Running and hydration: Everything you need to know (2024)

Knowing exactly how much to drink when training and racing can be tricky to work out. Not only does fluid and sodium loss (dehydration) vary considerably from person to person but is also dependent on a number of factors, from the outside temperature to the intensity you are running at.

Drink too little and you’ll find yourself battling the negative, and potentially dangerous, physiological effects of dehydration. Drink too much and you'll upset your sodium balance, which, too, can cause unpleasant symptoms and lead to health issues.

Thankfully, it's possible to hydrate optimally when running and racing by following some expert, but surprisingly simple, guidelines. We asked sports dietician Renee McGregor for everything runners need to know...

Why is adequate hydration while running so important?

Before we dig into the guidance, it's helpful to understand why adequate hydration is so important – both in our day-to-day lives and while on the run.

As humans, we are only designed to deal with heat or limited water for a certain period of time. While we can survive for as long as a month in a moderate climate without food, we would struggle to stay alive for longer than two days in desert conditions without water.

After oxygen, water is a close second on the list of essentials for life. Water makes up approximately 60-70% of the human body and performs many crucial functions, including nourishing cells; carrying food through the body; eliminating waste; regulating body temperature; cushioning and lubricating joints; and maintaining blood volume and pressure.

Every day we lose fluid by sweating, breathing and urinating. But it's the sweating in particular that runners need to pay attention to, because as soon as you start to run, you start to dehydrate.

About 75% of the energy you put into exercise is converted into heat and is then lost. This is why exercise makes you feel warmer. Extra heat has to be dissipated to keep your core body temperature within safe limits – around 37-38°C – and your body keeps cool by sweating.

Our core body temperature therefore rises if we become dehydrated, as the body cannot sweat enough, and this decreases our plasma (blood) volume. This then results in an increased heart rate, which accelerates fatigue, impairing our running performance. Dehydration also impairs cognitive function, which can make it harder for us to make decisions.

How much should I drink before a run?

One of the best ways to prevent dehydration during training and racing is to make sure you're hydrated before you start, meaning you have a larger reservoir of fluid to draw from during your session.

But this doesn’t mean chugging lots of water before heading out the door. ‘It’s not efficient to just drink a load in one go and it will be sloshing around in your stomach,’ says McGregor. ‘You need to regularly hydrate throughout the day or, if you’re going out first thing in the morning, just make sure that you have hydrated well enough.’

How many litres of water should I drink per day then?

So how much should you drink per day to ensure you’re optimally hydrated? ‘It’s very individual because we all have different water losses, particularly if we’re running,’ explains McGregor. ‘And things like whether you’re running indoors or outdoors are all going to make a difference to what your fluid losses are.’

We’re frequently told to drink eight glasses of water a day, but this doesn’t account for lots of individual and lifestyle factors, says McGregor. ‘The recommendation of 1.5 litres minimum per day is based on what the body needs fundamentally just to function, but obviously lifestyle factors come into play. If you’re in a heated office you’re probably going to lose more fluid and if you’re drinking lots of tea, you’re going to get additional fluid through that, but equally if you’re drinking lots of coffee and caffeine you might dehydrate yourself, so I don’t think we can put a number on it because it’s very individual.’

We can also get fluids from the foods we eat – some fruits, for example, have a very high water content, such as watermelon and strawberries, but foods such as plain yoghurt and cottage cheese are also rich in water.

Individual and genetic factors like your body size will impact how much fluid you need, too, as smaller people will lose less fluid than bigger people.

Your can gauge your hydration levels by thirst, but also the colour of your urine. ‘If you’ve got really dark urine you’re obviously not drinking enough, but if you’ve got that pale straw coloured urine generally throughout the day, then that’s a pretty good indicator that you’re hydrating enough for you,’ says McGregor.

How much should I drink during a run?

Most runners sweat between 400 to 2,400ml per hour of exercise, with the average value being around 1,200ml per hour, explains McGregor. However, she points out that, as with your fluid consumption outside of running, it will vary dependent on your age, sex, weight, exercise intensity and the environmental temperature.

A good way of measuring your individual fluid loss on the run is by doing an at-home sweat test: weigh yourself in the nude before a run, then run at race pace for one hour and then weigh yourself after the run. Make sure you go for a wee before weighing yourself and towel off any sweat before weighing yourself again afterwards. Subtract your post-run weight from your pre-run weight and convert to millilitres (rule of thumb is that 1 gram = 1 millilitre) – and that’s roughly how much fluid you’ve lost. This is a method that has been around for many years but is the still the gold standard – and still used by professional sports teams.

So, once you’ve worked out exactly how much fluid you lose, should you aim to replace that on the run? ‘You don’t need to replace that amount while you’re running, but we do encourage for that to be replaced within one and a half to two hours of finishing,’ says McGregor. ‘It would be impossible to drink that volume while you are running but that’s why recovery is so important.’

However, if you’re running for over an hour, it’s still important to hydrate as you go, but again, the amount you aim for will depend on a number of factors. For example, some people drink much less than others generally in their day-to-day life, points out McGregor.

‘The general rule of thumb appears to be a minimum of 50ml every 20 minutes up to 250ml every 20 minutes,’ says McGregor. ‘The maximum amount most people can consume in an hour roughly is about 750ml – maybe a litre if you're a bigger individual – but that seems to be the maximum the body can actually absorb.’

Although McGregor says that being guided by the body and drinking to thirst can be useful, she highlights the importance of being mindful of much how fluid you’re taking on – particularly during longer races.

What are the symptoms of dehydration?

As well as experiencing a loss of energy and stamina on the run, other main symptoms of dehydration include: dry mouth, dark yellow urine, decrease in urination, muscle cramping and headaches. Chronic dehydration symptoms include low blood pressure, rapid heart rate, rapid breathing and sunken eyes.

Do you need to hydrate if you’re running for less than hour?

If you’re running for under an hour, you probably don’t need to take a drink with you, as long as you’re hydrated beforehand, says McGregor. ‘Once you start getting into running for 90 minutes, that’s when you need to think about carrying a drink around with you,’ she says.

What about electrolytes?

When we lose fluid, we also lose electrolytes – the main one being sodium – and so maintaining a good sodium balance is crucial. ‘The general recommendation for sodium balance – especially for endurance running – is to aim for a minimum of 700mg of sodium per litre of fluid you consume,’ says McGregor.

However, the sodium content of sweat also varies substantially, from 115mg to greater than 2,000mg per litre of sweat, and so if very individual. The easiest way to work out how much sodium you’re losing when you sweat is by doing a sweat test, but you can also understand how your body responds to different levels of salt and fluid intakes via trial and error. ‘It’s about knowing you,’ says McGregor. ‘For example, when I finish a run, there’s salt crystals everywhere on my kit, so that’s pretty big evidence that I’m a salty sweater.’

You can consume electrolytes via various sports nutrition products in different forms, such as tablets, powder and energy drinks, as well as through real foods, such as salted peanuts. 'Drinks like Lucozade and Gatorade are often supplied at races, and those will help with consuming a bit of salt, and you can also use energy gels that contain electrolytes,' advises McGregor. 'Taking some salt caps with you on training runs and races will also help.'

How much should I drink after a run?

Modest dehydration is normal following a race and is a temporary condition for many runners that doesn't lead to any serious medical conditions. Elite athletes, for example, don't have time to drink very much at sub-5:00min/mile pace during a marathon, and are probably the most dehydrated runners on the course – a state that is easily and quickly reversed afterwards as long as you rehydrate properly afterwards. But exactly how much fluid should you take on when you’ve finished your run?

‘There used to be guidance about trying to get on board 1.5 times the amount you've lost, but obviously if you don't know exactly how much you've lost then that's a bit tricky to work off,' says McGregor. 'So I would say definitely try and rehydrate in the first hour immediately after finishing your run. After then keep an eye on your urine colour because that is a really good indicator of how your body's coping, fundamentally.’

It is possible to overhydrate?

In some cases, it’s possible to overhydrate – and this becomes an issue if you’re not keeping sodium levels balanced. It’s far more likely to happen during longer runs, such as marathon distance and beyond. ‘Say you’re running the London Marathon, and it’s a particularly hot day, and you’ve trained through the winter months where you probably haven’t sweated that much, but then April comes around and it’s really hot, you understandably feel thirsty because you’ve lost a lot of fluid, so you keep drinking but you’re not replacing salt at the same time.’

This is where you need to be mindful of how much water you’re consuming and the potential salt losses that could be happening at the same time, says McGregor.

Symptoms of overhydration include: a sloshing around in your stomach, which indicates that you’re not absorbing fluid as well, nausea, vomiting, dizziness. In more serious cases, you may not be able to think clearly and eventually you can collapse, warns McGregor.

The bottom line? Listen to your body, get to know what works for you and be diligent about keeping on top of your fluid and electrolyte intake – especially during longer training runs and races. The key is in your pee!