If you're a runner, you'll know how important it is to fuel your body with the right foods. But what about liquids? Elite sports nutritionist James Collins explains exactly why your body needs to be hydrated during a long run or marathon, how often you should drink, which drinks to enjoy, those to stay away from and how to get your fill of electrolytes.


Next check out our marathon nutrition and training tips, including our marathon meal plans, carb-loading guidance and how to eat for recovery.

Male runner drinking water on a scenic run

Why is hydration important?

The overall goal of any hydration strategy is to replace the body's water and electrolytes that have been lost through sweat. However, one size does not fit all – sweat rates vary greatly between individuals so you need to develop a strategy that is unique to you.

When we lose more water than we take on, we become dehydrated and the effects and symptoms we experience can range from mild to more serious including, in serious cases, confusion and lack on consciousness. Sweat rates during exercise vary from between 0.3 to 3 litres per hour, with a loss of 1kg of body weight equalling around a litre of sweat loss. If you experience fluid losses of greater than two per cent of body weight (1.4kg for a 70kg person), then you may show reductions in both physical and cognitive (decision-making, concentration) function.

Symptoms of dehydration

There is no one measure for dehydration. Symptoms include a loss of body weight, producing a small amount of dark coloured urine and a sensation of thirst. If two or more of these symptoms are present then dehydration is likely. Evaluating these measures helps you develop an appropriate strategy for maintaining your hydration.

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During training, runners should attempt to estimate sweat rates. This is easily done by weighing themselves (in minimal clothing) before and after a run (following towelling down), then subtracting any fluid consumed – remember every 1kg of body weight lost is roughly equal to 1 litre in sweat.


a) Two-litre loss from a two-hour run equals a sweat rate of 1 litre per hour
b) One-litre lost from a one-hour run equals a sweat rate of 500ml per hour

Note: This estimation should be repeated in alternative temperatures as sweat rates will be affected by environmental conditions.

Because sweat and urine losses will continue after your run, aim to replace a greater volume of fluid (e.g. 1.25-1.5 litres for every 1 litre lost). Take your time with this, don’t gulp down large volumes. Alongside this, runners should listen to their body and drink according to thirst. The body's physiology is tightly regulated so when there is excessive sweat loss (affecting blood levels) the thirst mechanism is triggered.

Can I drink too much?

Don’t forget over-hydration can also be a problem in long races like a marathon. Recreational runners drinking large amounts of water at each opportunity can lead to hyponatremia (a dilution of blood sodium), which may have serious consequences.

Runners running across a bridge

How often should I drink during a run?

First off, make sure you start your race fully hydrated – drinking approximately 500ml of fluid two hours before the race should suffice. This will allow any excess to be passed as urine before you line up at the start line.

Just before the start, plan to drink a small amount of fluid (about 150ml). During your run, drinking should match sweat and urine losses as closely as possible, taking care not to over-hydrate. Aim to drink small amounts regularly, rather than gulping down large volumes and listen to your body when you are thirsty.

What are electrolytes and where can I get them?

Electrolytes are found in blood, sweat and other bodily fluids and have an important role to play in maintaining fluid balance, activating our muscles and preventing cramp. Like fluid, electrolytes are lost in sweat and may need to be replaced.

Sodium, body levels of which are tightly controlled, is the most important electrolyte for hydration because it enables the body to absorb and retain fluid, while other electrolytes include potassium, magnesium and chloride.

You can top up your electrolyte levels with sports drinks and gels, which are an easy way to absorb and retain fluid during heavier sweat losses (e.g. during a marathon or when running in intense heat).

Can I drink water during a marathon, as I don't like sports drinks?

If you prefer, you can stick to water but preferably only in the early stages of the race. You are advised to consume electrolytes, from sports drinks or gels, later in the race and especially if it is a hot day.

Try out sports drinks or gels during training to see if you can find one that works for you. During an event, consume sports drinks at the drinking stations on route – you could rinse your mouth out after, if it's the taste you don't like. Whether you're running in the London Marathon or another, the event website should tell you what products will be available on the day.

Can isotonic gels play a part in my hydrating strategy?

Also known as electrolyte gels, these products are formulated to contain the correct water and electrolyte balance so they can be a helpful part of your hydration strategy. They are thinner in consistency to energy gels and include electrolytes such as potassium, sodium and magnesium. Their goal is to aid hydration and prevent adverse symptoms like cramping. Although you can consume these with additional water, it is generally not advisable to do so. As with all products, it's important to trial these during training first.

Pints of beer and ale on a wooden table

Can I continue to drink alcohol leading up to a race or should I cut it out?

It's best to cut out (or at least cut down) alcohol consumption leading up to an event, as it can interfere with the body's maintenance of blood glucose and hinder your recovery (including sleep quality) after training. If you really can't resist a tipple, it's best that you drink after you have fully recovered from your last run.

Want more like this? Try...

What to eat for a run
What to eat for a swim
What to eat on normal training days
What to eat on rest and easy training days
What to eat on heavy training days

Are you training for an event this year? Share your tips and experiences below.

This article was reviewed on 29 December 2023 by Kerry Torrens.

James Collins is recognised as a leading performance nutritionist through his work with Olympic and professional sport. Over the last decade he has worked with Arsenal FC, the England and France national football teams and Team GB. He has a private practice in Harley Street where he sees business executives, performing artists and clients from all walks of life. He is the author of the new book The Energy Plan, which focuses on the key principles of fuelling for fitness.


All health content on bbcgoodfood.com is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. If you have any concerns about your general health, you should contact your local health care provider. See our website terms and conditions for more information.

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