Knowing how much to drink and when during a marathon can be tricky. It’s easy to go overboard and the science isn’t all in agreement either. Here are some tips that might just help you out.


How much fluid you’ll lose

During intense exercise you can lose around 1 litre of fluid per hour as sweat and through your breath. In hot or humid conditions, you’ll naturally lose a little more, and in cold conditions you’ll lose a little less.


A marathon is an intense activity due to its duration, but the actual rate of exertion is much lower than you’re capable of, so you’re unlikely to be losing a litre per hour. Depending on your own perspiration levels, you may instead be losing around 700-800ml per hour.


You don’t need to replace it all

One of the by-products of cellular respiration (the process that releases energy) is water, and when running a marathon your cells will be doing a whole lot of respiring! This means you don’t need to drink as much as you perhaps thought.


Dehydration is inevitable

You’d have to drink more than you’d feel comfortable with to avoid a net loss of fluid during a marathon. It would be sloshing around and probably make you feel pretty sick.

A study of elite and non-elite marathoners was undertaken a few years ago with runners being weighed before and after the race, and those who lost a small percentage of their bodyweight as fluid during the race had faster times on average. Essentially those who drank more, ran slower.


You do need to drink

It would be irresponsible of me to not point out that fluid intake is important in a marathon, and ultimately the chances of ill health from drinking too little are far more common than drinking too much. Both can contribute to fatal consequences unfortunately, so please, do drink.


How much to drink

If we go with the estimate of 700-800ml of sweat loss per hour, with some of this being replaced by water from respiration, we can set a goal of around 600ml of fluid per hour as a sensible target that should be perfectly safe, and not leave you feeling too bloated or in desperate need of a roadside portaloo. Simply multiply 600ml by the number of hours you’ll likely be running and you’re there.


What to drink

If your fluid is also your carb source such as Lucozade or Tailwind, you may want to still take water from drink stations occasionally, especially in warm weather since you’ll lose more fluid but won’t be losing more energy.

You can add electrolytes to drinks as well if you find you prefer this or tend to lose a lot of salt (often seen by white stains on the arms, neck and forehead), although check any gels you’re using as some brands such as Torq have electrolytes in them already, so adding more could be a bad idea.


When to drink

Hopefully it goes without saying, but make sure you’re well hydrated before the race starts.

When you get underway, start drinking early. You won’t need it just yet, and may not feel you want it, but start keeping on top of things in the first few miles and it’ll make things go much more smoothly. I tend to take a bottle or cup of water from almost every station, excluding the last one usually as it’s a distraction when I need to focus on fighting the pain of the last 2 miles or so.

If I run with a hydration pack such as during a training run or ultramarathon, I’ll have a few sips every mile rather than a larger drink every 3 miles or so (where the water stations tend to be situated). There’s not a particular reason for this other than I haven’t got to remember to drink if I just do it when my watch beeps for the mile, and it stops any sloshing or long periods of time where I’m breathing ineffectively in order to drink.


To recap, aim for around 600ml/hour in normal conditions, start early and drink regularly.


Written by Kyle Brooks, Running Coach based in Norwich, Norfolk