In 30 seconds
- It is blood sodium below 135 mmol/L during exercise or in the following 24 hours.
- In the Boston Marathon, 13% of 488 runners finished with hyponatraemia, and 0.6% with critical levels.
- In Boston, gaining weight during the race multiplied the risk by 4.2, and taking more than 4 hours by 7.4.
- Drinking to thirst is the recommended prevention. Sodium slows the drop, but does not prevent hyponatraemia if you drink too much.
Exercise-associated hyponatraemia is a drop in blood sodium during sport or in the hours that follow. The studies agree on the main cause, drinking more fluid than you lose, and on the most effective way to prevent it: drinking to thirst.
What it is
Exercise-associated hyponatraemia is diagnosed when blood sodium falls below 135 mmol/L during activity or in the following 24 hours 1. Most cases cause no symptoms; those that do are rare, but they can be serious 1.
The mechanism is dilution. If you drink much more than you sweat, the extra water dilutes the sodium in your blood. On top of that, during exercise the body can release vasopressin, the hormone that makes the kidneys hold on to water, even when there is already too much 1.
Level of evidence by outcome
| Outcome | Evidence | What the data show |
|---|---|---|
| Overdrinking as the main cause | High | Grade 1A in the international consensus; almost all symptomatic cases gained or maintained weight. |
| Drinking to thirst to prevent it | Moderate | Consensus recommendation: it limits excess fluid without causing significant dehydration. |
| Events of more than 4 hours and a slow pace | Moderate | 7.4 times higher risk with more than 4 hours in Boston; a risk factor according to the consensus. |
| Salt tablets in long events | Low | Weak relationship with final blood sodium in an ultramarathon; no differences in an Ironman. |
| Drinks with sodium to prevent it | No clear effect | They slow the drop when you drink what you sweat, but do not prevent it if you drink too much. |
The rating is our own, based on the type of studies, their number and the consistency of the results.
The key studies
| Study | Type | Sample | Outcome |
|---|---|---|---|
| Hew-Butler et al., 2015 | International consensus | Review of the evidence | Main cause: drinking more fluid than is lost, together with inappropriate release of vasopressin. At least 14 deaths of athletes since 1981. 1 |
| Almond et al., 2005 | Observational study | 488 runners, Boston Marathon | 13% with hyponatraemia and 0.6% with 120 mmol/L or less. Higher risk with weight gain, more than 4 hours and extreme BMIs. Water or sports drink did not change the risk. 3 |
| Kipps et al., 2011 | Observational study | 88 runners, London Marathon | 12.5% with asymptomatic hyponatraemia (128-134 mmol/L). They drank more and gained more weight, although 4 of the 11 lost weight. 4 |
| Twerenbold et al., 2003 | Crossover trial | 13 women, 4 hours drinking 1 L per hour | Mild hyponatraemia in 92% with water, 69% with 410 mg/L of sodium and 46% with 680 mg/L. Severe only with water, in 2 women. 5 |
| Hoffman and Stuempfle, 2015 | Observational study | 161 km ultramarathon, 296 finishers | 6.6% with hyponatraemia, even though 93.9% took sodium tablets. The main feature was overdrinking. 6 |
| Hew-Butler et al., 2006 | Randomised trial | 413 triathletes, Ironman | About 3.6 g of extra sodium in tablets did not change blood sodium compared with placebo. 7 |
| Hew-Butler et al., 2017 | Review | Update of the evidence | 11% of 1,089 Ironman triathletes and 6% of 33 endurance cyclists with hyponatraemia. Severe cases treated with 3% saline. 2 |
How many athletes get it
It depends very much on the event. The international consensus gives these figures in athletes without symptoms, measured at the finish 1:
- Marathon: from 0% to 12-13%.
- Ironman: up to 25%.
- 161 km ultramarathon: from 0% to 51%.
- Others: 33% in rugby players and 70% of elite rowers during a 28-day training camp.
Cases with symptoms are much less common: 1% in a study with 2,135 athletes from eight endurance events 1. Even so, the consensus counts at least 14 deaths of athletes since 1981 1.
Boston Marathon
488 runners analysed at the finish
The two reference marathons
In Boston, 488 runners were analysed at the finish. 13% had hyponatraemia and 3 (0.6%) had critical levels, of 120 mmol/L or less 3. There were more cases in women (22%) than in men (8%) 3.
What mattered most was gaining weight during the race, which multiplied the risk by 4.2. Taking more than 4 hours multiplied it by 7.4 compared with finishing in under 3 h 30 min 3. Drinking water or a sports drink did not change the risk 3.
In London, of 88 volunteers, 12.5% finished with asymptomatic hyponatraemia, with values of 128-134 mmol/L 4. They had drunk more and gained more weight, although 4 of the 11 had lost weight. That is why the authors suggest that vasopressin also plays a part 4. To put this into practice, see our hydration plan for the Barcelona Marathon.
Who is most at risk
The international consensus lists these factors 1:
- Overdrinking, whether water, sports drinks or other fluids.
- Gaining weight during exercise.
- Efforts lasting more than 4 hours, such as the Matagalls-Montserrat.
- Little experience or little training.
- A slow pace.
- A very high or very low BMI.
- Having fluid available at all times.
Most documented cases are women, but in Boston the difference between the sexes disappeared once BMI and race time were taken into account 2. The 2017 review also reports it in endurance cyclists: 6% of 33 2.
4 hours, 1 litre per hour
Mild hyponatraemia by the sodium in the drink
What about sodium?
Sodium helps, but it is not the solution. In one trial, 13 women ran for 4 hours drinking 1 litre per hour. Mild hyponatraemia occurred in 92% with water, 69% with 410 mg/L of sodium and 46% with 680 mg/L 5. Severe cases occurred only with water 5.
Note that, even with the saltiest drink, almost half developed it: they were drinking a lot. The same has been seen in real events:
- 161 km ultramarathon: 93.9% of runners used sodium tablets, and even so 6.6% of finishers had hyponatraemia. The common feature of those who developed it was overdrinking 6.
- Ironman: about 3.6 g of extra sodium in tablets did not change blood sodium compared with placebo 7.
The consensus conclusion is clear. Sports drinks contain much less sodium than blood: they slow the drop when you drink what you sweat, but do not prevent it if you drink too much 1. We go into more detail in sodium and performance: what the studies say.
Symptoms and what to do
- Mild: dizziness, nausea, bloating and weight gain compared with the start 1.
- Severe: vomiting, headache, confusion, seizures or loss of consciousness 1.
With severe symptoms, seek urgent medical help and do not keep drinking water. Severe cases are treated by healthcare staff, with 3% hypertonic saline 1 2. Until the person passes urine, water and other low-sodium fluids should be avoided 1.
Prevention
How to prevent hyponatraemia
How to prevent it
- Drink to thirst. This is the consensus recommendation: it limits excess fluid without causing significant dehydration 1.
- Do not gain weight. If you weigh more at the end than at the start, you have drunk too much 2.
- Do not overdo it before you set off. Drinking too much before exercise also increases the risk 8.
- Accept losing some weight. In cool or mild temperatures, a loss of up to 3% of body weight is well tolerated 1.
To find out how much you sweat, and therefore how much you need to drink, use the hydration calculator.
What we still do not know
- The role of vasopressin: it is put forward to explain cases without weight gain, but this still needs confirming 4.
- Anti-inflammatory drugs: they could contribute, but the data are contradictory 1.
- Very long events in the heat: cases with weight loss also occur there 1.
What about drinks with sodium?
A drink with sodium helps you replace what you sweat. The key to avoiding hyponatraemia, however, is not any particular drink: it is not drinking more than you sweat.
Frequently asked questions
What is exercise-associated hyponatraemia?
A drop in blood sodium below 135 mmol/L during exercise or in the following 24 hours. In athletes, the main cause is drinking more fluid than is lost in sweat.
What are the symptoms of hyponatraemia?
Mild symptoms are dizziness, nausea, bloating and weight gain. Severe ones are vomiting, headache, confusion, seizures or loss of consciousness. With severe symptoms, seek urgent medical help and do not keep drinking water.
Does taking salt prevent hyponatraemia?
Not if you drink too much. Drinks with sodium slow the drop in blood sodium when you drink roughly what you sweat, but excess fluid outweighs the sodium you add.
Who is most at risk of hyponatraemia in a race?
People who drink a lot and gain weight during the event, those who take more than 4 hours, those who go at a slow pace or have little experience, and people with a very high or very low BMI.
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
- Hew-Butler T, Loi V, Pani A, Rosner MH. Exercise-associated hyponatremia: 2017 update. Front Med. 2017;4:21. View publication
- Almond CSD, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med. 2005;352(15):1550-1556. View publication
- Kipps C, Sharma S, Tunstall Pedoe D. The incidence of exercise-associated hyponatraemia in the London marathon. Br J Sports Med. 2011;45(1):14-19. View publication
- Twerenbold R, Knechtle B, Kakebeeke TH, et al. Effects of different sodium concentrations in replacement fluids during prolonged exercise in women. Br J Sports Med. 2003;37(4):300-303. View publication
- Hoffman MD, Stuempfle KJ. Sodium supplementation and exercise-associated hyponatremia during prolonged exercise. Med Sci Sports Exerc. 2015;47(9):1781-1787. View publication
- Hew-Butler T, Sharwood K, Collins M, Speedy D, Noakes T. Sodium supplementation is not required to maintain serum sodium concentrations during an Ironman triathlon. Br J Sports Med. 2006;40(3):255-259. 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
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.
