Hyponatremia in Endurance Exercise and Hot Weather: Overdrinking, Sweat Sodium and Official Advice

Urgent signs first

Low blood sodium during or after exercise can be a medical emergency. A review calls severe exercise-associated hyponatremia (EAH) with encephalopathy a life-threatening emergency. [1] In the US, call 911 for confusion, seizures, collapse or loss of consciousness. [2] Heat stroke is a separate emergency that needs rapid cooling and clinical care. [3] Only a clinician can tell which is happening, because EAH can look like low blood sugar, fainting or heat stroke. [4] Do not give salt tablets to someone who is unwell; MedlinePlus says they can cause serious complications. [2]

What it is

Hyponatremia means blood sodium below the normal range, which MedlinePlus gives as 136 to 144 mEq/L (ranges vary slightly by lab). [5] When sodium in body fluids falls, water moves into cells and brain cells swell, which causes many of the symptoms. [5] EAH is low sodium that develops during exercise. [1] It is mostly reported after endurance sports but is also seen after shorter events and in team sport athletes. [1]

What causes it

A 2019 review describes a spectrum from heavy overdrinking to modest sweat sodium losses with relative dilution. [1] A 2026 review says causes include excessive fluid intake and non-osmotic arginine vasopressin (a hormone linked to abnormal water retention by the kidneys [1]) released during exercise, and that sodium loss through sweat appears to play a less important role. [4] MedlinePlus lists both not drinking enough fluids during intense exercise or very hot weather and drinking too much water among reasons for an electrolyte imbalance, along with severe vomiting or diarrhea, heavy sweating, certain medicines and heart, liver or kidney problems. [6] Diuretics also lower sodium. [5]

Symptoms

The signs are vague: bloating, vomiting, headache and altered mental status. [1] MedlinePlus also lists confusion, irritability, restlessness, convulsions and fatigue. [5] A diagnosis can only be confirmed by a blood test. [1] Hyponatremia that develops in less than 48 hours is more dangerous than one that develops slowly. [5]

Prevention: what the reviews say

A review of EAH says that drinking according to thirst is the most individualized strategy against life-threatening dysnatremia during exercise, regardless of sport. [1] Another says adequate hydration has been shown with drinking to thirst and a typical race diet, without supplemental sodium. [7] A 2019 review adds avoiding excessive sodium supplementation aimed at replacing all sodium losses. [8] A 2025 narrative review found thirst-driven drinking was safe and effective for many experienced athletes with no increase in EAH, but that it may underperform in people with high sweat rates or impaired thirst perception, and that neither strategy seemed universally superior. [9]

We could not read the full text of the 2015 international consensus statement on EAH (the publisher page was blocked and the abstract was empty), so we do not quote it. This page does not give personal fluid plans: needs differ by person, event and weather.