Water is the most abundant, yet possibly the most overlooked, substance in the body. 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 (this is the key premise used for bioelectrical impedance analysis). Women will therefore 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, but for a sedentary adult in this country, an expected total water loss would be approximately 2500 ml per day and would comprise:

• 1500 ml/day excretion by kidneys in the form of urine
• 500ml/day evaporation and perspiration from the skin
• 300ml/day from the lungs
• 200 ml/day 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 thus leaving 2300 ml/day which has to be ingested in either liquid or food form. Exercise increases muscular work and consequently the demand for oxygen. Three quarters of muscular activity is lost as heat and therefore, 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 which can reach levels of up to 4 liters per hour during heavy work-outs. A loss of body mass through dehydration of as little as 2% significantly impairs performance (Armstrong et al 1985) which emphasizes the importance of the maintenance of hydration, especially during exercise.

But how does dehydration affect the body and how does the body recognize it has a problem with fluid loss? Blood plasma is 92% water, and therefore, dehydration reduces the volume of blood in the body which makes the cardio-respiratory 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 by 5% reduction in total body weight, performance has dropped by 30%. Fluid losses greater than this creates weakness, confusion and dizziness leading ultimately to coma and death if fluid isn't replaced immediately.

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.