH)

Exercise-associated hyponatremia (EAH) is a drop in blood sodium to below 135 mmol/L that occurs during or within 24 hours after physical activity. It develops when total body water expands relative to sodium, typically due to drinking more hypotonic fluid than is lost in sweat, urine and respiration.

Why it happens and how to avoid it

EAH results from dilutional or depletional mechanisms; the dilutional form is most common and is caused by sustained overconsumption of water or low‑sodium beverages combined with inappropriate antidiuretic hormone secretion, which impairs water excretion. Risk factors include excessive fluid intake, weight gain during exercise, events lasting more than four hours, low or high body mass index, hot weather and the use of non‑steroidal anti‑inflammatory drugs. Prevention focuses on drinking to thirst, monitoring body weight and avoiding positive fluid balance. Fluid intake generally should not exceed about 1.5 litres per hour; pre‑event salt supplementation has little benefit.

Staying safe during endurance events

EAH can produce mild symptoms such as nausea, headache, bloating or weight gain and may progress to confusion, seizures and pulmonary oedema if blood sodium falls well below normal. Education is key: athletes should learn the signs of hyponatremia and distinguish it from dehydration. During long races, weigh‑ins or calculating sweat rates can help tailor intake. Support crews should offer smaller sips of sports drink or water and encourage concurrent consumption of salty foods rather than forcing large volumes of fluid. On‑site testing of serum sodium and rapid treatment with fluid restriction or hypertonic saline can be lifesaving in severe cases.

EAH reminds athletes that more fluid is not always better. By respecting thirst cues, planning sodium intake through foods instead of tablets and limiting drinking rates, endurance athletes can maintain hydration without diluting their blood sodium.

Related Terms: Drink to thirst, Scheduled drinking, Salt capsules, Hypotonic, Isotonic