Running dry?

While it’s helpful to stay hydrated when running, drinking too much fluid poses significant health risks and is unlikely to improve performance, says ISEH consultant Dr Courtney Kipps and Assistant Medical Director for the Virgin Money London Marathon. Here he explains how to manage hydration during a race to maximise the health and performance benefits.

“Drinking to thirst is a very effective and safe way to drink during any endurance sport, including the marathon. Drinking too much, which can happen when you disregard your thirst, carries real health risks. Several half-marathon and marathon runners have died of exercise-associated hyponatraemia (“over-hydration”) in recent years directly caused by drinking too much. On the contrary, dehydration is not the enemy it is often made out to be. Research shows that weight loss and hypo- or under-hydration is related to improved performance.

Dehydration in the sports setting is measured by percentage of body weight loss. The published scales suggest that anything more than two percent body weight loss over the course of the marathon is undesirable, with increasing body weight loss (up to 9-10%) related to increasing risk and severity.  To avoid these risks, the advice has been to drink before you're thirsty. Everybody will be familiar with the old saying: "By the time you're thirsty, it's too late", implying that thirst was an ineffective signal of potential dehydration.

However, the risks of dehydration when running are not as severe as is often made out to be the case. In a marathon setting you may lose sufficient body weight such that you may be classified as “dehydrated”. But you may not be clinically dehydrated.

In the medical tents at the Marathon we are unlikely to see someone who is severely unwell because of dehydration. Dehydration as experienced under race conditions may well cause you to feel poorly and may slow you down, but it's very unlikely to cause collapse. It's difficult to properly dehydrate yourself, in a clinical sense, over the course of four or five hour marathon. In fact, over the course of a marathon runners should expect to lose body weight. Up to two per cent body weight loss comes from  utilising carbohydrate reserves. Interestingly, the carbohydrate breakdown process actually releases water back into the system. So a two per cent body weight loss is unlikely to lead to any change in your hydration status.

In endurance events such as running and cycling , there is strong correlation between better performance time and increasing body weight loss. For example, a study carried out on 640 runners in a French marathon in 2009 found that those who finished quickest lost the most weight, and those who finished slowest lost the least weight. The fastest racers routinely drink very little and lose more body weight as a result. This may be because they  are running so fast that they don't have time to take more than a few sips of water, but it doesn't seem to impair their performance in the process.

There are also risks associated with drinking too much. The major risk is developing Exercise-Associated Hyponatremia (EAH). This is a dilution of the sodium concentration in the blood due primarily to consuming too much water and, to a lesser extent, through the kidneys holding on to water whilst you're exercising. Fluid moves into adjacent tissues in an attempt to equalize the concentration between the blood and adjacent tissues. Signs of this are puffy hands and feet, with, for example, tight rings, tight watches, and sock marks. When fluid enters the brain and the lungs causing swelling in these vital tissues it becomes dangerous. In recent years several half-marathon and marathon runners have died of exercise-associated hyponatraemia (“over-hydration”) directly caused by drinking too much.

Thirst is the best guide

The best way to reduce the risk of hyponatremia is to pay attention to thirst, using thirst to inform when and how much to drink. Unless you’ve had one of a handful of very rare medical conditions which affect your thirst, there's no reason for thirst to be ineffective. If you listen to your thirst, you will drink as much as you need and should not drink more than necessary. After all, thirst has been a reliable indicator of hydration needs in humans for thousands of years, if not more.

You should expect to lose up to two per cent of your body weight, if not more, over the course of the marathon. Make sure you're well trained and don't drink too much. There's no need to start the race feeling full from drinking too much, and there’s no need to drink at every aid station. At the London marathon, for example, because there are so many runners on the course there are lots of water stations to ensure that everybody gets a chance to get the water they need. You should not feel like you have to drink at every station. You should drink whenever you feel thirsty.

Don't drink too much fluid after the marathon either, just drink what you feel you need to.

Sports drinks or water?

The rules of safe drinking apply whether you're drinking sports drinks or water. If you drink too much of either, you'll be putting yourself at risk. Sports drinks provide a useful supply of energy through carbohydrates. Some sports drinks also contain a small amount of sodium, but they are still dilute relative to the blood so drinking too much will still risk hyponatraemia.

On a hot day you will sweat more, which is a normal response to allow effective cooling. If you feel more thirsty as a result, then you should drink more. Drinking itself will not cool you down, although cold drinks, or even crushed ice, will certainly help to cool you down from the inside.