Hydration for a Marathon

Hydration for a Marathon

How much water do I need? This depends on quite a few factors if we look on a general daily basis there can be some debate on how much water to have but looking at around 0.033/ml/kg for an everyday sedentary individual should keep you in a fluid balance. For example, weight 70kg*0.033 = 2.31L water a day at a normal temperature and no activity.


How can you determine how much water you need to take on per hour during a marathon? Water requirements are very individualised and based off of each person’s individual sweat rate. This will also be affected by such things as heat, humidity and intensity of exercise. Though a general rule of thumb of 400-800ml an hour should suffice. But saying that there is a better way to work this out as mentioned.


Is there an equation or rough guide people can use to work this out, in relation to their height, weight and running intensity? How much will I be losing in sweat?  You can in fact perform your own sweat test which is easily done:

  1. Take a standard session you might do – ideally at race pace- for this let’s say 1 hour. Weigh yourself before (naked).
  2. Drink a controlled amount of water with you – for example let’s say 1 bottle of 500ml, ensure you drink all of this over the session.
  3. After the hour session before consuming anything else, undress, wipe yourself down with a towel and weigh yourself again.


This number at the end will let us know your average sweat rate per 1Hr at race pace at that outside humidity and temperature.


Pre-race hydration – how much extra water should you be taking on each day in the week before your race to ensure you’re adequately hydrated? Over hydration protocols have been shown to not be an effective form of hydrating in the lead up to an event. You’re best off making sure you meet your daily needs and then on the morning of just ensuring you’re adequately hydrated as I describe below.


How much water should you consume in the hours before your race? Start sipping about 600ml of water with an electrolyte in 3 hours before the race until your urine is clear if it still isn’t clear have another 400ml on top of this.


How can you tell if you’re adequately hydrated? Urine colour is the easiest way to check with the darker it being the more dehydrated you are and the lighter/clearer it is being more hydrated. You can if you want though


Should you avoid ‘dehydrating’ tea and coffee? Tea or coffee does not dehydrate you in itself – it would be illogical to think taking on more fluid will dehydrate you. But caffeine is a diuretic meaning it will cause you to pass urine more frequently. So, every time you go the bathroom you should be topping up. If anything, caffeine is proven for its performance enhancing abilities and I would recommend it at 3mg/kg/bodyweight 30-60mins prior to the event. Though this should be practiced and/or consulted with your GP if you have any reason to be wary of high caffeine intakes


How will the weather on the day impact your hydration? Will be there be enough water stations on course, or should you carry some water with you? How can you plan for this? It takes 2 weeks to acclimatise to a certain temperature. Running regularly and sessions at “race pace” will ensure you’re acclimatised but if it’s hotter on the day understand that your fluid requirement with alter with an increased sweat rate.


What are the signs of dehydration and overhydration – how do you know if you’ve had too little or too much water? Is the risk of overhydration (hyponatremia) worse than the risk of dehydration? There are quite a few signs of dehydration and overhydration with hyponatremia being a place that can be caused both by having too much fluid or not enough as the main issue is electrolyte balance, symptoms of this can be causes fluid movement into the brain, causing swelling with symptoms that can progress from feeling strange, to mental confusion, general weakness, collapse, seizure, coma and death.


One noted sign is to see your heart rate raise 5-8bpm for every 1% of dehydration.

Using the sweat test making sure you don’t lose more than 2% of your bodyweight is important as past this point you will start to struggle and start to get issues.

How to work this out:

70kg * 0.02 = 1.4kg or 1.4L of water

If in your 1 hour sweat test while consuming 500ml of water at the end weight 69.5kg we know you lose 1L/1kg of water an hour at the race pace, at the temperature/humidity.

If you carried on drinking at that rate for 3 hours you would now be 1.5L dehydrated which is over the 2% which we want to avoid.


Is the ‘drink when you’re thirsty rule’ reliable while running? Typically, if you’re drinking when you are thirsty rule probably means you’re already a bit dehydrated – but as explained over hydration isn’t good either. Steady consumption of water that is matched with your hydration testing and making sure you’re not losing more than 2% bodyweight should be the focus.


What’s the best method for alternating water with energy drinks (London usually has Lucozade Sport stations every 4-5 miles) and why should you be drinking Lucozade Sport/energy drinks as well as water? Muscle glycogen stores (stored carbohydrate) typically will only provide enough energy for ~90mins of exercise so taking on carbohydrate each hour will enable you to maintain blood glucose levels and keep performing. Carbohydrate solutions of 3-5g per 100ml also improve your uptake of water hydrating you better and typically sports drinks contain electrolytes too.


How important is it to take on electrolytes both during and before the race? Incredibly important – water on its own doesn’t hydrate you as well as if it contains electrolytes as mentioned but things like hyponatremia only happen with excessive water intake and no electrolyte intake upsetting the balance and essentially diluting yourself. You can get your sodium loss rate tested but if you’re leaving salt lines on your face or clothes you are probably a salty sweater.


Recommendations of 0.5 to 0.7 g/L of sodium for exercise lasting <3hours with longer durations going up higher per hour. Your typical electrolyte tab is around 0.4g.


Can you explain how energy drinks can contribute towards grams of carbs you need to fuel yourself per hour, and how many they can supply? Well it depends on the drink used, but performance improvements have been seen from as little as 20g an hour. I would suggest aiming for 40-60g an hour at least and more if you’re running intensely. Getting this from a glucose:fructose mix at a 2:1 ratio with improve your ability to digest it and reduce incidence of gastro-intestinal distress. The reason for carbohydrate-based drinks is simple the maintenance of blood glucose enables you to keep performing the exercise you’re doing. The amount we need will vary depending on exercise intensity, duration and also how well trained an individual you are.


Post-race hydration – how much water should you take on after your race? Ensuring you’ve replaced what you’ve lost should be the main goal so continuing to drink water with electrolyte for a while after the event should be expected – ensuring you’re getting the right mixture as described above to help.

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