The dangers of over-hydrating during a run

A guide to hyponatraemia – and how taking in too much fluid can be even more deadly than dehydration.


Although runners know how important it is to keep hydrated during a race, the dangers of over-hydration are far less known. The symptoms can often be very similar to dehydration, making it difficult to spot before it’s too late.

What are the symptoms?

Swollen fingers, nausea and a painful stomach may by signs of over-hydration, which occurs when you drink too many fluids. A combination of high water intake and low renal output (urinating) can cause abnormally low blood sodium levels, leading to a dangerous condition known as hyponatraemia.

Sodium is an electrolyte that plays a key role in regulating many important functions in the body, like the body’s fluid balance, and supports the central nervous system.

In an effort to be well-hydrated on race day, runners often drink too much water in the last few days leading up to the race, or drink too much during and after the run, making themselves susceptible to over-hydration. It often occurs after running for four or more hours – the idea being that the longer you’re out on the course, the more fluids you may be taking in.

Slower runners tend to be more susceptible to the condition because they sweat less and have more opportunity to drink with ease. By comparison, faster runners only tend to gulp down a few glugs of water every now and then, and sweat more because their heart rate is higher. As such, they are less likely to have too many fluids in their body.

What is hyponatraemia?

Hyponatraemia can cause many health issues that range from mild to life-threatening. The brain is especially sensitive to changes in your body’s sodium levels, which is why often the first symptom of hyponatraemia is confusion. Other symptoms include dizziness, headaches, fatigue, irritability, muscle weakness, cramping, nausea and vomiting. Runners may even become disoriented or unconscious, or experience seizures.

When there is too much fluid in the body, the sodium content becomes diluted. This can be dangerous, because sodium is an important component of osmosis – a process which regulates the amount of water that’s in and around your cells.

‘The low sodium content causes osmolality of the blood to go down, which pushes more water into the cells,’ explains Lewis James, a reader in human nutrition at Loughborough University. ‘This causes swelling, which is particularly dangerous in the brain because it can’t swell indefinitely as it in incased in a hard shell. Swelling can lead to problems with brain function and, in certain scenarios, death.’

Oddly enough, the symptoms of hyponatraemia, dehydration and heat illness are very similar and can often only be differentiated by a blood test. It’s very important to distinguish between the conditions because, while symptoms are similar, the treatments are different. Get to the medical tent immediately if you experience any of these symptoms again during a race or training run.

How to avoid hyponatraemia

Although drinking to thirst if often a recommended solution to avoiding hyponatreamia, it is not fail proof. That’s because one of the symptoms of the condition is the feeling of dehydration.

It’s important to note that cases of hyponatreamia are extremely rare and it is far more common for runners to under-hydrate than over-hydrate. Hyponatreamia is also more likely to happen if you’re out running for a long time, so it is rarely a concern for people running distances less than a marathon.

The easiest way to avoid the condition and to ensure you are hydrating appropriately is to do a simple sweat test.

Typically, sweat rates range between one litre and three litres per hour, but they can vary depending on the temperature, wind strength and how hard you are running.

Alan Ruddock, sport and exercise scientist at Sheffield Hallam University, says conducting a sweat test can be easily conducted at home. ‘Weigh yourself before and after exercise and either drink nothing or account for the amount of water you have ingested,’ he explains.

Ruddock recommends calculating this after an hour-long run, so you can more accurately measure your sweat rate per hour. One gram of lost weight equates to around a millilitre of water, so a loss of 1kg will be the equivalent of one litre. And, according to James, you don’t have to replace exactly the same amount of fluid.

‘If you are losing a litre an hour, then 500 to 800ml of water an hour will be more than enough,’ he says.

And don’t be fooled into thinking that drinking electrolytes or chewing salt tabs instead of slugging water will help you to avoid hyponatreamia. James explains that while the best electrolytes are great for recovery and rehydration after a race, the amounts of sodium are too low to counterbalance hyponatreamia.

‘The truth is that most electrolyte supplements are very low in electrolytes,’ he says. ‘The salt content is way below what is in the blood, so it has very little effect.’

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