How to stay hydrated during a marathon

Precision Hydration

You’ve nearly completed all your training. Just weeks before your marathon, your hard work should essentially be in the bag.

However many runners still overlook their hydration strategy and leave it to chance on the day. Last year’s Virgin Money London Marathon saw record temperatures of 24.1°C (75.3°F) and this affected many runners’ performances and, in the worst case, left one mum of 4 in a coma due to over hydration (a condition known as hyponatremia).

This is why the Run Deep team hooked up with the hydration experts from Precision Hydration to ask their advice on what to do this year. This is what their founder, Andy Blow, had to say on the subject.


When you’re racing for two hours or more, you’re right on the threshold at which good hydration practices start to become crucial if you want to perform at your best.

Here’s some things to consider when thinking about your marathon hydration strategy.

How to start hydrated (and why that’s important)

When people talk about hydration, most of the time it’s about what and how much athletes should drink during exercise. These are clearly important questions, but your performance is also massively influenced by how hydrated you are when you start exercising in the first place.

Once you begin sweating, you’re generally going to be fighting a losing battle against fluid and electrolyte loss, so starting off properly hydrated can be extremely beneficial. When you’re properly hydrated, you have a larger reservoir of fluid to draw from over time than if you’re dehydrated.

Starting well hydrated has other benefits too. Optimal hydration maximises your blood volume and this helps general cardiovascular function and your ability to dissipate the heat produced by your working muscles. This reduces fatigue and enables you to maintain your performance for longer.

Despite the relatively obvious benefits of starting exercise well hydrated, a recent study of over 400 amateur athletes showed that around 31% of them were turning up to training sessions (and, in some cases, competitions) dehydrated!

Here are some tips to help you start well hydrated

DON’T just drink lots of water in the build-up to the race. You can end up diluting your body’s sodium levels before you start, increasing the risk of hyponatremia – which we know is a potentially race-ruining (and even dangerous) condition. At best you’ll end up with a lot of fluid sloshing around in your stomach/bladder.

Drink a stronger electrolyte drink the night before the race to boost your blood plasma volume. Aim for drinks containing >1,000mg of sodium per litre, like our PH 1500 supplement.

Drink another bottle of stronger electrolyte drink about 90 mins before you start to top-up your blood plasma volume. Finish your drink >45 minutes before you set off to give your body time to fully absorb what it needs and remove any excess.

Adding additional sodium to your pre-race drinks (and meals) helps you absorb and retain more fluid in your bloodstream. This means you’ll have a bigger reservoir of electrolytes/ fluids to draw upon once the race begins and you start sweating some of it out. Having more blood makes it easier for your cardiovascular system to meet the competing demands of cooling you down and delivering oxygen to your muscles.

Read more about how to start hydrated.

What to drink during the race

How much should you drink?

Every athlete is different but, as a rule of thumb, very few runners can comfortably drink much more than 750ml (24oz) per hour – especially when running hard – so unless experience tells you otherwise, it’s unlikely you’ll need to drink more than that, especially if you got to the start line well hydrated.

What should you drink?

Personalising your hydration strategy could really make a tangible difference to your result.

That’s because everyone loses a different amount of sodium in their sweat and, if you don’t get replacing that right, you risk hydration-related issues like cramp, dehydration and hyponatremia.

Maintaining the sodium levels in your blood is crucial to performing at your best when you’re working hard. Sodium helps you absorb and retain fluid, which keeps your blood volume up, reducing cardiovascular strain, fatigue and potentially helping you avoid cramping up.

Just drinking water when sweating over long periods dilutes your sodium levels, which can really impact your performance and could lead to hyponatremia, which can have some nasty consequences as highlighted in last year’s London Marathon where the runner we mentioned at the start of this article, ended up in a coma with the condition.

Of course, this is an extreme and rare example but it does highlight how bad things can get if you get your hydration strategy badly wrong.

How much sodium you lose is largely genetically determined and doesn’t change too much due to acclimation, diet etc. So, a ‘one-size-fits-all’ approach to hydration just doesn’t work.

Take the free online Precision Hydration Sweat Test as a starting point for tailoring your hydration plan to your needs:



How to drink during a marathon

Getting fluids in without choking or spilling most of it down your front whilst running is not an easy task! Here’s some advice on how to overcome the issue of drinking on the move during the Virgin Money London Marathon.

At the 2019 London Marathon there will be drink stations offering water at every mile between miles 3 and 25 (apart from miles 7 and 11). That lovely H20 is supplied in 250ml race bottles with a flip-top.

Here’s how to nail drinking on the move.

1) Line up on the correct side of the road

No-one benefits from a last minute sideways surge across the path of other athletes, so move across with a couple of hundred metres to spare and try to establish yourself in a gap in the traffic if you can.

2) Slow down for the grab, and make eye contact

Whether volunteers are handing out the drinks, or you’re taking them from a table, it’s a good idea to slow down a bit as you approach ‘the grab’. The loss of time is insignificant but it massively reduces the risk of simply spilling the drink all over the aid station workers, yourself and the pavement!

The water will be supplied in 250ml race bottles with a flip-top but three Drinks Stations will be trialling compostable cups instead of bottles this year. Water stations at miles 10, 19 and 23 will offer drinks in compostable cups. Try drinking from a cup in training before Race Day.

If you’re being handed a drink by another person, it’s a good idea to make eye contact with them on the approach, maybe even pointing to them to signal your intent so they know they should give the drink to you.

A breathless ‘thank you’ as you run off is an optional but often appreciated gesture if you can muster the energy!

3) Don’t rush

Many athletes feel the need to drink the contents of whatever they pick up from an aid station within about 10 metres of collecting it.


Once you have the drink in hand, gather yourself, calm your breathing and take in small sips of fluid over a few hundred meters until you feel you have had what you need.

Do be mindful of where you end up dumping the cup or bottle (different races have different rules and clean up processes) so you’re not littering.

4) Dump the leftover water on your head

There are a few potential benefits to dumping water on your head, especially in hot conditions. If you end up with spare water after an aid station pickup, it may be better to dump it on your head rather than throw it away.

Doing the same with a sports’ drink is not recommended though, as things can get a bit sticky…

Bonus tip: Make sure you rehydrate properly once you’ve finished your race.

You should be able to top up on the fluids and electrolytes lost in your sweat through the food and drink you normally eat in the hours after the race.

If you struggle with cramp, or feel particularly dehydrated, some more deliberate fluid intake and sodium supplementation might be helpful. Here’s some advice on how to speed up your recovery by rehydrating more effectively.

If you want to put any of the above into action in training, visit and use the code RUNDEEPMAG to get 15% off your first order of Precision Hydration electrolytes before the end of April 2019.

Enjoy your race!

Precision Hydration