The body's fluid status is a truly delicate balance, so much so that experts rank water second only to oxygen as essential for life. Water plays a vital role in all bodily processes by providing a universal medium for chemical reactions, lubrication, nutrient delivery, waste disposal, heat dispersion and temperature regulation. In order to consider what fluids to drink, when to drink and how much to drink, it is first necessary to consider the basis of fluid regulation.
Water represents 45 to 60% of an adult's total body weight. The variation is explained by the difference in body tissue proportions between individuals, with muscle comprising 80% water compared to fat which is only 20% water. The lower a persons percentage of body fat the higher their percentage of body water is so women will tend to have a lower percentage of body water compared to their male counterparts.
In order to maintain the fluid balance within the body water gain needs to equal water loss. Fluid loss is heavily dependant on a person's activity level as well as environmental temperature and humidity. For a sedentary adult in the UK, an expected total water loss would be approximately 2500 ml per day and would comprise of -
1500 ml/day - excreted by kidneys in the form of urine
500 ml/day - evaporation & perspiration from the skin
300 ml/day - lost from the lungs
200 ml/day - lost from the gastrointestinal tract
In order to main a constant body fluid volume these water losses have to be matched by water gain. Only a small amount of water (200ml/day) is produced within the body through metabolic reactions. This leaves 2300ml/day which has to be ingested in either liquid or food form in order to maintain fluid balance. Exercise increases muscular work and this results in an increased demand for oxygen and the production of heat. Three quarters of muscular activity is lost as heat and sweat production is increased to maintain body temperature via evaporation. The rate of respiration increases to supply sufficient oxygen to the muscles which results in greater water loss from the lungs as the air is moistened to provide more efficient diffusion from the lung space into the blood. It can therefore be seen that exercise significantly increases fluid loss - up to 4 litres per hour during heavy exercise. A loss of body mass through dehydration of as little as 2% significantly impairs performance (Armstrong et al. 1985) which emphasises the importance of the maintenance of hydration, especially during exercise.
So how does dehydration affect the body and how does the body recognise it has a problem with fluid loss? Blood plasma is 92% water and dehydration consequently reduces the volume of blood in the body. This makes the cardiorespiratory system work harder to pump the blood around the body and deliver sufficient oxygen to the working muscles. Exercise feels much harder as your body's systems aren't working as efficiently as they do with good hydration and performance is impaired. As dehydration progresses, nausea and vomiting may be experienced and a 5% reduction in total body weight results in a performance drop of 30%. Fluid losses over 5% of total body weight results in weakness, confusion and dizziness that can lead, ultimately, to coma and death.
The body's response to fluid loss is that of thirst and the stimulation of thirst by dehydration is what is known as a negative feedback system.
1. Decreased flow of saliva
2. Decreased blood volume
3. Increased blood osmotic pressure
These three reactions to dehydration stimulate the thirst centre, which is situated in the hypothalamus in the brain, producing a the sensation of thirst. However, the thirst mechanism is relatively slow. Although a person can only survive a few days without water, and yet the body can survive without food for around 30 days, the drive to eat is much greater than the drive to drink.
To the majority of recreational exercisers thirst will be viewed as an indicator that they need to drink or they will become dehydrated. In actual fact, thirst is a sign that the person is already dehydrated, albeit in the initial stages. This is particularly pertinent as the thirst mechanism also tends to be depressed during exercise which increases the level of dehydration which occurs before thirst is sensed. This is why fluid replacement should be considered before, during and after physical activity rather than relying on thirst to indicate that the body needs fluid.
There is always a tendancy to think that water is plain and that additional ingredients will improve it. However, in actual fact, water is the best fluid you can choose for exercise of an hour or less in duration. If you think water is just water then think again. It is worth considering what type of water to use.
Tap water: is not only recycled on average 9 times before it even gets to you but its contents are very variable. Soft water contains higher levels of sodium and has been linked to a greater incidence of high blood pressure. Hard water contains high concentrations of calcium and magnesium. When bottled, tap water may be termed 'table water'.
Natural mineral water: Comes from an undergroung source. It must comply with UK Natural Mineral Water Regulations 1985 and EC Natural Mineral Water Directive 80/777 which stipulates that the water must be bottled at source, have a constant mineral content and cannot be treated in any way.
Spring water: Comes from an underground source and must meet Drinking Water in Containers Regulations and can be treated if necessary.
Purified water: Can be drawn from any source and can be artificially processsed and chemically treated to make it drinkable.
* Don't rely on thirst. Drink before, during and after any physical activity. Get into the habit of taking on fluids during training as well as competition.
* If the exercise lasts for less than an hour the body should have sufficient electrolyte and carbohydrate supplies to maintain optimal performance. Therefore, for short periods of exercise water is just as good as sports drinks.
* If exercise lasts for over an hour a drink with electrolytes and carbohydrates will aid performance by supplying additional energy.
* Contrary to popular opinion a relatively small amount of sweat is salt (0.2 to 0.4%) so, unless the climate is extremely hot or the activity is over an hour in duration, additional salt should not be needed.
* Always try to drink more fluids than you need. Smaller quantities at frequent intervals helps optimise hydration.
* The World Health Organization recommends drinking 6 to 8 large glasses a water a day. But it must be remembered that this is only to maintain normal fluid balance and does not take into account the extra fluid demands of exercise.
Drink at least half a litre of water before exercise to ensure your fluid levels are up to start and then continue taking in as much as you can during exercise.
* Finally if you are a health care professional a key role for you will be to educate and motivate your clients to adopt 'good hydration practices'.
References:
American College of Sports Medicine (1996) advise on fluid intake
Armstrong et al (1985) , Medicine and Science in Sports & Exercise, 17: 456-461
Davis & Brewer (1997), Journal of Sports Sciences, 15(1):44-45
Fallowfield et al (1995), Journal of Sports Sciences, 13(1):26-27
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