Water is the main constituent of the body: it makes up approximately 50-80% of the body weight, depending on lean body mass. Water is so essential to life as humans can survive only a few days without it.
In the body, water becomes the fluid in which all life processes occur. Functions of water can be summarised as:
• Carries nutrients and waste products throughout the body.
• Maintains the structure of large molecules such as proteins and glycogen.
• Participates in metabolic reactions.
• Serves as the solvent for minerals, vitamins, amino acids, glucose and many other small molecules.
• Acts as a lubricant and cushion around joints and inside the eyes, the spinal cord and in pregnancy, the amniotic sac surrounding the foetus in the womb.
• Aids in the regulation of body temperature.
• Maintains blood volume.
To support vital functions of our bodies, we need to create water balance: balance between water intake and output. Dehydration occurs when water output exceeds water input.
There are three levels of dehydration: mild dehydration, chronic mild dehydration and acute dehydration. The symptoms of mild dehydration include:
• dry skin and membranes
• rapid heartbeat
• low blood pressure
The reported health effects of chronic mild dehydration include increased risk of kidney stones, urinary tract cancers, colon cancer, childhood obesity, mitral valve prolapse and diminished physical and mental performance and salivary gland function. In addition to these health effects, mild chronic dehydration and poor fluid intake may have detrimental effects on proper elimination. There are certain processes in the body –such as intestines, kidneys, skin and lungs—require adequate water to function properly.
Acute dehydration manifests even more serious symptoms such as dizziness, spastic muscles, loss of balance, delirium, exhaustion and collapse.
The body must excrete a minimum of about 500 millilitres of water each day as urine. In addition to urine, water is lost from the lungs as vapour and from the skin as sweat and some is also lost in faeces.
How Much Water
According to Australian Guidelines, to be properly hydrated in a temperate climate like ours in Australia, adults require some 2500-3000 millilitres of fluid a day, depending on body size. Every day, solid foods we consume contribute approximately 1000 millilitres (1 litre) of water and 250 millilitres of water is produced by the body’s metabolism. However, the remainder needs to come from free water or other fluids or both.
There are circumstances, however, where there is an increased need for water such as exposure to hot weather or high temperatures, physical activity, exercise, strenuous work, exposure to air-conditioning, exposure to heating more than short periods, pregnancy, breastfeeding, fever, diarrhoea and vomiting.
1. Robinson J. 2002 Water, electrolytes and acid-base balance in Essentials in Human Nutrition edited by Mann J. & Truswell A. Oxford University Press. U. S. A. pages 113-128.
2. Baghurst K. 2003 Drink Plenty of Water in Food for Health: Dietary Guidelines for Australian Adults. Commonwealth of Australia. pages 95-105.
3. Whitney E. N., Cataldo C. B., Rolfes S. R. 2002 Understanding Normal and Clinical Nutrition Wadsworth /Thompson Learning U. S. A. pages 387-388.