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


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

Q: What are the different strategies for staying hydrated during a marathon?

Standing on scales

James says:This is often a contentious issue, and ultimately takes some planning and a bit of common sense. The overall goal of the hydration strategy is to replace the body's water and electrolytes that are lost through sweat.

When we lose more water than we take in, we become dehydrated and the effects can range from the mild to the much more serious, including in extreme cases, death. Sweat rates during exercise vary greatly, between 0.3 and 3 litres per hour. With 1kg of body weight loss equalling around a litre of sweat loss, fluid losses of greater than 2% of body weight (1.4kg for a 70kg person) have shown reductions in both physical and cognitive (decision-making, concentration) function.

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 are present then dehydration is likely. Both of these measures can help runners develop an appropriate strategy to stay hydrated – sweat rates can vary greatly between individuals and one size doesn't fit all.

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. Every 1kg lost is roughly equal to 1 litre in sweat. For example, a two litre loss from a two hour run would equal a sweat rate of one litre per hour. For example, 1 litre lost from a two hour run would equal a sweat rate of 500ml per hour.

Note: This estimation should then be repeated in hot temperatures as sweat rates can vary greatly in hot conditions.

Because sweat and uring losses will continue after the run, once you have finished 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, instead of gulping down large volumes.

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Once this has been worked out, a basic strategy can then be formed to reduce dehydration during the race. Alongside this, runners should fundamentally listen to their body, and drink according to thirst. The body's physiology is tightly regulated so that when there is excessive sweat loss (affecting blood levels) the thirst mechanism is triggered.

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 can have serious consequences.

Q: How often should I drink during a run?

James says: 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 the start.Just before the start, plan to drink a small amount (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.

Q: What are electrolytes and where can I get them?

James says: Electrolytes are found in the blood, sweat and other bodily fluids and have an important role in maintaining fluid balance within the body. Sodium is the most important electrolyte for hydration, while others include potassium and chloride. Sodium levels in the blood are tightly controlled. Fluid and electrolytes are lost in sweat and therefore need to be replaced. Sodium allows the body to absorb and retain more fluid, thus helping to maintain hydration.You can top up your sodium levels with sports drinks and gels, which are an easy way to absorb and retain fluid during heavier sweat losses (e.g. during the race or running in the heat).

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

James says: If you prefer, you can base your hydration strategy around water earlier in the race. It is, however, advised to consume electrolytes, from available sports drinks or gels, especially if it is hot on the day.

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

Q: Can I continue to drink alcohol in moderation leading up to a race or is it better to cut it out altogether?

James says: It's better to cut out (or at least cut down on) alcohol consumption leading into an event, as it can interfere with the body's maintenance of blood glucose and recovery (including sleep quality) after training. If you really can't resist the odd tipple, ensure that you drink after you have fully recovered following your last run.

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

This article was reviewed on 7 September 2021.

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.


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