Why Water is Important

• Humans can survive several weeks without food, but only a few days without water.  The exact amount of water a human needs is highly different individually, it depends on the condition of the subject, the amount of physical exercise, and on the environmental temperature and humidity.  In the United States, the reference daily intake is 3.7 litres per day for human males older than 18, and 2.7 litres for human females older than 18 including water contained in food, beverages, and drinking water.  It is a common misconception that everyone should drink two litres of water per day and is not supported by scientific research.  For example, people in hotter climates will require greater water intake than those in cooler climates.  An individual's thirst provides a better guide for how much water they require rather than a specific, fixed number.  A normal person should urinate 4 times a day, and the urine should be a light yellow color.
• A constant supply is needed to replenish the fluids lost through normal physiological activities, such as respiration, perspiration, and urination.  Food contributes 0.5 to 1 litre, and the metabolism of protein, fat, and carbohydrates produces another 0.25 to 0.4 litres, which means that 2 to 3 litres of water for men and 1 to 2 litres of water for women should be taken in as fluid in order to meet the reference daily intake. 
• Water is essential for growth and maintenance of our bodies, as it is involved in a number of biological processes.  Abundant sweating can increase the need for electrolyte (salt) replacement.  Water intoxication, the process of consuming too much water too quickly, can be fatal.
• The human kidneys will normally adjust to varying levels of water intake.  The kidneys will require time to adjust to the new water intake level.  This can cause someone who drinks a lot of water to become dehydrated more easily than someone who routinely drinks less.  Survival classes recommend that someone who expects to be in an environment with little water (such as a desert), not to drink water excessively, but rather to drink gradually decreasing amounts for several days before their trip to accustom the kidneys to making concentrated urine.  Not using this method has been known to be fatal.