Marathon – Importance of Hydration

January 18, 2014 | By | Reply

General recommendations for fluid intake for marathon runners:


It is important to start the marathon hydrated. However, drinking more than you need does not appear to add any advantage, and may lower sodium levels. You can monitor your hydration status by checking your body weight on a daily basis and paying attention to the colour and frequency of your urine.

General guidelines for fluid intake pre-event are:

  • 400-600ml, or 5-7 millilitres per kilogram of body weight, of fluid approximately 2-4hrs before the start, to normalise total body water volume and return urine output to normal.
  • a further 5-8ml per kg of body weight 10-20 minutes before you start running.


During the marathon, rehydration takes priority over energy provision to reduce the risk of dehydration and excessive loss of electrolytes. You should feel comfortable with your fluid intake and that is why you really need to work out your hydration strategy in training.

Recommended intake is:

  • 3-4ml per kg/body weight every 15-20 minutes during running, to a probable maximum of 225ml every 20 minutes. Remember the stomach can only empty 800-1000mL of fluid/hour and your own requirements may be less than this. Slower runners and walkers should keep to the lower recommendations as they will take longer to complete the event and therefore have a higher risk of hyponatremia.


After the event, you will need to consume slightly more fluid than you have lost in sweat to cover ongoing obligatory urine losses and glycogen. This is particularly so if the weather is warm and/or humid. The suggested intake is 1.5 litres of hypotonic or isotonic drink for every 1kg of total weight loss (including fluid consumed) over several hours. Although not essential, including glucose at less than 2% in rehydration drinks can improve palatability and encourage you to drink. Rehydration drinks should also contain at least 50mmol of sodium/litre (check the label). Potassium can generally be replaced through food.

Personalising the Guidelines

As mentioned above, these are general guidelines and runners must take personal responsibility for developing their own hydration plan in order to drink just enough to replace fluid lost at their personal sweat level. Some people sweat a great deal while others hardly sweat at all, and those who drink the most are not necessarily those who sweat the most.

In addition, thirst is not necessarily closely related to sweat loss and is probably not a reliable guide to fluid needs particularly since thirst is often not noticed until a runner has already incurred a deficit of 2% BWL. There is some evidence that the people who drink most begin a training session already dehydrated and so this might also apply in a race situation.

So how do you personalise the guidelines?

Remember, as a rule of thumb, you are aiming to drink just enough to lose no more than about 2-3% of your pre-race weight. The following approach can help to identify how much to drink.

Identifying Your Personal Hydration Needs:

Step 1:
Record your body weight before and after as many of your long runs as you can. Weigh yourself at the last minute before going out, after you have been to the toilet and without any clothes.

Step 2:
Weigh yourself again as soon as you get back and write the two weights in your training diary along with the approximate distance and duration of your run, how you felt while running, the weather conditions, what you were wearing and how much effort you put into your run on the day.

Work in kilograms and litres since 1kg of weight loss is roughly equal to one litre of sweat. You do also use some stored fuels in the form of carbohydrate and fat, but these can be ignored with no great loss of accuracy.

Step 3:
After a few weeks of recording this information, you will probably start to see some patterns emerging, and if you collect enough measurements, you’ll be able to allow for factors such as the weather, distance, clothing etc to get an idea of what to drink in different conditions.

Step 4:
Once you know what your sweat losses are for a particular set of conditions, you can plan your fluid intake for race day, and work through the guidelines below.

Category: Articles

Leave a Reply