There's a great deal of information already available about staying hydrated as you train, and we'd hate to add to the noise! It seems, though, that much of it is confusing or contradictory, so this month the Extra Mile looks at why it's so important to drink, what to drink and even when to drink it...

The scientific bit, simplified: As you train, heat builds up in your muscles, which raises your temperature. The body compensates by sweating. However, since the fluid you're then losing comes from blood plasma, essential to the process of carrying oxygen to the muscles, your heart has to work harder to pump more blood round. And because there's now less fluid in the body, you can't produce as much sweat, which causes heat to build up in the muscles...

So?: So we have to drink fluids to improve heart function, maintain kidney function and lower our body temperatures. To continue without replenishing may lead to a drop in blood pressure, fatigue, faintness, nausea and, eventually, unconsciousness.

What you shouldn't drink: There's no doubt at all about alcohol, I'm afraid. This is to be avoided when you're training! As with sugary drinks, it's dehydrating. There's some debate about caffeine drinks. Whilst tea and coffee are certainly diuretic, there's research to suggest you don't lose all the fluid you take in. Fizzy drinks aren't great, either – they are better for running if you let them go flat first.

What you should drink: Most nutritionists generally agree that water is the best way to replenish lost fluids. Having said that, anyone exercising vigorously for an hour or more needs to replace certain minerals. These minerals, carried in the body fluid, carry an electric charge and are called Electrolytes. There are eight major ones in your body; the most likely to need replacing after prolonged exercise are Potassium and Sodium. Many sport's drinks provide these as well as essential carbohydrates; a full range is available here or any specialist running retailer. Finally, fruit juice offers some benefits, and is best diluted with water.

BUT!: It's best not to alternate too much. Some people who've trained for a race 'on water', then switched to a sport's drink on the day, actually find this upsets their stomachs. You need to establish a level of consistency.

How much to drink...: Well, this is where it gets tricky! There are many factors that need taking into account, such as gender, weight, exercise duration, how much sweat you normally produce and external temperature. So, as we've said before, there's no 'one size fits all' answer to the question of how much water you should drink on a day-to-day basis. However, Professor Heinz Valtin came up with an equation, designed to establish an approximate daily intake for moderately active people:
Men: Bodyweight x 10.36 = ml required
Women: Bodyweight x 9.176 = ml required
So a moderately active man weighing 140lb would need around 1450ml (140 x 10.36 = 1450.4), increasing this amount in warm weather or if he became more active. Runners should know their minimum intake requirement and compensate for the fact that they sweat more, especially in hotter temperatures.

But I'm more than moderately active!: Indeed! So there's another calculation that many people find useful. If you weigh yourself before you train, then weigh yourself again afterwards, you'll probably find the difference is between 1/2 kg and 2kg, sometimes more in hot weather. Based on this, a popular solution is to replace each kg of weight lost with about 1 litre of fluid.

...and when to drink: Once you know what you need, it makes sense to spread the intake out over the course of training sessions, drinking some before, some during and some after. And when you're not exercising, the cliched answer is to drink before you're thirsty! Your brain monitors the levels of salt in your body and will send signals of 'thirst' when the concentration of salt goes up... But this is after you've started to dehydrate. You should be drinking consistently throughout the day and increase your intake before a session.

Also: One way to check you're getting enough water is to check your urine. It should, in fact, be nearly colourless or pale yellow. It should also be nearly odour free, so if the urine is dark, or has a strong smell, there's a good chance you're dehydrated. One important thing to remember, though, is that some Vitamins will add a yellow tinge to urine. If you're taking water-soluble Vitamin B, for instance, this will darken your urine.

Finally...: It stands to reason that you should carry a drink with you when you train. These days, there is a wonderful array of equipment that will let you do so, from a simple bottle with an easy to grip design to specially designed belts and 'bladder packs'. You can browse our range here. Find something to suit your needs rather than settling for something that's got only novelty value.