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In physiological psychology, water intake is an aspect of fluid intake and is necessary for all life on Earth. We are largely composed of water. By weight, mammals are approximately two-thirds water. Humans can survive for several weeks without food, but for only a few days without water. A constant supply is needed to replenish the fluids lost through normal physiological activities, such as respiration, perspiration, defecation and urination and prevent dehydration. Unlike minerals or energy, in adipose tissue, little water is actually stored in the body.
When water use exceeds water intake, the body tries to conserves liquid, mainly by reducing the amount of water excreted from the kidneys. Eventually, physiological conservation can no maintain the balance triggering thirst and the motivation to drink.
The body relies on signals from osmotic and volume pressure baroreceptors in the hypothalamus to inform the brain of body fluid status and to activate the homeostatic process of osmoregulation through specific neurohormonal systems (e.g., the renin-angiotensin system) to restore fluid balance. As with food intake, signals that stimulate drinking behavior, as well as those that terminate drinking, interact to ensure that adequate amounts of both water and electrolytes are consumed. However the signals for water satiety, and how satiety changes the taste and motivation for seeking water are unclear.
The exact amount of water a human needs is highly individual, as it depends on the condition of the subject, the amount of physical exercise, and on the environmental temperature and humidity. In the US, the reference daily intake (RDI) for water 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. 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 RDI. Because in general, RDI values incorporate a safety margin to account for individual variations, it does not mean that this amount is necessary for every person. The folk wisdom that everyone should drink two litres (68 ounces, or about eight 8-oz glasses) of water per day is not supported by scientific research. Various reviews of all the scientific literature on the topic performed in 2002 and 2008 could not find any solid scientific evidence that recommended drinking eight glasses of water per day.   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 more flexible guideline is that a normal person should urinate 4 times per day, and the urine should be a light yellow color. [How to reference and link to summary or text] Profuse sweating can increase the need for electrolyte (salt) replacement. Water intoxication (which results in hyponatremia), the process of consuming too much water too quickly, can be fatal.
The human kidneys will normally adjust to varying levels of water intake. If a person suddenly increases water intake, the kidneys will produce more diluted urine, even if the person did not happen to consume water excessively.[How to reference and link to summary or text] 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 can, and has been known to be, fatal.
- Main article: Fluid balance
The cells of animals require water for virtually all metabolic processes. Water acts as a solvent for sodium, chloride, and potassium ions, as well as sugars, amino acids, proteins, vitamins etc, and is therefore essential for the smooth functioning of the nervous system and for other bodily processes
In terms of mineral nutrients intake, it is unclear what the drinking water contribution is. However, inorganic minerals generally enter surface water and ground water via storm water runoff or through the Earth's crust. Treatment processes also lead to the presence of some mineral nutrients. Examples include fluoride, calcium, zinc, manganese, phosphate, and sodium compounds. Water generated from the biochemical metabolism of nutrients provides a significant proportion of the daily water requirements for some arthropods and desert animals, but provides only a small fraction of a human's necessary intake. There are a variety of trace elements present in virtually all potable water, some of which play a role in metabolism. For example sodium, potassium and chloride are common chemicals found in small quantities in most waters, and these elements play a role (not necessarily major) in body metabolism. Other elements such as fluoride, while beneficial in low concentrations, can cause dental problems and other issues when present at high levels. Water is essential for the growth and maintenance of our bodies, as it is involved in a number of biological processes.
Animal water intake
- Main article: Animal water intake
Like humans animals are largely made up of water and must maintainn their water intake
Water is needed by many birds although their mode of excretion and lack of sweat glands reduces the physiological demands. Some desert birds can obtain their water needs entirely from moisture in their food. They may also have other adaptations such as allowing their body temperature to rise, saving on moisture loss from evaporative cooling or panting. Seabirds can drink seawater and have salt glands inside the head that eliminate excess salt out of the nostrils.
- Animal drinking behavior
- Drinking behavior
- Drinking water
- Osmometric thirst
- Water deprivation
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- ↑ US daily reference intake values
- ↑ Swedish DFA (in Swedish)
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