Run Mummy Run guide to hydration when running

How much should I drink when I run? Making sense of fluid replacement

Have you ever wondered about fluid replacement?  The Run Mummy Run community often sees questions about “How much should I drink when I run”?  Now that spring has sprung Helen Metcalfe, a G.P. RMR and keen runner has written us an article to explain what to drink and when for runners.

There is a lot of conflicting information out there for runners with regard to fluid replacement. How much should you drink before, during or after a run?

There is the obvious danger of becoming dehydrated. Yet there is also the risk of suffering from excessive water consumption if we drink too much. Drinking too much water can cause hyponatraemia, or low blood sodium, which can cause nausea and bloating in its mildest form, but is dangerous if severe.

This can sometimes be a cause of collapse in marathon runners. Dehydration remains more common than excessive water consumption, but we should bear in mind both.
Fluid Loss
Fluid requirements during and after a run are directly related to how much we sweat and therefore how much fluid we lose. We sweat more in heat, when running for longer, and with increased intensity of exercise. Some of us simply sweat more than others, however unfair that may seem! Whoever said that women ‘glow’ hasn’t seen me after a long run.

The best way of assessing how well hydrated, or dehydrated, you are is the good old simple test of looking at your pee.

Light yellow pee means that you are most likely well hydrated.

Something resembling coca cola, or your high vis neon running jacket, that you have to hold your nose to get close enough to look at, means that you almost certainly need to drink more.

Reducing risk of dehydration

The best way to lower your risk of dehydration, is to ensure that you are well hydrated BEFORE you run. If you have a large glass or around 500 ml of water or sports drink a couple of hours before you run, followed by a small glass of water or 100 – 150 ml just prior to running, you should have time for a couple of toilet stops before you set off.

This will ensure that you’ve emptied your bladder and won’t need to keep stopping to locate bushes every 10 minutes.  This is never ideal – and can already be a bit of an issue after having children (reminder to do your pelvic floor exercises, ladies!).

But before you start thinking that men have it easy in comparison, us women actually have an advantage when it comes to hydration – we sweat less, tend to have lower body mass, and therefore need to drink less to avoid dehydration.

Reducing risk of hyponatraemia

We are however more likely to over hydrate than men, in part for this reason, and also because we tend to be more conscientious when it comes to safety issues, and are more likely to try to drink a small lake in order to avoid dehydration.

Our bodies are designed to be able to cope with a little bit of dehydration, so long as we correct it fairly quickly.  Likewise, you have to go to a fair bit of effort in knocking back the water to suffer the effects of excessive water consumption.
Fluid Replacement
So how much should we drink, and what drink should we be having?

If you’re running a marathon, keep well hydrated in the week prior to the race. If you’re running for under an hour, then you may well get away with not drinking during your run, and rehydrating afterwards. Or just having sips of water.

Be guided by your thirst and by how much you are sweating. If you are running in heat, then certainly you should take fluid with you.

If you are sweating a lot or running for longer than an hour, sports drinks containing a bit of sugar and sodium are generally better than water. They won’t dilute your blood and lower the sodium content.

Research done for the manufacture of Gatorade found that there was benefit to drinking fluid which has some sodium in after you’ve been running for 90 minutes. This only needs to be the same concentration as the normal concentration of sodium in our bodies, not a higher level – this equates to ‘isotonic’ sports drinks.

After your run, have a large drink or around 500 ml of ideally something with a bit of carbohydrate in it, such as a sports drink, squash or juice, which will give you some energy too.

I love Gatorade for fluid replacement after a long or hot run, and chocolate milk for protein recovery. Be guided by your trips to the toilet, and if your urine is looking darker then drink some extra fluid. Keep checking throughout the day, following your run, and drink accordingly!

This guide does not cover ultramarathon running, which is far more complex, and whole books have been written about nutrition and hydration for ultras!

For longer distances, however, a rough guide might be to consider taking on board 300 – 500 ml of fluid per hour of running. Again, it is very individual, so best to test out what seems to work for you on your training runs.

If you’re particularly interested in finding out how much fluid you’ve lost on a run, you can weigh yourself before you run, without clothes on, then again as soon as you get back, after peeling off all your sweaty gear. Any weight loss is pretty much all water so soon after exercise. (Sorry! But you’ll continue to burn fat after you’ve run).
Key Points

  • Sadly the “I don’t sweat, I sparkle!” slogan doesn’t apply to many of us, and we all need to increase our fluid intake when we run!


  • If we are going to be losing quite a lot of sweat with the activity we have planned, then we need to think about hydration a bit more carefully.


  • Don’t get bogged down with fluid replacement – if you’re on a long run and only have water, don’t not drink it because it isn’t sports drink and you’re worried about hyponatraemia. It’s fine to drink water, just not gallons of it.


  • If you’ve lost fluid, you need to replace it with some fluid, whatever that may be. You can always have something with sodium and carbohydrate in it when you get home.


  • Very basic rules – drink; don’t drink vast amounts of just water while on a long run (switch to something with sugar and sodium in also, if you’re needing to drink a lot); remember that fluid replacement does vary on an individual basis so test out what works for you, and remember to always check and be guided by your pee! (I don’t recommend drinking that).

Basic Guide to Sports Drinks

The terms hypotonic, isotonic and hypertonic essentially equate to the osmotic pressure of these different fluids – the relevance for the runner, is that these fluids differ in how quickly they are absorbed by the body.

Hypotonic drinks have a lower concentration of solutes / salt than the normal concentration that exists in our cells in our bodies, which means we absorb hypotonic drinks quickly.

Isotonic drinks contain similar concentrations of solutes to the human body, and are also still absorbed quickly by our bodies (as quickly as we absorb a drink of water) – so isotonic sports drinks are excellent during exercise because they replace the salt we are losing as well as rehydrating us quickly.

Hypertonic drinks are absorbed more slowly by our bodies and contain a higher concentration of solutes than we have in our cells – examples of hypertonic drinks are coca cola or fruit juice. These are better saved as post-run drinks, if you are wanting some extra carbohydrate.

Hypertonic sports drinks have extra sodium in – research has shown this is probably unnecessary unless you have been exercising at high intensity for over four hours.

Consuming too much sodium or salt when you’re exercising can cause ‘salt-induced cramps’ or stomach upset. A sensible amount of water and isotonic drink should cover you just fine for most runs.

About the author:

Helen qualified as a GP in 2007, after doing her hospital training in the UK and New Zealand. She has a diploma in Women’s Health and also a special interest in sports medicine. Her hubby got her into running two years ago when he bought her running tights for her birthday. Before that she’d only run short distances at the gym, although she used to run for Thames Valley Orienteering Club when she was younger. Helen thought running would be cheaper than her gym membership… But the mountains of running gear in her chest of drawers might contradict that! Once she started running outdoors she was hooked, and loves the freedom and the challenges that it gives her. Helen has done a number of 10k’s, but suffered an injury training for the Sydney half marathon last year, when she was living in Australia. Helen is due a knee operation in a month or two’s time, then can’t wait to get back to proper training. She’s a big fan of trail running and the Lakeland Trails in particular, so she’s hoping to get back to running those as well as improving my 10k time and doing some local half marathons. Helen joined Run Mummy Run in September 2014, just after her first ever 10k race, and loves our awesome and supportive community!

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Image credits: sanja gjenero and Zsuzsa N.K.

Run Mummy Run guide rehydration for runners

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