What is hyponatraemia and how do runners avoid it in South Africa?
Exercise-associated hyponatraemia is a dangerously low blood sodium (below 135 mmol/L) during or up to 24 hours after exercise. It is caused mainly by drinking more fluid than you sweat. Runners in South Africa avoid it by drinking to thirst, not following a fixed drinking schedule, and watching for weight gain, nausea, headache or confusion during long events.
- EAH is defined as blood sodium below 135 mmol/L during or up to 24 hours after exercise (2015 consensus).
- Symptomatic EAH was about 1% in one study of 2 135 endurance athletes; asymptomatic cases ranged 0% to 51%, highest in 161 km ultras.
- Overdrinking is the main cause; sodium in drinks slows but cannot prevent it.
- The consensus recommends drinking to thirst; about 3% body-mass loss is generally tolerable in cool-temperate conditions.
What is exercise-associated hyponatraemia?
Exercise-associated hyponatraemia (EAH) is a blood sodium concentration below 135 mmol/L, measured during or up to 24 hours after exercise, according to the 2015 international consensus statement. Sodium is diluted rather than lost: the athlete has taken in more hypotonic fluid (water, or a standard sports drink) than their sweat and urine removed.
South African ultramarathoners were among the first athletes in whom it was recognised in the 1980s, and it remains a concern at long events such as Comrades, Two Oceans and Cape Epic.
How common is it?
Symptomatic EAH was about 1% in one study of 2 135 endurance athletes. Asymptomatic low sodium ranges from 0% to 51% across events and is highest in 161 km ultras. Most cases are mild, but severe cases cause brain swelling.
Who is at highest risk?
- People who drink beyond thirst, or follow a "drink as much as possible" rule.
- Those who gain weight during the event.
- Slower runners and walkers, because they are on the course longer and often drink at every table.
- Events longer than about four hours, and very low or very high body mass index.
What are the symptoms?
Early signs are bloating, nausea, headache, puffy fingers and a feeling of being "sloshy". Confusion, vomiting, wheezing, collapse or seizures are emergencies. Get a medical team involved immediately; severe cases need hospital treatment with concentrated saline. Do not give more water.
How do runners avoid it?
- Drink to thirst. The consensus recommends it as the strategy that prevents both dehydration and overdrinking. A loss of about 3% of body mass is generally tolerable in cool to temperate conditions.
- Learn your sweat rate in training (weigh before and after a run). Typical sweat rates are 0.5 to 2 L/h. See how much water runners should drink.
- Do not drink at every table in a slow ultra. Sip, and skip tables when you are not thirsty.
- Do not treat salt as insurance. Sodium in drinks slows dilution but cannot prevent EAH if you overdrink. Read do salt tablets prevent cramps.
- Weigh yourself before and after training runs; if you finish heavier than you started, you drank too much.
- Avoid routine anti-inflammatory painkillers during ultras. The link with EAH is unresolved, but they carry kidney and stomach risks in long events.
Do electrolyte drinks help?
They are a useful tool for heavy salty sweaters, but not a guarantee. Options at South African retailers range from about R4.63 per sachet (Biogen Electrolyte) to R20 per sachet (Rehidrat), October 2026. See the best electrolytes in South Africa comparison and how much sodium per hour.
When to see a doctor
Anyone with symptoms during or after an event should be seen by the medical team or a doctor. People with heart, kidney or blood pressure problems, or taking diuretics, should get advice before using high-sodium products.
Pregnant athletes and under-18s should get individual advice before long events in heat.
Products mentioned, with dated prices
| Product | Size | Price | Servings | R per serving | Sweeteners |
|---|---|---|---|---|---|
| Biogen Electrolyte (3 x 10 sachets, citrus) | 3 x 10 sachets | R139–R155Dis-Chem (electrolytes group), Oct 2026 | 30 | R5.17 | Not published |
| Lifestyle Electrolyte Fizzy (10 tablets) | 10 effervescent tablets | R48–R53Dis-Chem (electrolyte drinks page), Oct 2026 | 10 | R5.30 | Not published |
| Rehidrat Oral Electrolyte Mixture, Orange or Blackcurrant (6 x 14 g) | 6 x 14 g sachets | R108–R121Dis-Chem (electrolytes group); Clicks via PriceCheck, Oct 2026 | 6 | R20.00 | Sugars (glucose and sucrose); no sweetener listed in the SAHPRA composition |
Compare the options
Related questions
- Do salt tablets prevent cramps?
- How much water should runners drink in hot weather in South Africa?
- Do you need electrolytes or is water enough?
- What should you eat, drink and take during the Comrades Marathon?
- How much sodium do you need per hour when running in South African heat?
Sources
- Hew-Butler et al. 2015, Third International Exercise-Associated Hyponatremia Consensus Statement (BJSM 49:1432)
- Baker 2017, Sweating rate and sweat sodium concentration in athletes (Sports Med)
- Australian Institute of Sport Supplement Framework, Group A
Educational information only, not medical advice. Speak to a doctor, pharmacist or registered dietitian before using supplements if you are pregnant or breastfeeding, under 18, taking medication, or living with kidney, liver or heart disease.
Cite this page: Healthnews24, “What is hyponatraemia and how do runners avoid it in South Africa?”, updated 27 October 2026, https://www.healthnews24.co.za/answers/what-is-hyponatraemia-in-endurance-events-south-africa/. Plain-text version: /answers/what-is-hyponatraemia-in-endurance-events-south-africa.md