The average person can survive for about 40 days without food, but most people will die if they go for more than 5 days without water.
Water is found throughout all body tissues – body cavities, blood vessels, cells and organs – and three-quarters of the human brain is made up of water.
Water is necessary for all the chemical reactions that occur in our bodies; it lubricates our joints, regulates body temperature, transports dissolved molecules and assists with removing potentially poisonous waste products.
Water is an essential nutrient because our bodies need it in amounts that exceed our ability to produce it from our food.
Did you know?
- About 70% of the human body consists of water, with the average adult male body containing about 40L.
- Every day, men need 3L and women 2.2L of water from all sources: bodily breakdown of food, food we eat, beverages we drink.
- Solid foods contribute approximately 20% (about 500–600ml per day) of total water intake.
- It’s recommended we drink at least 6 to 8 cups of liquid – from a selection of water, milk, tea, coffee and other drinks – per day, enough to make you pee 4 to 5 times each day.
- It is important not to ignore your thirst even if it is only mild. And the elderly and young should drink frequently, even if they are not thirsty.
How much water do we need?
The digestive system uses about 12L of water every day to process food. Even without exercising we lose 2 to 3L a day through breathing, perspiration, urine and faeces. However, we do conserve water by reabsorbing it from the bowel when we need to.
For the body to keep functioning normally, it needs a steady supply of water from daily intake of food and drink.
Solid food, especially vegetables and fruits, contributes approximately 1L of that. The remainder of the water needs to come from fluids such as water, milk, tea, coffee and other beverages: 1.7L for men, 0.9L for women.
As a general rule for good health take an amount of fluid that requires you to urinate 4 to 5 times a day. Knowing this – you can relax a little over the pressure to consume 8 glasses of water (2L) every day!
The body has a very clever way of telling us when we are low on water – we get thirsty! Loss of water makes body fluids more concentrated (thicker) and this sends signals to the brain to make us feel thirst and to our kidneys to conserve water.
Thirst appears when there is just a 2% rise in the thickness of blood, whereas dehydration is defined as a 5% rise. So the majority of healthy people can rely on thirst without risk of serious dehydration.
However, there are exceptions to this. Sometimes in old age, and in young people, thirst is not such a reliable indicator. The elderly and young should drink frequently, even if they are not thirsty. And, when participating in vigorous sport, thirst is usually an indication that you are already dehydrated.
It is important not to ignore your thirst, even if it is only mild.
What causes chronic, mild dehydration?
Several factors increase this possibility, including: poor thirst detection, dislike of the taste of water, drinking a lot of caffeine and alcohol, and environmental conditions and exercise. Young people and the elderly are more at risk from dehydration.
Water & exercise
If you exercise vigorously you should drink a glass of water before starting and then have half a glass every 15 minutes. This will prevent dehydration and improve performance.
Tap water vs bottled water
In New Zealand, most tap water is reticulated (supplied by the local water authority) and has fluoride added. There is evidence to show that drinking water containing fluoride can help to prevent tooth decay.
Take care drinking non-reticulated water; water from tanks and other sources can easily be contaminated with such things as bird and animal droppings – and sometimes dead animals as well.
Bottled water may be a convenient alternative to tap water, and one that many people choose to drink, but there are no known benefits over reticulated tap water. In fact, bottled water is often just filtered tap water.
Water & sugar
Remember sports drinks, fruit juices and fizzy drinks contain lots of sugar. Know what you are drinking.
Too much water
We can suffer from an excess of water as well as a deficit, and both can threaten life. An excessive amount of water in the body is rarer than a deficiency, because our kidneys are very good at getting rid of water before it can be absorbed. When water intake exceeds our need, cells swell, the thirst message is switched off and our kidneys release urine.
Not enough water
Dehydration can be caused by a reduced intake of fluids or by an increase in loss of fluids. When 1% to 5% of body water is lost, symptoms include: thirst, vague discomfort, lessened movement, loss of appetite, flushed skin, impatience, increased pulse rate and nausea. When greater amounts are lost, symptoms can go from dizziness and headache, right through to finding it hard to breathe and swallow, and death.
Health effects of chronic, mild dehydration and poor fluid intake include increased risk of:
- kidney stones and urinary tract cancers and some colon cancers
- heart valve disorder and diminished physical and mental performance.