When I did the Championnat du Canigou race last year I noticed a lot of people at the finish in the medical tent lying on stretchers; they were not injured but they were seriously dehydrated. Even mild dehydration can impair your performance significantly as your body starts using more glycogen to make your muscles work (so you have to work harder to maintain the same pace).

So how much do you need to drink when running an ultra-marathon? If you are running for longer than 90 minutes you should aim to drink 300 to 800ml of fluid per hour depending on your size and your sweat rate. Ideally this will include some electrolytes to aid fluid absorption.

However, there are slight differences as not every one is average and sometimes the weather can be extremely hot and humid. People lose fluid at different rates depending on how much they sweat and peoples fluid needs vary on their size (a large person needs more than a small person as they have more blood volume). Women in particular are generally smaller, but also their blood plasma, in particular their salt content, varies at points in their cycle so their needs can change week by week.

Do a sweat test.

If you really want to be scientific about the amount you need to drink when doing an ultra event, you could do a sweat test.

The equation is: Weight loss (kg)/Time (hrs)

  1. Before the test make sure that you are generally adequately hydrated (that your wee is normally a light straw colour apart from the first wee of the day).
  2. To start the test, go to the toilet for a wee.
  3. Weigh yourself (dried and naked) and make a note of your weight in kg.
  4. Go for a run of about an hour in length.
  5. Do not urinate until after the next step.
  6. As soon as you return from your run dry yourself down and weigh yourself again (naked as before).
  7. If you choose to drink during the session make a note of the weight in kg that you have drunk.
  8. The weight you lose in kg (plus the weight of any liquid consumed in kg) divided by the time you were training in hours will equate to your sweat rate in litres per hour.

Whilst this might be a good starting point, it is worth noting that unless your race has the exact same conditions as your test (including air temperature, humidity and intensity of the effort) your sweat rate is unlikely to be exactly the same.

It is also worth bearing in mind that you are highly unlikely to either need or be able to replace 100% of your fluid loss during a long event like an ultra marathon. Aiming to drink about 80% may be more realistic.

In addition, the type of things you choose to drink during your event will effect your body’s ability to maximally absorb the fluid from them. Studies have shown that including some electrolytes (sodium, potassium and magnesium) in your drink maximises the body’s ability to absorb and use the fluid you take in. So it’s not as simple as ‘one out one in’ in terms of keeping hydrated.

Take regular drinks.

Although your body is pretty good at absorbing liquid (it will start within 5 minutes) it cannot absorb a massive amount of liquid straight away, so it’s no good waiting for ages then having a massive slug of a drink; it will take time for your body to absorb it, so the key is to drink little and often. Having regular sips of your drink will help you keep topped up and stop you falling into a hydration deficit. In practice this does mean that you will need to carry some liquid with you on your run, even if there are regular aid stations.

I would recommend taking a drink every 15 minutes. If, like me, drinking is not automatic for you, have a routine and stick to it, or set an alarm on your watch if you are able. Some people prefer to drink to thirst, but people’s thirst mechanisms can vary, especially the older we get, and it’s easy to override physiological needs in the excitement of a race.

Carry on hydrating after you have finished.

As previously stated you are unlikely to need or be able to completely replace all your liquids during your long ultra event so you are going to need to carry on drinking afterwards. Unless I feel really ill, I usually try to continue with an isotonic drink given that I know I will also have a deficit of calories to fill. Whilst a celebratory beer is also nice to end the race, remember alcohol is a diuretic so does not count as rehydration! Continuing to sip regularly after the finish will help aid recovery as you gradually help your body restore its own equilibrium; in particular adding some protein into your recovery drink can help with muscle repair.

Whilst there are many very good branded sports recovery drinks on the market, I have found that chocolate flavoured (plant based) milk has been as good as anything; it tastes nice, is gentle on stomach and has a good amount of protein. For events where I am having a kit bag transported to the finish, I go for a recovery powder that I can add to water as a more practical solution.

Be Flexible

Lots of things can effect how much you may need to drink including the weather on the day of your event, which, depending on where and what you are doing could vary from hot and humid to very cold. Whilst you may need to drink less when it’s cold, you will still need to drink and having a way of ingesting warm drinks can be a good way to motivate yourself to drink. Even if you do have a plan, if you find yourself feeling very thirsty despite drinking every 15 minutes try drinking a little more.

If you race at altitude, especially if you have not done any altitude training it is worth noting that your body’s need for fluids often increases. If you have no experience of altitude before your big race, be prepared with extra drinks should you need it and stick to a good drinking plan during your event.

The liquid in what you eat will also contribute to your overall hydration levels; if it’s on offer at aid stations, I will often take water melon as another source of both fuel and carbohydrate (and because it tastes so refreshing!).

A note for women.

It is worth noting that for women our hydration needs may differ according to where we are in our cycle. During the high hormone, luteal phase of our cycle (the last one and a half to two weeks) blood plasma is reduced by about 8% (giving us thicker blood). At these times our hydration needs will increase in order to help us maintain our core body temperature and our sodium levels naturally decrease. Drinking a drink well balanced with electrolytes and possibly some carbohydrates should help with all these changes. Again trying and practising this in training will help prepare you for wherever you might be in your cycle on race day.

Drink Electrolytes

Your body uses electrolytes to carry the fluid into the cells where it’s needed. The main electrolyte lost in sweat when we exercise is sodium, but Magnesium and Potassium as also important for the body and in particular muscle function.

It’s worth noting that not everyone has the same sweat composition, some people tending to sweat out more sodium (salt) than others. For people who’s sweat is particularly salty, drinking electrolytes is even more important. There are tests you can take to analyse the composition of your sweat, but in simple terms if you notice salt lines in your clothing and on your skin after exercise you are probably a salty sweater!

Including both some carbohydrate and electrolytes to your drink will increase the absorption rate of any liquid you take on. Most off the shelf isotonic drinks contain a good ratio of carbohydrates to electrolytes. The better ones will contain a mixture of glucose/fructose and maltodextrin.

In addition to optimising absorption, for long ultra events drinking isotonic drinks (with electrolytes and carbohydrates in with the same osmolarity as blood) also helps you take on some of the energy you will need in liquid form, which is typically easier to absorb. It will also reduce the risk of hyponatremia as you will be taking on salts thus be less at risk of over-diluting and damaging your body’s natural balance of salts in the cells and blood. Good isotonic drinks will deliver 20g of carbohydrate in a 1:3 ration of fructose to maltodextrin with a good balance of sodium, potassium and magnesium and have a neutral pH. Having a mixture of different types of carbohydrates optimises their absorption as the body is using different systems to absorb them; maltodextrin in particular is kinder on your stomach.

Practise Drinking in Training

If we accept that you are not going to be able to replace all your fluids during your main event the key really is to make sure that you manage this to the best of your ability. The only way to truly feel confident that you can do this in a race situation is to have tried and tested your methods for staying optimally hydrated for you in training. Given that you are likely to have to carry your own fluids for sections if not all of your race, this also provides a good opportunity for training with additional weight of the fluids you will have to carry. It will also help ensure that you carry enough liquid, but not too much.

Try out different types of drinks/drink combinations in your training and find one that works for you; having done your sweat test you will have a starting point in terms of how much you are losing in sweat so you can aim to replace about 80 percent of this whilst you are running.

On the day of your race, do not try any new type of drink that you have not trained with. If you are hoping to use the support stands, find out what products they are using and try some out in training to see if they agree with you; if they don’t go for water and take your own powder/tablet/concentrate to add to it.

Start your race hydrated.

If you are dehydrated an hour before your race it’s probably too late to do anything about it. It takes your body time to absorb liquids and rehydrate so having a good overall hydration strategy in every day life will ensure that you get to the start line hydrated and in shape to run your best.

A good way to monitor your general hydration levels is to have a good look at your wee. In well-hydrated adults wee is normally a light straw colour; the darker your wee is the more dehydrated you are. However, be aware that if you take certain medications and/or vitamin supplements these will effect the colour of your urine.

The best way to maintain good overall hydration is to sip regularly throughout the day; when not exercising water can be fine for this, but if you really don’t like the taste of water try making up a (weak) fruit juice or in winter herbal tea so that you are motivated to drink; I like to add a slice of lemon to my water and drink it either hot or cold depending on the season.

Additional Questions

What is hyponatremia?

Hyponatremia is a condition that occurs when your blood’s serum and sodium levels become over-diluted (less than 135mmol/l). This can happen if you drink too much plain water over a short period of time, and women in the luteal phase of their cycle can be particularly vulnerable. Unfortunately, the symptoms of hyponatremia (confusion, nausea, headache, fatigue, muscle weakness) are similar to those of dehydration so it can be easy to confuse the two. If the blood sodium levels continue to drop the condition can be potentially fatal. Drinking little and often and using electrolytes in your fluids during your race will reduce the risk of this condition.

What are the signs that I am dehydrated?

Dehydration symptoms include dry mouth, decreased urination, dry skin, dizziness/lightheadedness, headache; in severe cases you may stop sweating, your heart rate will increase and your wee will become very dark. An easy test to check if you are dehydrated can be to pinch the skin on your arm, if it does spring back in place you may well be dehydrated. Drinking fluids with electrolytes will help restore you, although in severe cases hospitals will attach a saline drip.

Should I drink water at all?

There’s nothing wrong with drinking water and for events of up to 90 minutes plain water is just fine. Some athletes prefer to drink a hypertonic solution/take a gel with electrolytes and drink plain water as well in their longer events. The main thing is to make sure you drink small amounts regularly, that you take on some electrolytes as well, and that you try and test things in training so you know what will work best for you on race day.

Clare Pearson
Post by Clare Pearson
May 27, 2020
A professional endurance coach since 2018, Clare Pearson has worked with endurance cyclists and runners to help them achieve their goals. Clare specialises in endurance events, she loves to work with people to help them succeed at their own goals; whether that's a personal best, a completion, a podium or better emotional health. Clare will work with you to design a plan that fits in with your day to day life and helps you get the most out of each session.