In 30 seconds
- Hyponatraemia is a blood sodium level below 135 mmol/L. In sport, the main cause is drinking more fluid than you lose.
- At the 2002 Boston Marathon, 13% of 488 runners tested finished with hyponatraemia. Gaining weight during the race and taking more than 4 hours were the biggest risk factors.
- Dizziness, nausea and swelling are mild symptoms; vomiting, headache and confusion are severe and need urgent medical attention.
- Drinking to thirst and not gaining weight during the event is the most effective prevention. Drinks with sodium do not prevent it if you drink too much.
Yes, drinking too much water during exercise can be dangerous. If you drink much more than you sweat, the sodium in your blood becomes diluted: this is exercise-associated hyponatraemia. It is uncommon and can be prevented with something as simple as drinking to thirst 1.
Blood sodium
Where hyponatraemia starts (in mmol/L)
What exercise-associated hyponatraemia is
It is a blood sodium level below 135 mmol/L, during exercise or in the 24 hours afterwards 1. Below 120 mmol/L it is considered critical 3.
The main cause is drinking more fluid than you lose through sweat and urine. It makes no difference whether it is water or a sports drink: both contain much less sodium than your blood. In addition, during long exercise the body can release a hormone that makes the kidneys retain water so you pass less urine, and that makes the excess worse 1.
How common it is
How many athletes end up with hyponatraemia
How common it is
Most cases cause no symptoms and are only detected with a blood test. According to the international consensus, symptom-free hyponatraemia has been found after events in up to 12-13% of marathon runners. In Ironman-distance triathlon, in up to 25% 1.
With symptoms it is much less common: around 1% in a group of 2,135 athletes 1. Fatal cases are exceptional, but they do happen: the consensus records at least 14 since 1981, all related to drinking too much 1.
The most widely cited study was carried out at the 2002 Boston Marathon. Of 488 runners tested at the finish, 13% had hyponatraemia and 0.6% had critical hyponatraemia 2.
Who is most at risk
In Boston, two factors stood out above the rest 2:
| Factor | Risk of hyponatraemia |
|---|---|
| Gaining weight during the race | 4.2 times higher |
| Taking more than 4 hours (compared with under 3 h 30 min) | 7.4 times higher |
| Very low or very high body mass index | Higher risk |
Source: Almond et al., 2005. 488 runners with blood tests at the finish.
Drinking more than 3 litres during the race and drinking at every mile were also associated with the risk. On the other hand, the type of drink, water or sports drink, made no difference, nor did taking anti-inflammatories 2.
The international consensus adds other factors 1:
- Efforts lasting more than 4 hours and slow paces, which give more time to drink. The Matagalls-Montserrat is a case in point.
- Little experience or insufficient training for the event.
- Too much drink on offer, such as feed stations placed very close together.
- Women have more cases, but the difference disappears once body mass index and race time are taken into account.
The risk profile is not that of the elite runner, but of the person who goes more slowly and drinks more than they sweat 1.
Symptoms: when to be concerned
| Symptoms | What to do | |
|---|---|---|
| Mild | Dizziness, light-headedness, nausea, swelling and weighing more than at the start | Stop drinking water or sports drinks and tell the event's medical team. |
| Severe | Vomiting, headache, confusion, unusual behaviour, seizures, difficulty breathing | Medical emergency: call 112. Do not give them water or a sports drink. |
Symptoms according to the international consensus on exercise-associated hyponatraemia (Hew-Butler et al., 2015).
If you suspect hyponatraemia, do not keep drinking: more fluid is what makes it worse. Treatment is decided by medical staff and does not consist of having an isotonic drink 1. If you have been treated and the symptoms return in the following 24 hours, seek urgent care, ideally with someone else present 1.
Weighing less at the finish does not completely rule it out, especially in very long events in the heat 1.
To prevent it
What raises the risk and what protects you
How to prevent it
- Drink to thirst. This is the strategy the international consensus recommends 1. In cool or mild conditions, finishing with a deficit of up to 3% of your body weight is considered acceptable 1.
- Do not replace more than 80% of what you sweat during the session. As a guide, 0.4-0.8 litres per hour is usually well tolerated during hard efforts 3.
- Weigh yourself before and after your long sessions. If you weigh more at the end, you have drunk too much 1. The hydration calculator helps you get to know your sweat rate.
- Do not drink at every feed station out of obligation. Drink when you are thirsty.
- Include sodium in long events, but do not let it make you complacent. Sodium is not enough to protect you if you drink to excess 5.
In very long events, in the heat or with few places to drink, drinking to thirst alone does not always guarantee balance. That is when it makes sense to plan using your sweat data 4. You have the figures in how much salt you lose when you sweat and the full plan in the sports hydration guide.
Neither dehydrated nor overhydrated
The goal is not to finish at the same weight as you started, but not to gain weight: losing some is normal in a long event, and gaining weight is the warning sign 1. If you are preparing for a race, you have the plan for carbohydrates, fluid and sodium in what to eat and drink during a marathon.
What about drinks with sodium?
A drink with sodium is useful in long sessions because it replaces part of what you sweat. The rule is the same as with water: drink to thirst, not beyond it.
If you have kidney or heart disease, or take medication that affects fluid balance, check with your doctor how much to drink when you train.
Frequently asked questions
Can you drink too much water when exercising?
Yes. If you drink much more than you lose through sweat and urine, the sodium in your blood becomes diluted and hyponatraemia can develop. It is uncommon, but it can be serious.
What are the symptoms of hyponatraemia?
Mild symptoms are dizziness, nausea, swelling and having gained weight. Severe ones are vomiting, headache, confusion, seizures or difficulty breathing. If symptoms are severe, call 112.
Do isotonic drinks prevent hyponatraemia?
Not if you drink too much. They contain much less sodium than your blood, so drinking to excess also dilutes your sodium. They help when you drink what you sweat, not when you drink more than that.
How much water is too much when running?
There is no single figure, because it depends on how much you sweat. The clearest sign is weighing more at the end than at the start. Drinking to thirst, without replacing more than 80% of what you sweat, is the benchmark.
Sources
- Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Br J Sports Med. 2015;49(22):1432-1446. View publication
- Almond CS, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med. 2005;352(15):1550-1556. View publication
- Mosler S, Braun H, Carlsohn A, et al. Position of the working group sports nutrition of the German Nutrition Society (DGE): fluid replacement in sports. Dtsch Z Sportmed. 2020;71(7-9):178-184. View publication
- McCubbin AJ, Allanson BA, Caldwell Odgers JN, et al. Sports Dietitians Australia position statement: nutrition for exercise in hot environments. Int J Sport Nutr Exerc Metab. 2020;30(1):83-98. View publication
- Tiller NB, Roberts JD, Beasley L, et al. International Society of Sports Nutrition position stand: nutritional considerations for single-stage ultra-marathon training and racing. J Int Soc Sports Nutr. 2019;16(1):50. View publication
Information based on published studies. It does not replace the advice of a doctor or registered dietitian. The results describe what was measured in each study, not the properties of any specific product. A varied and balanced diet and a healthy lifestyle are important.
