- Water is the most abundant constituent of the human body and regular fluid intake is essential for our bodies to function properly.
- The amount of fluid needed varies between people and according to age, time of year, climatic conditions, diet and levels of physical activity.
- We can obtain our fluid requirements from a number of sources such as water and other drinks, as well as the food we eat.
- Dehydration can impair physiological and performance responses, and in extreme cases can be fatal.
- It can be dangerous to drink too much water as water intoxication can lead to hyponatraemia.
- We have sensitive mechanisms to maintain our body water but attention should be given to children and older people who may not recognise the sensation of thirst so easily, to ensure they consume enough fluids.
- Regular fluid intake and water replacement are essential before, during and after exercise.
Water is essential for life and although humans can survive for a number of weeks without food, they cannot go without fluids for more than two to three days. On average, water makes up 60% of body weight (range 45-75%) and is essential for the correct functioning of all the cells in the body. Water is made up of hydrogen and oxygen (H2O).
Why is water good for us?
Water is essential for our bodies to work properly. Water is the major component of body fluid and has many functions in the body:
- It acts as a lubricant for joints and eyes
- It is the main component of saliva, which helps us swallow
- It provides the medium in which most reactions in the body occur
- It acts as a cushion for the nervous system and helps get rid of waste
- It helps regulate body temperature.
The body cannot produce enough water to meet all of its needs, so most of the water we need must be provided by our food and drink. If we do not consume enough water, we become dehydrated.
How much water do we need?
The amount of water and other fluids that we need to drink each day varies from person to person, depending on age, time of year, climatic conditions, diet and the amount of physical activity we do. Water requirements are particularly increased in hot climates and when physically active.
Dehydration can be very serious; a loss of 15-20% of body weight as water is fatal, and a loss of as little as 2% (equivalent to an overall loss of about 1.4L water in a 70kg man) can result in impaired cognitive, physiological and performance responses. Symptoms of dehydration include headache and fatigue.
However, drinking too much can also be dangerous as it can lead to water intoxication with potentially life threatening hyponatraemia (caused when the concentration of sodium in the blood gets too low). Severe cases can result in lung congestion, brain swelling, headache, fatigue, lethargy, confusion, vomiting, seizures and eventually coma.
The FSA recommends we drink about 6-8 glasses of fluid a day, equivalent to about 1.5-2L. We have sensitive mechanisms in the body to control hydration, triggering the kidneys to conserve water and stimulating a feeling of thirst when body water decreases, even by a small amount. Thus most people living in a temperate climate, such as that in the UK, who have ready access to food and drink are not at high risk of dehydration. However, children and older adults may not recognise the signals of thirst so readily, and care should be taken that they consume sufficient fluids. Those who are extremely physically active should also take care to consume enough fluid and replace that lost as sweat.
Sources of water in the diet
It has been estimated that roughly 20% of water consumed is from food, and the remaining 80% from beverages. The water content of food varies widely; it is usually less than 40% in cereal products, 40-70% in hot meals, more than 80% in fruits and vegetables, and approximately 90% in human breast milk and cows’ milk.
We can get our fluid requirements from a number of sources as water is contained in most foods and all drinks consumed. It is not necessary to drink only ‘pure’ water, although this is a good choice. Other drinks such as squash, fruit juice, fizzy drinks, tea and coffee contribute to our daily requirements too. Like everything we consume, selection depends on personal preference and availability. In addition to their contribution to our water needs, beverages may also have a wider effect on our health. Some, such as fruit juice, milk and tea contain nutrients and other substances that are beneficial for health. However, it is important to be aware of the sugar and overall energy content of beverages. Drinking a lot of energy-containing beverages may cause over consumption of energy, potentially leading to weight gain and obesity.
Dental health is also a consideration, and frequently consuming beverages containing sugar, such as fruit juices and sugar-containing fizzy drinks, can increase the risk of dental caries. In addition, dental enamel erosion can be caused by frequent consumption of acidic beverages, such as fruit juices and carbonated beverages.
Caffeine is a mild diuretic (in that it increases urine output to a small extent), but drinks that contain caffeine (such as tea, coffee, cola) also contribute to fluid intake under normal circumstances. You do not need to drink more water to compensate for the caffeine present in these drinks. Women who are pregnant should limit their caffeine intake to 200mg/day or less .This is because high levels of caffeine can result in babies having a low birth weight, or even lead to pregnant mothers miscarrying.
Other caffeine-like substances are present in chocolate (theobromine) and tea (theophylline) which also have a mild diuretic effect.
With alcoholic drinks, the dehydrating effect can be greater, depending on the type of drink consumed. Spirits consumed alone can cause dehydration, while more dilute alcoholic drinks such as shandy can have a net hydrating effect. Drinking water alongside alcoholic drinks will minimise any dehydrating effects. It is important to take account of safe guidelines for drinking.
Bottled, filtered or tap water?
A large number of bottled waters are now available, some of which are flavoured, carbonated and/or have added vitamins and minerals. Bottled waters are a convenient way to keep hydrated and are recommended where a clean, safe water supply cannot be guaranteed, for example when travelling abroad. There are no significant nutritional differences between bottled and tap water in the UK, and regulations can be tighter for tap than bottled water.
Some people choose to drink filtered water because they prefer the taste, odour and colour. Filtering water decreases its hardness and so reduces the scum sometimes seen on the surface of hot drinks. The filtering process also reduces chlorine along with other impurities in the water, which may affect taste, but filtered water offers no special benefits in terms of nutrition.
Tap water in the UK is safe to drink. Boiled and cooled water is recommended when making up infant formula.
Hard and soft water
Water naturally contains dissolved minerals such as calcium, magnesium and potassium. These minerals are picked up by rain water as it passes through the ground. If water seeps through hard rocks (such as granite) or peaty soils, it does not pick up these minerals. But, if water seeps through soft rocks, such as chalk and limestone it picks up both calcium and magnesium.
Water that contains lots of calcium and magnesium is called hard water and causes deposits to develop in kettles and other household equipment. Water with low levels of these minerals is known as soft water.
It has been suggested that one of the health benefits of drinking hard water may be a protective effect against cardiovascular disease and stroke. However, results from a number of studies have been inconsistent and the risk of developing cardiovascular disease is more strongly influenced by other factors such as high blood pressure and cholesterol, and smoking, diet, obesity, alcohol and physical activity.
Water and sport
Dehydration can occur amongst athletes, particularly those exercising in hot climates and/or at altitude. This is due to increased water loss due to sweating, and sweat rates can reach 3L/hour under extreme conditions. Sweat also contains sodium, and sodium losses can be significant. For example, an athlete who produces 5L of sweat during a day will lose the equivalent of about 10g of salt as sodium. Thus, replacing lost sodium can also be important. Fluids are essential, before, during and after exercise, as dehydration of as little as 2% of body weight lost as water (equivalent to a net loss of 1.4L in a 70 kg man) can impair performance.
Plain water is suitable for replacing any lost fluids following mild or moderate exercise, but research has demonstrated that water alone is not the best solution for fluid replacement during or after vigorous or prolonged exercise. Drinks that are specially prepared for this purpose contain carbohydrates (often glucose) and sodium to replace sodium lost in sweat. Hypotonic drinks are those with a lower osmolality (concentration of solutes) than body fluids and usually contain some sodium and small amounts of carbohydrate (less than 4%). Isotonic drinks have a similar osmolality to body fluids. They contain 4-8% carbohydrate and sodium so provide some energy as well as replacing water and sodium. Many commercially available sports drinks are isotonic. Hypertonic drinks contain more than 8% carbohydrate and have a higher osmolality than body fluids. These are more slowly absorbed than hypo- or isotonic drinks and are designed for use when energy needs are high and sweat rates are lower.
The amount of fluid and sodium lost when exercising will vary according to the intensity of exercise, the environmental conditions and the sweat rate of the individual, but it has been suggested that the addition of carbohydrate and sodium to drinks, either as a commercial sports drink or home-made version, are only necessary when exercising for an hour or longer.
Last reviewed July 2009. Next review due December 2012