How much should I drink during exercise?

One of our great interests is fluid replacement during exercise. In fact, Jonathan got his PhD looking specifically at this question, and my own work included two studies on the effects of the heat on performance, so this is something of a life's work for both of us.

So in the coming weeks, we'll give substantial attention to this question. To begin with, the most basic question of all - "How much should I be drinking when I exercise?"

· The simple answer is to drink as much as your body tells you to!

· This issue is a controversial one and there is a great deal of conflicting information on it.

· However, what is undoubtedly true is that if you obey your thirst and drink only when thirsty, then you will be safe and it will not affect your performance at all.

· The only time that dehydration becomes a risk is when you have restricted access to fluids, and so for that reason, if it is a particularly hot day, plan your route so that you will be able to stop off and take in some fluid if you do feel that you need it.

· But you do NOT need to force yourself to drink every so often, rather listen to your body and you will be fine.

· In fact, the risks of drinking too much far outweigh the risks of not drinking enough – if you drink too much, you can develop a lethal condition known as hyponatremia, in which the sodium levels of the blood are decreased by excessive water intake. This condition has in fact led to more deaths than dehydration and so is far more dangerous than not drinking.

· Generally, there is a range of fluid intakes that you would find you take in if you listen to thirst. This range is between about 400 and 1000 ml per hour, depending on the conditions and your running speed.

· As far as performance goes, all the evidence suggests that when you are exercising outdoors and under normal conditions, performance is only affected when you become very thirsty.

· The discomfort of being very thirsty causes you to slow down and so should be avoided. Again, therefore, plan to have access to water or fluid should you need it.

Over the weeks, we'll look at this in more detail - why you don't need to drink as much as you may have been told, what the dangers of too much drinking are, and where that theory came from.

So join us for a series on the ins and outs of the science of drinking and dehydration