Aim to consume around 400ml to 800ml of water during a marathon
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Marathon

How to perfect your marathon hydration strategy

Drinking the right amount of water is crucial to race success. But how much is too much? And what happens if you don’t drink enough? We asked an expert nutritionist for their advice.
Written by Howard Calvert
6 min readPublished on
You’ve trained for months, feverishly following your schedule to the letter. All the early morning runs in the dark when your bed seemed so tempting, and the increasingly long Sunday sessions have built up to this: the day you run 26.2 miles.
You’ve got your nutrition plan locked in place – you know your preferred brands, and what you plan on consuming both the night before and during the race.
But there’s one thing you might not have considered: hydration. Often, runners don’t think about it until the morning of the race. And considering that 83% of you is water, failing to prepare could make or break your run.
We asked leading sports and performance nutritionist Will Girling for his advice on how much liquid you should be taking onboard before, during and after the big event.

The minimum amount

Girling suggests hydrating with 0.033ml/kg a day for a sedentary individual
Girling suggests hydrating with 0.033ml/kg a day for a sedentary individual
There’s a raft of conflicting advice regarding how much water our bodies need and how much water we should drink day to day, but as a guide, Girling suggests 0.033ml/kg for an everyday sedentary individual. So, for example, if you weigh 70kg, that equals 2.31 litres of water a day when it's a normal temperature and you're not doing any activity.
The tricky part comes when attempting to calculate how much water you should consume during a marathon. “Water requirements change based on each individual’s sweat rate,” says Girling. “This will also be affected by such things as heat, humidity and intensity of exercise. However, as a general rule of thumb, consuming 400ml to 800ml of water an hour should suffice.”

Do the sweat test

Doing a sweat test will help you perfect your hydration strategy
Doing a sweat test will help you perfect your hydration strategy
It is possible to get more accurate than that. Girling says that you can perform your own sweat test to determine how much sweat you lose during an hour of exercise and, therefore, how much water you should be drinking to replace that.
“To do this test, run at race pace for 60 minutes. Weigh yourself beforehand with no clothes. Take a controlled amount of water with you on the run – for example 500ml – and ensure you drink all of it during the session. After the run, undress, dry yourself with a towel and weigh yourself again. The difference in your weight will let you know your average sweat rate per hour at race pace at that specific humidity and temperature.”
The reason for doing this is to make sure you don’t lose more than two percent of your bodyweight during the race, as anything past that point will cause you to experience issues and potentially start to struggle. Work out this figure by taking your weight and multiplying it by 0.02, so 70kg would equal 1.4 litres of water. “If after the sweat test you weigh 69.5kg we know you lose a litre of water an hour at the race pace, at that temperature/humidity,” says Girling. “If you carried on drinking at that rate (500ml per hour) for three hours you would be dehydrated by 1.5l, which is over the two percent we want to avoid.”

The day of the marathon

Checking your urine colour will determine how hydrated you are
Checking your urine colour will determine how hydrated you are
According to Girling, studies have shown that over-hydrating in the run-up to a marathon has proved ineffective. The best practice, he says, is to hydrate normally in the run-up to the day of the event. “Start sipping about 600ml of water with an electrolyte tablet in it three hours before the race, and drink until your urine is clear. If it still isn’t clear, drink another 400ml on top of this.” The 'check the colour of your urine' test is a reliable way to determine how dehydrated you are: the darker it is, the more dehydrated you are, and the lighter/clearer it is, the more hydrated you are.

Don’t avoid tea or coffee

The caffeine in your pre-run coffee can help give your race a boost
The caffeine in your pre-run coffee can help give your race a boost
“Tea or coffee does not dehydrate you in itself, but caffeine is a diuretic meaning it will cause you to pass urine more frequently,” says Girling. “So every time you go to the bathroom you should be topping up on water.”
But there’s more to your pre-run caffeine than just hydration benefits. “Caffeine is proven in its performance-enhancing abilities. I would recommend 3mg/kg/bodyweight 30-60mins prior to the event. Though check this with your GP if you have any reason to be wary of high caffeine intakes.”

Try to acclimatise to the race-day weather

Acclimatising can be hard, but training at race pace should help
Acclimatising can be hard, but training at race pace should help
With many races, the weather can prove tricky to predict, as 2018’s hottest London Marathon on record showed. “Running regular sessions at race pace will help to ensure you’re acclimatised,” says Girling. “But if it’s hotter on the day, understand that your fluid requirement will alter with an increased sweat rate.”

Don’t always live by the ‘drink when thirsty’ rule

Hydrating little and often is a good strategy to employ
Hydrating little and often is a good strategy to employ
“Typically, if you’re drinking when you’re thirsty, it means you’re already a bit dehydrated,” says Girling. “Steady consumption of water that’s matched with your hydration testing, and ensuring that you’re not losing more than two percent bodyweight, should be the focus.”
At the London Marathon, there are water stations every two miles from mile three. They will supply 250ml bottles, so you can keep an eye on exactly how much water you are drinking.

Electrolyte balance is crucial

A lack of electrolytes can cause fatigue, dizzyness or worse
A lack of electrolytes can cause fatigue, dizzyness or worse
There are multiple signs of dehydration and overhydration, with hyponatremia – where your blood's sodium levels are too low – being a condition that can be caused both by having too much fluid or not enough. "The main issue is electrolyte balance,” says Girling. “It can cause fluid movement into the brain, causing swelling with symptoms that can progress from feeling strange to mental confusion, general weakness, collapse, seizure, coma and, ultimately, death.”
One sign to look out for is your heart rate rising 5-8bpm for every one percent of dehydration.
It’s incredibly important to take on electrolytes while running. “Water on its own doesn’t hydrate you as well as it does if it contains electrolytes, but hyponatremia only happens with excessive water intake and no electrolyte intake, which upsets your body’s balance and you essentially dilute yourself,” says Girling. “You can get your sodium loss rate tested, but if you’re leaving salt lines on your face or clothes you are likely to be a salty sweater.”
Girling recommends 0.5 to 0.7g/l of sodium for exercise lasting less than three hours, with longer durations requiring more. Average electrolyte tabs are around 0.4g.

Make sure you grab some energy drinks, too

Repartiendo energia en los aid stations
Most long-distance runs will hand out energy drinks along the route
As well as providing electrolytes, energy drinks provide carbohydrates, which help replenish glycogen stores (which generally run out after two hours of exercise), meaning you can maintain blood glucose levels and avoid 'hitting the wall'.
“Performance improvements have been seen from as little as 20g [or carbohydrates] an hour,” says Girling. “I would suggest aiming for 40-60g an hour at least, and more if you’re running intensely."

Keep drinking after the race

Replace each 1kg of your weight (sweat) loss with 1.2 - 1.5 litres of fluid
Replace each 1kg of your weight (sweat) loss with 1.2 - 1.5 litres of fluid
On reaching the finish line, your marathon effort is over, but your hydration strategy isn't (just yet).
“Ensuring you’ve replaced what you’ve lost should be the main goal after finishing, so continue to drink water with electrolytes for a while after the event,” says Girling.