Paul conway Wednesday 9th August 2006

As soon as you being exercising, you lose water in the form of sweat. Sweating is acually a very good thing as it rids your body of the excess heat produced in your muscles. If this didn't happen you would quickly over-heat and die. You lose about 500ml in 30 minutes, depending on the heat, humidity and exercise intensity.

If you don't replace that water, your core temperature will rise and your performance will suffer. As your blood becomes 'thicker', your heart needs to beat faster to pump the blood around your body, and your body is put under undue stress.

Losing the equivalent of 2% of your body weight as sweet - that's a mere 1.3kg loss if you weight 65kg - results in a 10-20% drop in your performance (or aerobic capacity). Whatever kind of exercise you are doing, once you become mildly dehydrated it will be harder to keep going at the same intensity, and you will be forced to drop your pace.

During exercise you should aim to match your fluid loss with your intake. Studies with athletes at the University of Aberdeen have shown that if you can replace at least 80% of your fluid loss or keep within 1% of your body weight, then your performance won't be affected. Exactly how much you need to drink depends on how heavily your are sweating.

The harder and longer you are working out, the more you sweat. Training in hot humid conditions also makes your sweat more, and some people simply sweat more than others.

In general, you can count on losing around 750-1000ml per hour so you'll need to put back around 600-800ml in that time. Now that may sound like a tall order - particularly if you are out running or playing a team sport - but it can be done. You will need to schedule in drink breaks and learn to drink on the run. If you can manage only a few sips at a time, then make sure you do that frequently, say every 10 minutes.

The American College of Sports Medicine and American Dietetics Association recommends drinking around 150 - 350ml every 15 - 20minutes. You should start drinking early during your workout as it takes about 30minutes for the fluid to be absorbed into your bloodstream.

To work out how much fluid you lose in a typical workout and therefore how much you ought to drink weight yourself before and after exercising. You can assume that all of your weight loss is fluid. A weight loss of 1/2kg represents a fluid loss of 500ml. So if you have lost 1/2kg, that indicates a loss of 500ml, aim to replace that fluid loss with 1 1/2 time that volume of fluid.

This accounts for the fact that you continue sweating after exercise (and losing fluid) and that urination is usually increased during this time. So if you lose 1/2kg you should aim to drink 750ml during after your workout. Divide this volume into manageable amounts, according to the activity you are doing. For example, you could drink 125ml (about half a cup) four times during your workout, and a further 250ml immediately after your workout.

DON'T WAIT UNTIL YOU FEEL THIRSTY AS THIS INDICATES THAT YOUR ARE ALREADY ON YOUR WAY TO DEHYDRATION!

I hope this article has been of interest to you.