The human body can last weeks without food, but only days without water. The body is made up of 55 to 75 per cent water. Mature adults are about 70% water; this drops to about 60% in the elderly and continues to drop into very old age. Water forms the basis of blood, digestive juices, urine and perspiration. The water content of the body breaks down along these lines:
· 80 per cent of blood is made up of water
· 73 per cent of lean muscle (including brain tissue) is water
· 25 per cent of fat is water
· 22 per cent of those solid-looking bones are water.

The body is unable to store water for any length of time and needs fresh supplies every day due to losses from lungs and skin, accounting for 50% of water loss; losses from urine and faeces account for the rest of the total losses. The amount we need depends on our metabolism, the weather, the food we eat and our activity levels. Heavy or obese people carry less body water than people of a healthy weight. As fat content increases, lean tissue decreases, leading to an overall decline in total body water. Body water is higher in men than in women and falls in both with age. Most mature adults lose about 2.5 litres (women) to 3 litres (men) per day and the elderly lose about 2 litres per day. This water loss needs to be replaced through food and beverages. Foods provide about 1 litre of fluid and the remainder must be obtained from beverages.

Water is needed in the body to:
1. Maintain the health and integrity of every cell in the body.
2.Keep the bloodstream liquid enough to flow through blood vessels.
3.·Help to eliminate toxins (such as those found in tea, coffee, alcohol, refined foods and soft drinks) through urine and faeces.
4.Regulate body temperature through sweating.
5.Keep mucous membranes moist, such as those of the lungs and mouth.
6.Lubricate and cushion joints.
7.Reduce the risk of cystitis by keeping the bladder clear of bacteria.
8.Aid digestion and prevent constipation.
9.Work as a moisturiser to improve the skin's texture and appearance.
10.Carry nutrients and oxygen to cells.
11.Serve as a shock absorber inside the eyes, spinal cord and in the amniotic sac surrounding the foetus in pregnancy.

Chronic mild dehydration and poor fluid intake can:
1.Increase the risk of kidney stones and constipation
2.Increase the risk of urinary tract cancers
3.Increase the risk of breast and colon cancers
4.Increase the risk of childhood obesity
5.Diminish physical and mental performance
6.Diminish salivary gland function

On a normal day, the body loses 2.4 litres of water (or 10 cups) and this figure is higher on warmer days, or when exercising. When the water content of the body drops below optimal levels, the result is dehydration. This is easily remedied by increasing fluid intake. Mild dehydration is often observed because many people do not consume enough fluids. About 30-40% of Australians were having less than 6-8 cups of fluid on the day of the Nutrition survey conducted in 1995.

Symptoms for dehydration include headaches, lethargy, mood changes and slow responses, as well as dry nasal passages, and dry or cracked lips. Other symptoms of dehydration include dark-coloured urine, weakness, tiredness, confusion and hallucinations. Eventually urination stops, the kidneys fail and toxic waste products can't be removed by the body. In extreme cases, this may result in death.

The various causes of dehydration include:
1. Increased sweating due to hot weather/humidity, exercise, fever
2. Lack of drinking water
3. Insufficient signalling mechanisms in the elderly; sometimes they do not feel thirsty even though may be dehydrated
4. Increased output of urine due to a deficiency of pituitary or adrenal hormones, diabetes, kidney disease, or medications that increase the output of urine like diuretic drugs for the treatment of high blood pressure.
5. Increased output of faeces (diarrhoea) or vomiting due to illness such as cholera, dysentery, food poisoning
6 . Recovering from burns

Elderly
Kidney function can decline as part of the normal ageing process with decrease in kidney mass. This together with hormonal changes and factors such as decreased thirst perception, medication, cognitive changes, limited mobility, and increased use of diuretics and laxatives can increase their risk of dehydration or decrease their requirement for fluid. It is estimated that 6 household glasses or cups (at least 150millilitres each) in combination with an adequate intake of food will provide more than the required 2 litres a day in a temperate climate. Although healthy older Australians living independently appear to drink sufficient fluid their risk of dehydration increases with medication use, chronic illness and frailty.

Babies
Children are very susceptible to dehydration, particularly if they are ill. Vomiting, fever and diarrhoea can quickly dehydrate a baby. This can be a life threatening condition. If you suspect dehydration, take the child immediately to the nearest casualty department. Some of the symptoms of dehydration in a child include:
· Cold skin
· Lethargy
· Dry mouth
· Depressed fontanelle on the skull
· A blue tinge to the skin as the circulation slows.

Increased water requirements are observed :
1. for people consuming a high protein diet because it requires more water for digestion and removing nitrogen from protein
2. for people consuming high fibre diets - fluids are important in preventing constipation especially if wholemeal or wholegrain foods are consumed in large amounts.
3. for Children
4. during episodes of vomiting and/or diarrhoea
5. during physical activity
6. when exposed to warm/hot conditions

Water regulation
The kidneys regulate the amount of water in the body. A special gland in the brain called the hypothalamus monitors water levels. If there is too little water in the body, the hypothalamus asks the nearby pituitary gland to communicate the need to conserve water to the kidneys. This is achieved by the secretion of a chemical called antidiuretic hormone. Once the kidneys receive this message, they reabsorb more water and concentrate the urine. At the same time, we are prompted to drink by the sensation of thirst, which is signalled by the brain. Alternatively, if there is too much water in the body, the hypothalamus instructs the kidneys - via the pituitary gland - to dilute the urine and get rid of the excess.

Water Intoxication
Drinking too much water can also damage the body. If too much water is consumed, the kidneys cannot excrete enough fluid. Water intoxication can lead to headaches, blurred vision, cramps and eventually convulsions. For water to reach toxic levels in the body, an individual would have to consume many litres a day. Water intoxication is most common in people with particular diseases or mental illnesses (for example, in some cases of schizophrenia), and in infants who are fed high quantities of fluid with low electrolyte levels (for example, infant formula which is too diluted).

Fluid retention
Many people believe that drinking water causes fluid retention. In fact, the opposite is true. One of the main causes of fluid retention is a high concentration of sodium (salt) in the body. Drinking water helps the body rid itself of excess sodium - this results in less fluid retention. The body also retains fluid if there is too little water in cells. If the body receives enough water on a regular basis, there will be no need for it to conserve water - and this will reduce fluid retention.

Water content in food
Most foods, even those that look hard and dry, contain water. Generally, the higher the water content, the lower the kilojoule count. Uncooked meat is two thirds water; most fruits are many vegetables are 90 percent water. The body can get about half of its water needs from food alone. The digestion process also produces water as a by-product and can provide around 10 per cent of the body's water requirements. The rest can come from liquids.

Mineral water contains salt
Commercially bottled mineral water typically contains spring water, salts and carbon dioxide. The sodium in mineral water can lead to fluid retention and swelling. It can even increase blood pressure in susceptible people. It is advisable to limit the amount of mineral water in the diet, or at least make low sodium varieties your preference. The label should state less than 30mg sodium per 100ml, or 300ppm (parts per million). Mineral waters that are sweetened and flavoured are more or less equivalent to regular soft drinks as far as kilojoule content and artificial additives are concerned.

How much water/fluids are recommended daily?
It has become a common belief that we need "2 litres or 8 glasses" of water daily and that beverages containing caffeine or alcohol do not count because they increase the excretion of water or have a diuretic effect. This belief has never been scientifically tested, and there is little evidence to support it. A recent review of the evidence was published in the American Journal of Physiology: Endocrinology and Metabolism (August 2002) http://ajpregu.physiology.org/cgi/content/abstract/00365.2002v1. The review found that such a large amount of liquid is not needed because diet surveys with thousands of adults have shown that many adults did not consume this much fluid and were still considered to be healthy and not dehydrated. Also, the system that regulates water balance is precise. The review concluded that some people may need large amounts of fluids, particularly if they are physically active and in hot climates and that caffeine and alcohol containing beverages can be counted as "water" because most of the water in these beverages does get used by the body.

So how much water is recommended daily? Approximately six to eight glasses (of at least 150 millilitres each) of a variety of "fluids" can be consumed each day.

More than eight glasses may be needed for the physically acitive, for children, for people in a hot/humid environments and for breastfeeding women (who need an extra 750-1000ml per day). Less water may be needed by the sedentary, for the elderly, for people in a cold environment, or if many high water content foods are consumed.

"Fluids" includes fresh water and all other liquids like juice, soft drinks, coffee, tea, milk, soup. Fresh water is the best drink of all because it does not contain calories and has fluoride that is good for the teeth. The next most important fluid is milk (especially for children) and tea (especially for adults). Tea, especially green tea, is an important source of antioxidant polyphenols which appear to offer protection against heart disease and cancer. However, the additional energy from milk and sugar added to tea can be substantial in some people. Fresh fruit is preferable to fruit juice because the former has more fibre and nutrients and less sugar; for these reasons it is recommended that juice consumption be limited to one to two glasses a day. Soft drinks and other sweet drinks with added sugar should be limited because they add substantial dietary energy to the diet without additional nutrient value.

Healthy Beverage Pyramid

The Healthy Beverage Pyramid for adults was created by Dr John Weisberger of the American Health Foundation. The guide follows the same principles as related Healthy Eating Pyramids (please note: 1 glass in this pyramid is 250ml)

Drink MOST of the drinks featured at the base of the Pyramid (this means water, 3-4 glasses a day)
Drink MODERATELY of the drinks featured in the second tier of the Pyramid (tea and/or vegetable soup, 2-3 glasses a day)
Drink MODESTLY of the drinks featured in the third tier (milk, juice, coffee 1-2 glasses a day)
Drink LEAST, if at all, of the drinks at the tip of the Pyramid (this means red wine, 0-1 glasses a day)

The pyramid provides a minimum of 6 OR a maximum of 10 glasses of fluid a day.
The added advantage of choosing a variety of fluid is that a variety of nutrients will be consumed. For example vitamin C (juice), antioxidants (from tea, juice and wine), potassium and other minerals (juice and soup), folate (tea and juice) and calcium (milk). The fluid pyramid guidelines were designed to moderate the intake of sweetened drinks such as soft drinks, cordial and sports drinks to encourage responsible alcohol consumption and to limit caffeine intake. Since the pyramid was designed for adults, children would switch milk for tea/coffee and have no alcohol.

References

Review of the literature and evidence on which the Dietary Guidelines for Australians are based 2001 adults & 1999 elderly

Article co-authored by Better Health Channel
(Australian -Victorian Government website)

Last Updated: June 2006