Around 75 per cent of the energy you put into exercise is converted into heat and lost. As your body temperature has to remain in the safe zone of 37-38°C, your body keeps you cool by sweating.
This makes the replacement of fluids crucial - if you don't take on enough fluid, you'll become increasingly dehydrated, which will raise the body's temperature, quickly reduce your heart's efficiency and increase your heart rate. This in turn will make exercise much harder.
How do you know how much fluid you need to take on?
Begin your race in a hydrated state
You should prehydrate at least several hours before the race. The American College of Sports Medicine (ACSM) produced guidelines on exercise and fluid replacement in 2007.
The ACSM guidelines recommend consumption of 5-7ml of fluid per kg of body weight at least four hours before exercise. If the individual produces dark urine (or none at all) then more fluid should be consumed about 2 hours before the event. Aim for 3-5ml per kg of body weight.
The goal of drinking during exercise is to prevent excessive dehydration and changes in mineral balance, which will have a knock-on effect on performance. Try to ingest fluid throughout, little and often: approximately 150ml every 10-15 minutes.
Consumption of beverages containing minerals and carbohydrates can help maintain the fluid-mineral balance and therefore improve exercise performance.
Fluid intake during exercise should, as near as possible, match the fluid losses so that the overall fluid deficit remains low.
However, every runner sweats at a different rate, produces different amounts of sodium and reacts differently to heat. To work out how much fluid you need, there are a couple of simple tests you can do at home. First, check the colour of your urine before heading out on a session - you should have pale yellow urine.
You can also perform a sweat test to see how much water you lose on the average run. Weigh yourself before and after a short run (one on which you haven't consumed any fluids) - the difference is how much water you lost during your run. The results might surprise you!
Once you've worked out how much fluid you need, it's time to learn how to drink while running. The best way is to take on small amounts throughout the session - you can do this by carrying an isotonic sports drink.
You could also find out where the water stations will be located on your target marathon course and drink at these same points on your training run to replicate your experience on race day. If you're using gels or sweets, be sure to wash them down with water to avoid digestive problems while you're running.
If you're targeting a marathon later in the spring or in summer you'll need to adjust your hydration plans as the weather gets warmer. Make sure you perform another sweat test on a warm day to estimate how much more water you'll need in warmer weather.
Reduction in body fluids, unavailability of carbohydrates and an inadequate mineral balance during prolonged training sessions and exercise events will hamper performance. Fluid intake and adequate hydration during and after exercise are essential and, more importantly, critical during prolonged training sessions (lasting more than one hour) and competition events.
Carbohydrates are the primary fuel source for muscles. The more intense the exercise, the more carbohydrates you need to consume to ensure successful refueling and offset the depletion of the body's glycogen reserves.
When you sweat your body loses more than water; you lose minerals, too. Sodium is the main mineral lost in sweat. It's vital to replace it because it helps to offset dehydration. In addition, Sodium stimulates carbohydrate and water absorption, helping the body to hold on to water for a more effective rehydration both during and after the exercise session.
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