RW’s complete guide to running and hydration

Running and dehydration often go hand in hand as people are only designed to deal with heat or limited water for a certain period of time. While we can survive for as long as a month in a moderate climate without food, we would struggle to stay alive for longer than two days in desert conditions without water. After oxygen, water is a close second on the list of essentials for life. Water makes up 60 per cent of your total body weight and performs many crucial functions, including nourishing cells; carrying food through the body; eliminating waste; regulating body temperature; cushioning and lubricating joints; and maintaining blood volume and pressure.

Every day we lose fluid by sweating, breathing and urinating. It's the sweating in particular that runners need to pay attention to because as soon as you start to run, you start to dehydrate. About 75 per cent of the energy you put into exercise is converted into heat and is then lost. This is why exercise makes you feel warmer. Extra heat has to be dissipated to keep your core body temperature within safe limits – around 37-38°C. Your body keeps cool by sweating, which makes the replacement of fluids crucial. Fail to consume enough fluid and your blood will thicken, reducing your heart's efficiency, increasing your heart rate and raising your body temperature.

Dehydration is normal

Modest dehydration is a normal and temporary condition for many runners and doesn't lead to any serious medical conditions. Elite athletes, for example, don't have time to drink very much at sub-five-minute mile pace, and are probably the most dehydrated runners on the course – a state that is easily and quickly reversed within minutes of finishing, by ingesting fluid.

Replacing fluid after a run is just as important. For every kilogram of bodyweight you lose, you need to drink one-and-a-half litres of fluid. Try to drink around 500ml in the first 30 minutes after your run and keep gulping every five to 10 minutes until you have reached your target. If you pass only a small volume of dark yellow urine, or if you have a headache or feel nauseous you need to keep drinking – a sports drinks or diluted juice (with a pinch of added salt) are your best options.

Related: Do you need to drink during an hour long run?


Ultra-distance runner, Professor Tim Noakes, author of The Lore of Running and leading researcher at the University of Cape Town, is worried that runners are following out-dated hydration plans: "The reason why athletes drink too much is almost certainly due to hysteria attached to the supposed dangers that dehydration poses to athletes and to the belief that fatigue is caused by dehydration so that replacing more fluid than is lost will ensure optimum performance. I find no scientific support for either belief," he says.

Since running requires you to support your body's weight while trying to complete a race in the shortest possible time, Noakes believes it could be more useful to measure whether keeping levels of dehydration at less than five per cent of body weight would lead to a better performance.

Not everyone would agree with Noakes but all researchers do agree on one thing: you need to start a run or race hydrated. By drinking 500ml of fluid two hours before a run – try water, a sports drink or diluted fruit juice – and another 150ml of fluid just before you run, you'll have enough time for your body to clear what you don't need before you set off.

Related: sports drinks vs energy gels

Your body has a finely tuned thirst mechanism that lets you know when you need to drink, but how do you know if you're drinking too much? Excessive consumption is also a potential danger and has started to become an issue as marathon running has broadened its appeal to attract more recreational runners. Hyponatraemia means "low blood sodium" and is caused by excessive water consumption, which lowers the concentration of sodium in the blood. In its mild form, hyponatraemia will cause bloating and nausea; in extreme cases it can lead to brain seizure and death.

The group most at risk are women. Why? They're smaller and less muscular than men, on average, so they sweat less and need to drink less. "Women may be more fastidious in following rules," says Noakes. "So if they're told to drink as much as they can, they may be more likely to do that than men." An average woman needs to drink up to 30 per cent less than an average man – this will ensure blood doesn't become diluted, lowering sodium to a dangerous level.

Anyone running for more than four hours should be guided by thirst, avoid drinking huge amounts of water, and use sports drinks that contain sodium. You can also increase your risk of hyponatraemia by using drugs such as aspirin and ibuprofen. Researchers at Baylor College of Medicine in Houston studying marathon runners concluded that these drugs impair the body's ability to excrete water.

How To Replace Fluids

You will undoubtedly need to replace sweat with fluids during some training runs and races so what should you drink and when? Water, diluted juice and sports drinks are all good fluid replacers. If you've been running for less than an hour, plain water is a good choice, but, if you have been running hard for longer than an hour, drinks containing sugar or maltodextrin (a slow-release carbohydrate) and sodium may speed your recovery. Researchers at Loughborough University found that when runners drank a sports drink (5.5g carbohydrate/100ml), they improved their running time by 3.9 minutes over 42km compared with drinking water.

Sports drinks containing carbohydrate also increase water absorption into your bloodstream, according to research at the University of Iowa, and that counts when you're sweating heavily. Researchers found that drinks containing approximately 6g carbohydrate/100ml are absorbed the most rapidly. The taste of a flavoured sports drink will also encourage you to drink more of it – compared to plain water – according to a study at the McMaster University in Ontario, Canada.

Researchers at the University of Iowa originally thought that sodium speeds water absorption in the intestines, but they discovered that after you consume any kind of drink, sodium passes from the blood plasma into the intestine where it stimulates water absorption. In other words, the body sorts out the sodium concentration of the liquid in your intestines all by itself, so the addition of sodium to sports drinks is unnecessary.

While sodium in sports drinks does not have a direct effect on performance, it does have one key benefit: it increases the urge to drink and improves palatability. That's because the increase in sodium concentration and decrease in blood volume that occurs when you exercise increases your thirst sensation, with the result that you want to drink. If you drink plain water it dilutes the sodium, thus reducing your urge to drink before you're fully hydrated.

Despite growing concerns about hyponatraemia, it's important to remember that for most runners, there is a bigger risk of dehydration than overhydration. Each of us sweats at a different rate, produces varying amounts of sodium in our sweat, and reacts differently to heat. Exactly how much you need to drink depends on how heavily you are sweating. The harder and longer you are working out, the more you sweat. Training in hot humid conditions also makes you sweat more, and some people simply sweat more than others.

Current guidelines recommend drinking anything from 300ml to 800ml of fluids per hour when you're exercising. As we've already stated, the upper end of that scale is almost certainly more than you need. However, you need to try different approaches to hydration in your training to establish a strategy that works for you and remember: you're an experiment of one.

Exactly how much is enough?

So how do you know if you're hydrated before you start a run? The easiest, most practical test is to check the colour of your urine. University of Connecticut researchers found that urine colour correlated very accurately with hydration status. Pale yellow urine indicates you're within one per cent of optimal hydration.

Try to drink one litre of water for every 1,000kcal you burn daily. (An average male burns around 2,500kcal a day, a runner covering five miles a day more like 3,000kcal.) In general we need two to three litres of liquid a day – half from food and half from fluids. This is a minimum: if you live somewhere hot or you know you sweat a lot, you'll obviously need more.

You can work out how much fluid you lose in a typical run by weighing yourself before and after. Remember to go to the toilet and remove your clothes before you weigh yourself, then remove your clothes and weigh yourself as soon as possible after you return. You can assume that all of your weight loss is fluid.

The recommendation to drink one-and-a-half times the fluid loss accounts for the fact that you continue sweating after exercise (and losing fluid) and that urination is usually increased during this time. This method of calculating dehydration does not take into account water that is metabolised from glycogen stores when you exercise. "This does not need to be replaced," says Professor Noakes. "According to one study, drinking to prevent any weight loss during a marathon would have caused the athletes to be overhydrated by 2.2kg." Some researchers have, however, concluded that you should aim to match your fluid loss with intake during exercise. A study at the University of Aberdeen, for example, revealed that by replacing at least 80 per cent of the fluid lost, or keeping within one per cent of your body weight, performance is not affected.

In the past, many of the studies that set out to measure dehydration and performance were poorly designed and impossible to reproduce. Some studies do show that perceived exertion is lower when fluid is ingested, but in these tests athletes are asked to exercise at a fixed intensity for as long as they can; their performance may change from day to day, and may be influenced by external factors such as boredom. "Drinking fluid might enhance performance simply because it alleviates boredom," Noakes concludes.

Studies where athletes are asked to complete a fixed amount of exercise in the shortest time possible – simulating a race situation, for example – do not show any beneficial effects of drinking at different rates. One study, led by Glenn McConell, senior lecturer at the School of Physiology at the University of Melbourne, found that replacing 100 per cent of the sweat lost during exercise did not improve exercise performance more than replacing only 50 per cent of fluid lost – which, again, suggests that if you follow accepted hydration wisdom you could be drinking too much.

So, while drinking might not lead to any physiological benefits that help you to produce a better performance, the majority of studies do agree that there may be psychological benefits. "The most consistent finding is that fluid ingestion markedly reduces the perception of effort during exercise at both low and high intensities," says Professor Noakes, adding that, "all the evidence indicates that ad libitum fluid ingestion during exercise appears to be as beneficial as higher rates of forced ingestion."

In other words, you should drink when you're thirsty. By experimenting in training and races, ingesting varying amounts of fluid, you will establish how your body responds to dehydration and find out what works best for you. Just be sure, as you crest a sand dune in the Marathon des Sables and spy a watery oasis in the distance, that you outpace any camels in the vicinity. After they've had their 200 litres, there won't be much left for you to slake your thirst, let alone refill your back-borne water bladder.

What's In A Name?

What's the difference between hypotonic, isotonic and hypertonic drinks?

A hypotonic drink is more dilute than your body fluids (ie there are fewer particles – sugars/electrolytes per 100ml), which means that it can be absorbed faster than plain water. For example, squash diluted at least 1:8 with water, or one part fruit juice diluted with three parts water.

Isotonic means it is the same concentration (ie the same number of particles per 100ml) as your body fluids – it is therefore absorbed as fast as or faster than water. They provide an ideal compromise between rehydration and refuelling. For example, Isostar, Lucozade Sport or fruit juice diluted half and half with water or squash diluted 1:4 with water.

Hypertonic drinks, such as cola, lemonade or neat fruit juice, are more concentrated than body fluids, and will be absorbed more slowly than plain water. Hypertonic drinks slow down the rate at which the stomach empties and therefore reduce the speed of fluid replacement. These are better post-run drinks that offer a higher dose of energy with the fluid.