In addition to exercise, it is important that your body gets enough water. Water is necessary for our body to operate efficiently. Water is vital to the body in temperature regulation, nerve impulse conduction, circulation, metabolism, immune system, eliminative processes, sensory awareness and perceptive thinking.

Irish people, on average, drink only eight ounces (one cup) of water per day. The rest of the water the body needs must be extracted from other liquids or foods that we eat. Not enough water is a real threat to the system. Many chemical reactions inside the body will not occur without the right amount of water.

It only takes a one percent fluid loss in the body to become dehydrated. This generally happens prior to any conscious sensation of being thirsty. Very small shortages of water can dramatically change and disrupt biochemistry. Water is considered by exercise physiologists as the single most important variable in peak performance. Your muscles can lose up to ten percent of their contractile strength and eight percent of speed from only a three percent dehydration.

A small change makes a big difference when it comes to water. If you do a lot of travel by air, you can lose as much as two pounds of water in a three to four hour flight. Stress, alcohol and caffeine all influence the amount of water and the speed in which your body loses it. Any of these factors, alone or in combination, could cause a small but critical shrinkage of the brain. This small shrinkage will impair neuromuscular coordination, decrease concentration, and slow thinking.

The average amount of water loss per day is two cups through breathing, two cups through invisible perspiration, and six cups through urination and bowel movements. That is a total of ten cups lost per day without taking into account perspiration from exercise or hard work, excessively dry air, or alcohol and caffeine consumption.

To maintain efficient operation, you want to drink the following:

Six-eight glasses (64 fl oz)
For every 20 pounds of body weight over 10.5 stone, drink an
additional glass. 
Thus, a person weighing 13.5 stone would drink a minimum of ten
glasses or 80 fluid ounces.
Over 250, drink the same as 17 Stone +, (12-13 Cups.)