Water is vital both as a solvent in which many of the body's solutes dissolve and as an essential part of many metabolic processes within the body. It is also a vital constituent of body cells that is essential to their form and functioning.
Water is also central to acid-base neutrality and enzyme function. An acid, can be neutralized by a base to form water. Water is considered to be neutral, with a pH (the negative log of the hydrogen ion concentration) of 7. Acids have pH values less than 7 while bases have values greater than 7.
From a biological standpoint, water has many other distinct properties that are critical to body func-tioning and these include:
- Regulating body temperature.
- Carrying nutrients and oxygen to cells.
- Protecting body organs and tissue.
- Lubricating joints.
- Lessen the burden on the kidneys and liver by eliminating waste.
- Moisten tissues such as those in the mouth, eyes, and nose.
- Help prevent constipation.
- Help dissolve minerals and other nutrients to make them accessible to the body.
- Helps form structures of large molecules such as protein and glycogen.
- All biochemical reactions occur in water, it also fills the spaces in and between cells.
Water covers some 70% of the Earth's surface. Approximately 97.2% of it is saline (contains a significant concentration of dissolved salts), and only 2.8% is fresh. Potable water is available in al-most all populated areas of the Earth, although it may be expensive and the supply may not always be sustainable.
Sources where water may be obtained include:
- Ground sources such as groundwater, springs, hyporheic zones and aquifers
- Precipitation which includes rain, hail, snow, fog, etc.
- Surface water such as rivers, streams, glaciers
- Biological sources such as plants.
- Desalinated seawater
- Water supply network
- Atmospheric water generator
Springs are often used as sources for bottled water.
Suggestions for Fluid Intake
- Drinking voluntarily before the thirst signals. Thirst and discomfort are experienced when the body loses 1% of body fluid. So if you feel thirsty, you are already dehydrated.
- Try to avoid caffeinated and alcoholic beverages. While they supply water to the body, they do contain diuretics (e.g. caffeine) that cause the body to lose water.
- Cold beverages are more palatable during and after exercise, and this greater palatability will increase fluid consumption by athletes. Drinking cold beverages (8-12°C) causes a slight transient cooling of the upper digestive tract.
- Sports drink contain minerals. Sports drinks are intended to replenish electrolytes, sugar, water, and other nutrients, and are usually isotonic (containing the same proportions as found in the human body). Non-athletes who use sports drinks should also be aware that sports drinks for athletes typically contain high levels of carbohydrates which will result in weight gain if consumed without a corresponding increase in exercise activity.
- Try to avoid concentrated juice, sweetened drinks, chocolate milk and soda. These sugared beverages will lower the rehydration rate.
- The choice of drink will depend on whether you need a drink to replace fluid losses or to provide more energy / carbohydrate or both. Either plain water or sports drink which contain 4 to 8% carbohydrate are suitable.
A deficiency of water in the body is known as dehydration. Dehydration occurs when water intake is not enough to replace free water lost due to normal physiologic processes, including breathing, uri-nation, and perspiration, or other causes, that may include diarrhea and vomiting. Dehydration can be life-threatening when severe and lead to seizures or respiratory arrest, and also carries the risk of osmotic cerebral edema if rehydration is overly rapid.
The term dehydration has sometimes been used incorrectly as a proxy for the separate, related condition hypovolemia, which specifically refers to a decrease in volume of blood plasma. The two are regulated through independent mechanisms in humans; the distinction is important in guiding treatment.
Most people can tolerate a three to four percent decrease in total body water without difficulty or ad-verse health effects. A five to eight percent decrease can cause fatigue and dizziness. Loss of over ten percent of total body water can cause physical and mental deterioration, accompanied by severe thirst. Death occurs at a loss of between fifteen and twenty-five percent of the total body water. Mild dehydration is characterized by thirst and general discomfort and is usually resolved with oral rehy-dration.
In humans, dehydration can be caused by a wide range of diseases and states that impair water homeostasis in the body. These occur primarily through either impaired thirst/water access or sodi-um excess.
Risk factors for dehydration include but are not limited to; exerting oneself in hot and humid weather, habitation at high altitudes, endurance athletics, elderly adults, infants, children and people living with chronic illnesses.
In the elderly, blunted response to thirst and/or inadequate ability to access free water in the face of excess free water losses (especially hyperglycemia related) seem to be the main causes of dehy-dration. Excess free water or hypotonic water can leave the body in two ways - sensible loss such as osmotic diuresis, sweating, vomiting and diarrhea, and insensible water loss, occurring mainly through the skin and respiratory tract.
Signs and symptoms
The hallmarks of dehydration include thirst and neurological changes such as headaches, general discomfort, loss of appetite, decreased urine volume (unless polyuria is the cause of dehydration), confusion, unexplained tiredness, purple fingernails and seizures. The symptoms of dehydration be-come increasingly severe with greater total body water loss.
Water intoxication, also known as water poisoning or hyper-hydration, is a potentially fatal disturb-ance in brain functions that results when the normal balance of electrolytes in the body is pushed outside safe limits by over-hydration (excessive water intake).
Water, just like any other substance, can be considered a poison when over-consumed in a specific period of time. Water intoxication mostly occurs when water is being consumed in a high quantity without adequate electrolyte intake.
Under normal circumstances, accidentally consuming too much water is exceptionally rare. Nearly all deaths related to water intoxication in normal individuals have resulted either from water-drinking contests, in which individuals attempt to consume large amounts of water, or from long bouts of ex-ercise during which excessive amounts of fluid were consumed. In addition, water cure, a method of torture in which the victim is forced to consume excessive amounts of water, can cause water intox-ication.
Excess of body water may also be a result of a medical condition or improper treatment; see "hypo-natremia" for some examples. Water is considered as one of the least toxic chemical compounds, with an LD50 of over 90 ml/kg in rats.
The amount of drinking water required is variable. It depends on physical activity, age, health, and environmental conditions. In a temperate climate under normal conditions, adequate water intake is about 2.7 liters for adult women and 3.7 liters for adult men. Physical exercise and heat exposure cause loss of water and therefore may induce thirst and greater water intake. Physically active indi-viduals in hot climates may have total daily water needs of 6 liters or more. The European Food Safety Authority recommends 2.0 liters per day for adult women and 2.5 liters per day for adult men.
In the United States, the reference daily intake (RDI) for total water is 3.7 liters per day (L/day) for human males older than 18, and 2.7 L/day for human females older than 18 which includes drinking water, water in beverages, and water contained in food. An individual's thirst provides a better guide for how much water they require rather than a specific, fixed quantity.