Daily consumption of drinking water
The recommended daily amount of drinking water for humans varies. It depends on activity, age, health, and environment. In the United States, the Adequate Intake for total water, based on median intakes, is 4 litres (140 imp fl oz; 140 US fl oz) per day for males older than 18, and 3 litres (110 imp fl oz; 100 US fl oz) per day for females over 18; it assumes about 80% from drink and 20% from food. The European Food Safety Authority recommends 2.0 litres (70 imp fl oz; 68 US fl oz) of total water per day for women and 2.5 litres (88 imp fl oz; 85 US fl oz) per day for men.
The common advice to drink 8 glasses (1,900 mL or 64 US fl oz) of plain water per day is not scientific; thirst is a better guide for how much water to drink than is a specific, fixed amount. Americans aged 21 and older, on average, drink 1,043 mL (36.7 imp fl oz; 35.3 US fl oz) of drinking water a day, and 95% drink less than 2,958 mL (104.1 imp fl oz; 100.0 US fl oz) [of what?] per day. Exercise and heat exposure cause loss of water and therefore may induce thirst and greater water intake. Active people in hot climates may need 6 litres (210 imp fl oz; 200 US fl oz) of water, or more, per day.
How much drinking water contributes to the intake of mineral nutrients is unclear. Inorganic minerals generally enter surface water and groundwater via stormwater runoff and through the ground. Water treatment also adds some minerals, such as calcium, zinc, manganese, phosphate, fluoride, and sodium compounds. Water generated by the biochemical metabolism of nutrients provides a significant part of the daily water needs for some arthropods and desert animals, but provides only a small fraction of a human's necessary intake. There are trace elements in almost all potable water; some of these affect metabolism, such as sodium, potassium, and chloride, which are common in small amounts in most water. Other elements, such as fluoride, while beneficial in low concentrations, can cause dental and other problems at high levels.
Fluid balance is important to health. Profuse sweating can increase the need to replace electrolytes (salts). Water intoxication (the consumption of too much water too quickly) causes hyponatremia, which can cause death in minutes or hours. Water makes up about 60% of the body weight in men and 55% of weight in women. A baby is about 70% to 80%; old people are about 45% water.