Hydration for Runners

Knowing how much fluid and electrolytes to take on board when training can be confusing. You've probably seen a lot online about electrolytes – they're everywhere just now – but do you actually need them? In this blog, I'll explain how to maintain optimal hydration and the role electrolytes can play in your training and racing.

Why hydration matters

The amount of fluid and electrolytes you need depends on several factors, including your individual sweat rate, environmental conditions such as temperature and humidity, along with running intensity and duration.

Generally, in hotter conditions or when you're working at higher intensities, you'll sweat more and therefore have greater fluid and electrolyte losses. Sweat rate is also highly individual – some runners lose significantly more fluid and sodium than others, even in similar conditions.

A small amount of dehydration can occur without a major impact on performance. However, it can quickly become a problem if hydration isn't given attention before, during, and after training or racing. Waiting until you feel tired or thirsty is far from ideal. As little as 2% dehydration can result in reduced cognitive function and impaired performance. When dehydrated, your body must work harder and the perceived effort increases. Drinking to thirst can be helpful, but thirst signals may be delayed or blunted in heavy sweaters or if you start exercise already mildly dehydrated. For this reason, having a hydration plan can be extremely beneficial.

Is water alone enough or do I need electrolytes?

Often water alone is sufficient to maintain hydration status during short or low-intensity training runs of less than 60 minutes. Therefore, taking a daily electrolyte drink is often not necessary – we usually get adequate sodium (salt) from everyday foods. Electrolytes are often needed when you are doing longer runs of 60–90 minutes or more, or when there is high-intensity work, or when running in hot and humid conditions.

What are electrolytes?

They are minerals which help maintain fluid balance, nerve signalling and muscle contraction. The main mineral of concern is sodium, although others such as potassium, magnesium and calcium have a role too. When you sweat you lose sodium, which is why it is important to replace it alongside water. If sodium levels are too low you may feel dizzy, fatigued, bloated and have impaired concentration. Only taking plain water over long, hot or intense running can in extreme cases lead to a life-threatening condition called hyponatraemia, caused by low blood sodium levels.

Where do I find them?

Many sports nutrition products such as drinks, gels, chews and electrolyte tablets all contain sodium, though the amounts vary widely. Some products contain little or none, so it's important to read labels carefully. Everyday foods also provide sodium, which can be especially useful during long training runs or ultradistance events where real food is often being consumed.

How much do I need?

This is very individual and there is no clear one-size-fits-all approach. However, as a general guide I would suggest that runners have at least 500mg of sodium per 1L of fluid when doing long training runs or events like marathons or ultra marathons. The actual amount needed will vary between runners so will require some trial and error. In addition, taking on board fluid intakes of between 400–800ml per hour tends to cover the needs of most runners.

You can often gauge if you are a heavier or saltier sweater if you notice salty-tasting sweat on your top lip, sweat stinging your eyes, and white salt marks on clothes. Sweat rate testing can provide more accurate, personalised guidance – and as a sports dietitian I have helped runners carry out simple sweat assessments to gauge their own sweat rates.

With hydration you often need to practice different things and see what works best for you, as everyone is different. Do take into consideration that when under more intense workloads or in higher temperatures you will typically need more fluids and electrolytes.

What next?

As with all nutritional information, this blog is a general guide and does not take into consideration any special medical conditions or therapeutic dietary needs which can impact advice. If you want further personalised advice and support, you can get in touch with me via the contact section.

References

McCubbin, A.J. Sodium intake for athletes before, during and after exercise: review and recommendations. Perform. Nutr. 1, 11 (2025). https://doi.org/10.1186/s44410-025-00011-9

Kleiner, S (1999) Water: An essential but overlooked nutrient. Journal of American Dietetic Association. 99 (2) pp. 200-206.