Guide to Rehydration


A two per cent loss in body weight caused by dehydration can lead to a 20 per cent drop in performance for runners and even a one per cent drop can diminish the performance of some runners.

Many runners taking part in marathons do so with a level of hydration of between two to five per cent, though runners do need to be careful about hydrating too much as they run. If you’re drinking as much as 800ml of fluid per hour in order to maintain your dehydration level at lower than two per cent, then you could be drinking too much.
For many runners, modest dehydration is a normal and temporary condition not leading to any serious medical conditions.

Drinking on the Run

If you’re a long distance runner, you need to learn how to drink while on the move as frequent small sips are best in order to prevent overloading your stomach. Tips to practice drinking while running include running a circuit around your home, stopping off each time you pass to grab a drink. You can also try stopping off at any shops that you pass to buy a drink, or if you’re running on a treadmill, take a bottle of water with you and take a drink every three miles or so.

During a Race

Most people are right handed and tend to veer off to the right, so consider practising veering off to the left to grab your drink as this may be less crowded. It may also be easier to head for the end of the table as there may be less people than at the front.

If you need to stop, don’t feel you have to keep running while you drink. You’ll only lose a few seconds if you walk while drinking but make sure you stick to the side of the road out of the way of other runners.

After a Race

It’s important to start a run or race hydrated and equally important to replace lost fluid after a run.
A good guide is to drink around 500ml of fluid two hours before a run – try water, a sports drink or diluted fruit juice – and another 150ml just before you set off. Your body will then have enough time to get rid of what it doesn’t need before you set off.

For after a run, general guidelines are that for every kilogram of bodyweight lost, you need to consume one and a half litres of fluid. During the first 30 minutes after a run, try to drink around 500ml and keep taking on small amounts every five to ten minutes. Listen to your body and if you have a headache or feel nauseous you need to keep drinking.


Runners do need to be careful not to drink too much after hard exercise as excessive consumption is a potential danger.

Hyponatraemia – “low blood sodium” – is caused by excessive water consumption which lowers the concentration of sodium in the blood and can be very dangerous. In mild cases, hyponatraemia causes bloating and nausea, but in extreme cases it can lead to brain seizure and even death.
Women in particular need to be careful about hyponatraemia as they tend to sweat less so need to drink less. An average woman needs to drink up to 30 per cent less than an average man to ensure their blood doesn’t become diluted, lowering sodium to a dangerous level. A safer alternative is to drink a sports drink which contains sodium.

Researchers have also found that drugs such as asprin and ibuprofen impair the body’s ability to excrete water and so can increase the risk of hyponatraemia.

Exactly how much you need to drink depends on how heavily you are sweating so you need to try different approaches to hydration in order to establish a strategy which works for you.

Exactly how much is enough?

To check if you are hydrated before you start to run, the easiest way is to check the colour of your urine which should be pale yellow.

Generally we need to drink two to three litres of liquid a day but runners taking part in hard training sessions or races need to drink more.

Researchers recommend that you drink one and a half times the fluid lost during a run – an easy way to work this out is to weigh yourself before and after a run.

Experiment during your training to establish how your body responds to dehydration and find out what works best for you.

How to replace Fluids

Runners need to replace sweat lost with fluids – water, diluted juice and sports drinks are all good choices. If you’ve been running for less than an hour, plain water is fine, but if you have been running hard for longer than an hour, drinks containing sugar or maltodextrin – a slow-release carbohydrate – and sodium may be better.

Researchers have found that sports drinks which contain carbohydrates increase the amount of water absorbed into the bloodstream.

The difference in sports drinks

There is a vast range of sports drinks on the market, from hypotonic, isotonic and hypertonic drinks. A hypotonic drink is more dilute than body fluids – there are fewer particles per 100ml, which means that it is absorbed faster than water.

Squash diluted at least 1:8 with water, or one part fruit juice diluted with three parts water are examples of hypotonic drinks.

Isotonic drinks have the same concentration as body fluids and are absorbed as fast as or faster than water. These drinks are a good compromise between rehydration and refuelling and examples include Isostar, Lucozade Sport or fruit juice diluted half and half with water or squash diluted 1:4 with water.

Hypertonic drinks offer a higher dose of energy with the fluid and reduce the speed of fluid replacement. These drinks include cola, lemonade or neat fruit juice and are more concentrated than body fluids and absorbed slower than plain water.