Health Lifestyle Women's Fitness Women's Health

How to Nail Your Marathon Race Day Hydration

58 total views

Hydration is something a lot of runners get wrong during the marathon, especially when we have a freak hot day in April and race day is a lot warmer than most of the long training runs we’ve done during the winter months!

The best thing you can do in preparation for your marathon (apart from tapering and carb loading!) is to make sure you are hydrating well during race week.

For most women our daily hydration needs are about 2.7L per day (20% of which we typically achieve through food intake). During race week I typically aim for around 2.5L fluid per day, through a combination of water, juice (carbs!), coffee/tea, carb drinks and electrolyte drinks. Don’t forget to take a refillable bottle to drink at the expo!!

Why is it important to hydrate on the run?

  • prevent dehydration
  • help absorption on carbs

Dehydration can not only impact our overall health and wellbeing during the race, but also negatively impact performance. Becoming dehydrated reduces blood volume, increases heart rate, reduces gastrointestinal blood flow, raises core temperate and increases the rate of muscle glycogen usage.

What are the signs/symptoms of dehydration?

  • nausea
  • fatigue
  • runners tummy
  • cramps
  • increased perceived effort levels

How much fluid should you drink during the marathon?

Your sweat rate is how much fluid you lose per hour while training. This can vary from person to person as well as based on the climate/intensity of workout etc. Average sweat rates are 500ml to 2500ml per hour!

Sweat Rate

You can work out your sweat rate by weighing yourself naked before an hour run (make sure to go to the loo first!). Ideally you’d do this at a time of day that will be similar temps/conditions to race day. If you drink anything before your run after weighing in make sure to document exactly how much in ml. Run for 60 minutes (or 30 mins and double it), then weigh back in naked!

Take your pre-exercise weight, subtract your post exercise weight = weight lost in grams per hour of exercise. Don’t forget to subtract any fluid drunk after weigh in. 1g = 1ml. Therefore if you lose 500g per hour, your sweat rate is 500m/hr.

The good news is that during the race, you should be aiming for a ‘like for like’ sweat replacement, as you will be starting hydrated and able to replenish post-race.

For most people, drinking to thirst, assessing how you feel and adjusting based on race day conditions should be sufficient. Aiming to drink 300ml – 600ml per hour for a race lasting longer than 60 minutes.

For very heavy sweaters or very hot temps, you might want to increase this to over 700ml – 1L/hr (the max rate of fluid absorption).

Are you a Salty Sweater?

Do you finish a run with salt streaks down your face and clothes? Or suffer with cramping, headaches after a long run/race? Does the salt sting your eyes?

Sodium is the electrolyte lost in greatest amount when we sweat, between 200 and 2000mg sodium per litre of sweat depending on the person!

Last week I was invited to the Porsche Human Performance lab at Silverstone to have some testing done by their specialist team thanks to Precision Fuel and Hydration. Whilst I sadly didn’t spot any of the F1 drivers that I have creepy crushes on, I did learn a lot.

Apparently I am classed as a salty sweater! I often have a salt crust on my face but never on my clothes so had guessed I was medium salty!

You can book to have a sweat test at one of 45 centres around the UK (no running or heat chambers involved!), or for a quicker, slightly more generic test, try the Precision Fuel and Hydration online Fuel and Hydration Planner here.

How much should you drink on race day morning?

Don’t try to chug water to compensate on race morning, you want to go into the day well hydrated!

On race morning try to have a 500ml electrolyte drink about 2 hours before the race. Ideally you want to have finished it at least 45 mins before the race start.

As a salty sweater, I was advised to have a Precision Hydration 1500 tablet containing 750mg sodium or a sachet of PH 1500 with 750mg sodium and 15g carbs, dissolved in 500ml. I would also drink your typical drink with breakfast – for me this would be a coffee (to help get things going!) and some water to help my pre-race bagel go down. I would take the electrolyte drink with me to sip on the way to the start line.

I’d also take an electrolyte drink for your drop bag to drink after finishing and before you get stuck into any of the post race celebratory drinks!

What does Preloading/Hyperhydrating mean?

The goal of preloading or hyperhydrating is to hold on to sodium and fluid the night before a race or hard training session to buffer sweat losses. Whilst not definitive, there is strong evidence that taking additional sodium with fluids before you start sweating can promote increased acute fluid retention and improved endurance performance, especially when it’s hot.

You can do this with high sodium electrolyte drinks, food or adding salt into your diet. For those in the US, Skratchlabs have a very high sodium electrolyte drink (apparently they aren’t allowed to sell it in the UK!) For a British option, try the Precision Fuel and Hydration 1500 tabs or sachets. Salty. sweaters may benefit from having one of these the night before the race, or big training run as well as the morning of. Alternatively have a 2 or 3 of the standard sodium electrolyte drinks.

You could also hyperhydrate by choosing salty food options as part of your carb load, eg pretzels, bagels or adding salt or soy sauce to your foods.

Can you overhydrate?

The simple answer…yes.

You can drink too much water in the lead up to the race and during the event, increasing the risk of hyponatremia (low blood sodium levels). In fact, at one Ironman, 10% of participants were found to have hyponatremia at the start!

The key to avoiding this is to get your sodium intake right!

Best Electrolyte Drinks

Salty sweaters will want to look out for higher sodium content in their electrolyte drink options – just like gels/fuel, not all electrolyte drinks are created equal. As a general rule, most runners will need to take between 700-900mg sodium per 1L fluid, which can come from drink, gels/chews or salt tabs, or real food options (especially in ultras).

I’d suggest looking for electrolyte tabs with 200mg per 500mls. I’ve had an electrolyte taste test requested so keep an eye out for that coming soon!


I have used Nuun for years (and years). I love the flavour options and their vitamin containing tablet options. Personal favourite flavours are Kona Cola, Watermelon, and Strawberry Lemon. Contains 360mg sodium per tab.

Precision Fuel and Hydration

I recently tried these after being recommended the diff strength options. They have 250mg/500ml, 500mg/500ml and 750mg/500ml options in their neutral flavour, as well as electrolyte drinks with added carbs for a real bonus!

ORS Hydration

I really like the lemon and strawberry flavours of these tabs. They seem to be more readily available than many other electrolyte drinks (eg Boots stores) and are also cheaper per tablet than Nuun and PFH. The ORS Hydration have 277mg sodium per 500ml and the ORS Sport have 260mg sodium per 500ml.

Best Carbohydrate Drinks

My personal favourites (although a more thorough taste test is definitely needed);


This was in my water bladder for most of my training runs for the London Marathon over the summer. I loved having an extra blast on carbs on the run (plus Caffeine in some of the flavours). I like the Naked, Cola and Lemon flavours. 50g carbs/200 kcals per 54g sachet.

Precision Fuel and Hydration 60g carb drink

I used this to carb load before the Chicago Marathon. Containing 30g carbs and 500mg sodium per 500ml, this is a really easy addition to your carb loading strategy and can also be sipped on race morning! Love that the flavour is really neutral (like all of their products).

Maurten 320

The marathoners fave, with nearly 80g of carbs in each sachet, this is a great option for those who struggle eating enough carbs in the pre race carb load. 80g carbs, 500mg sodium per 500mls. I haven’t personally tried Maurten drinks but know lots of runners that love them.

There are a wide variety of carb drink options available, and much like gels, can be trial and error as to whether you like the taste. I find Xmiles a good place to start your search or have a look at the race expo. You can also make your own by mixing water with orange juice and a pinch of salt.

How to carry your fluid on race day

This will depend on what you have practiced with, what is allowed on the race course and what provisions are available on course.

For the London Marathon, with water bottles almost every mile, I haven’t ever run with a hydration pack. However I did use one in Berlin, Paris and Tokyo (before they banned them!). A good option would be to take a handheld with you filled with carb drink to sip on for the first 4-6 miles, then hand to a spectator or ditch it in an aid station bin and switch to on course hydration stations. Alternatively, you could have your spectators hand you pre-mixed bottles when you see them.

Personally, i like to walk through the water stations, especially if they are cups, to ensure I get enough to drink before running again. I try to time my gels with the aid stations to take advantage of the water there to wash down the gels!

If you like using a hydration pack, you can use them for London Marathon, plus they have plenty of space to carry all your fuel too! If your race does not allow hydration vests, then working out what you’re going to do on race day is going to be an important part of your training.

What fluid is available on the London Marathon course?

There are Buxton Water stations providing bottles of water roughly every mile from mile 3 onwards. Lucozade Sport is provided in cups at mile 9, 15, 21 and 24.

What should you drink after the race?

Ideally you will take an electrolyte drink in your drop bag to drink after finishing to replace sodium and fluid. You also want a mix of carbs and protein to help with recovery post-race, although by this point you may prefer to enjoy this in food form rather than a recovery drink. For those that struggle to eat post-race, it might be worth bringing a recovery shake mix with you to sip on!

Any other questions about hydration – let me know in the comments or send me a DM on instagram!

Share this Post