Water

Water is also known as H2O, which refers to its molecular composition of two atoms of hydrogen and one of oxygen. Virtually every process in the body depends on having enough water, including metabolism, waste elimination, cellular life, temperature regulation and nutrient transport.
No RDA has been created for water, but adequate intake is considered to be 3.7 liters daily for men and 2.7 for women. All languages have words for water, and no organism on Earth can live without the liquid.
Civilization has essentially been built around the availability of fresh water from sources above and below ground.

Water Deficiency

The body can become deficient in water if intake remains lower than elimination for enough time. Diarrhea and vomiting, for example, can cause dehydration rapidly if not treated with water and electrolytes. Fever can also cause a water deficiency if it results in sweating, which is the body's attempt to cool itself through evaporation.

Long-term exposure to high temperatures, particularly in humid environments, can similarly result in dehydration.

Finally, increased urination caused by certain substances, such as alcohol and caffeine, or by health conditions, such as untreated diabetes, can also cause the body to become deficient in water.

Water Excess

Excessive amounts of water can occur because of abnormal water retention. Salt, a key player in the body's regulation of water levels, can increase water retention if it is overconsumed. The kidneys, which also regulate fluid levels, can drive water retention if diseased.

High levels of estrogen, caused by pregnancy, oral contraceptives or monthly hormonal fluctuations, are also known to cause water retention. Hypothyroidism is notorious for its ability to cause a bloated appearance, and this is due to increased retention of water. Congestive heart failure, allergic reactions, liver cirrhosis and lymph edema from cancer are a few other causes of water excess.