Why does exercise-induced hyponatremia deserve the attention of endurance athletes?
Exercise-induced hyponatremia (EIH) is a complication that is still too often confused with dehydration. However, when we talk about the diet of a trail runner, a triathlete, a marathon runner, or a cyclist, we often overlook the fact that drinking excessively can be more problematic than not drinking enough. Indeed, exercise-induced hyponatremia is this disruption of fluid balance that occurs when the sodium concentration in the blood falls below normal values, usually following an intake of fluids greater than actual losses.
While severe cases remain rare, this situation can lead to serious neurological symptoms, in which case prompt management is necessary. For endurance athletes, understanding the mechanisms of hyponatremia is therefore an integral part of the nutritional and hydration strategy.
What is exercise-induced hyponatremia?
Sodium is an electrolyte essential for the proper functioning of muscles, the nervous system, and the regulation of body fluids. It is when it becomes too diluted in the blood that we talk about hyponatremia. In the sports context, exercise-induced hyponatremia is defined by a blood sodium concentration lower than 135 mmol/L occurring during or within 24 hours following prolonged exertion. That's the medical definition, but don't worry, we'll clarify all of that below!
Contrary to what one might imagine, it is not solely related to salt loss through sweating. In the majority of cases, the main factor lies in excessive fluid consumption, often water, compared to the actual needs of the body. Furthermore, in some athletes, the body also continues to retain water under the effect of hormones involved in water regulation, which further accentuates the dilution of sodium.
Which athletes are most exposed to exercise-induced hyponatremia?
Specifically, even though all practitioners of physical activity can be affected, certain situations significantly increase the risk:
long endurance events (marathon, ultra-trail, Ironman, cycling events...),
efforts exceeding 4 hours,
high availability of aid stations,
regular water consumption without feeling thirsty,
hot weather conditions favoring an excessive hydration strategy,
inexperienced athletes or those following inappropriate recommendations.
The phenomenon has already been observed in a significant proportion of participants in ultra-endurance races, even when symptoms remain moderate.
The symptoms of exercise-induced hyponatremia
It can be difficult to identify this medical complication because the symptoms are often similar to those of heat stroke or dehydration.
The first signs to watch for to identify EIH
Headaches, nausea, a feeling of bloating, unusual tiredness, dizziness or a drop in performance may be early symptoms of exercise-induced hyponatraemia. It is important to emphasise that these kinds of symptoms should not be underestimated: they are still too often dismissed as trivial during a competition, when in fact they should be taken seriously.
Severe forms
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When sodium levels drop significantly, water enters the cells, particularly those in the brain. This can cause confusion, which may prevent you from making the right decisions in the field, as well as coordination problems, convulsions and, in extreme cases, a coma.
Fortunately, such situations remain rare, but they explain why exercise-induced hyponatraemia should never be taken lightly.
Exercise-induced hyponatraemia and hydration: why drinking more isn’t always better
For years, many athletes have been told that they should drink as much as possible to avoid dehydration. However, current understanding puts this message into perspective. Indeed, recent recommendations place greater emphasis on a hydratation tailored to individual needs. For the majority of endurance athletes, the sensation of thirst is an excellent indicator for guiding fluid intake. Consistently drinking beyond this sensation increases the risk of hyponatraemia. As we like to say at Baouw : Listen to yourself and trust yourself!
But be careful, this does not mean that hydration should be relegated to the background! The goal is rather to find the right balance between losses due to sweating and the intake during effort. Moreover, managing fluid intake follows the same logic as managing energy reserves: it is about providing what the body needs, neither too little nor excessively.
How to prevent hyponatremia during a marathon, a trail, or a triathlon?
The golden rule: learn to personalize your hydration
Every athlete has a different sweating profile, and on top of that, external temperature, effort intensity, heat acclimatization capacity, or body morphology strongly influence needs. So let's repeat our mantra: testing your hydration strategy during training helps avoid surprises on race day.
Do not neglect sodium intake
Sodium lost through sweating can be compensated by:
smart sports drinks, with intelligent composition,
salty foods consumed during refueling (like our tasty savory energy puree “vegetable curry”)
electrolyte tablets, specifically designed for endurance sports: full of essential minerals and vitamins!
However, it should be noted that sodium intake does not compensate for massive overconsumption of water. Prevention primarily relies on a global balance between water and electrolytes.
Monitor your weight during long preparations
A significant weight gain during an endurance event is sometimes a warning sign indicating excessive water retention. Conversely, a slight weight loss is generally normal during prolonged exertion. Yes, we can guess that you are not going to train for the trail with your scale under your arm! However, just after an intensive training session, you can weigh yourself to get an idea of your water consumption, and thus judge whether it is reasonable or excessive.
What to do in case of suspected exercise-induced hyponatremia?
In the face of suggestive symptoms, continuing the effort or drinking large amounts of water can worsen the situation. It should be noted that a medical evaluation becomes necessary when symptoms are pronounced or evolve rapidly. Severe forms require urgent management with specific protocols aimed at correcting blood sodium concentration.
Last point: self-diagnosis remains tricky since the signs can resemble those of heat stroke or dehydration. Finally, there is no need for paranoia regarding exercise-induced hyponatremia: let’s remember that drinking far too much water during exertion remains rare among athletes.
The right balance to perform and avoid EAH
In endurance, performance often relies on a succession of details. An appropriate nutritional strategy, good management of energy reserves, controlled recovery and consistent hydration together contribute to optimizing sensations and results. Exercise-induced hyponatremia reminds us that in sports nutrition, more is not always synonymous with better. In short, drinking according to your actual needs, integrating sources of sodium when the effort prolongs, and testing your strategy ahead of competitions are the best reflexes to fully exploit your potential on the field.
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