You can't make enough water in your body or get enough from food to stay healthy, so make sure you drink enough each day. If you don't, it can affect your health.
What is the role of water in your body?
About 70% of your body is made up of water and it's involved in many of your body's processes.
Water fills the spaces in and between your cells, as well playing a key role in digesting and absorbing the food you eat. Water also helps keep your body temperature within safe limits.
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.
What is the role of thirst?
Your body has a clever way of telling you when you are low on water – you get thirsty! Loss of water makes body fluids more concentrated (thicker). This sends signals to your brain to make you feel thirst and to your kidneys to conserve water. It is important not to ignore your thirst, even if it is only mild.
Thirst appears when there is just a 2% rise in the thickness of your blood, whereas dehydration is defined as a 5% rise. So most healthy people can rely on thirst to tell you when you need to drink more.
However, there are exceptions to this. Older adults have fewer reserves of water and your sense of thirst is less reliable. Young children can dehydrate easily. They often can't tell you that they're thirsty, nor can they get a drink for themselves. Older adults and young children should drink often, even if they are not thirsty.
How much water do you need?
For your body to keep functioning normally, it needs a steady supply of water, especially in hot weather, when exercising or playing sport. The amount you need depends on your age, size, the weather and how active you are.
Fluid comes from what you drink as well as many everyday foods (eg, watermelon and lettuce have a high water content).
As a rough guide, adults should aim for 1.5–2 litres (6–8 cups) of fluid each day and children 1–1.5 litres (4–6 cups). This includes most drinks, such as water, milk, tea or coffee, but don't count alcohol in this total as it dehydrates you.
If you are exercising or playing a lot of sport you will need more.
As a general rule for good health, have enough fluid to pee 4 to 5 times a day. If your pee is a pale yellow colour, you are drinking about the right amount. If it is dark yellow you are not drinking enough, and if it looks like water, you are drinking too much.
How much water do you need when exercising?
If you exercise vigorously you should drink a glass of water before starting and then have half a glass every 15 minutes. This prevents dehydration and improves performance.
When taking part in vigorous sport, thirst is usually an indication that you are already dehydrated.
Is bottled water better than tap water?
In New Zealand, most tap water is reticulated (supplied by the local water authority) and in many cases has fluoride added. There is evidence to show that drinking water containing fluoride can help to prevent tooth decay. This water is safe to drink.
If you drink water from tanks and other sources, take care. It can easily be contaminated with such things as bird and animal droppings and sometimes dead animals.
Bottled water can be a convenient alternative to tap water, but there are no known benefits over reticulated tap water. In fact, bottled water is often just filtered tap water.
Read more about how to find out if your water is safe to drink.
What is the best thing to drink?
Water is the best fluid to drink. Other low or no-sugar options include milk, tea or coffee. Sports drinks, fruit juices and fizzy drinks contain lots of sugar and are best avoided. Know what you are drinking:
Read more about drinks for kids.
What happens when you don't drink enough water?
When you don't have enough fluid, you get dehydrated. Dehydration can be caused by drinking less water or losing more fluids.
When 1% to 5% of body water is lost you will get symptoms of dehydration:
- vague discomfort
- lessened movement
- loss of appetite
- flushed skin
- increased pulse rate
When you lose even more fluid, symptoms can go from dizziness and headache, right through to finding it hard to breathe and swallow, and death.
Read more about dehydration.
What causes ongoing mild dehydration?
Several factors increase the risk of ongoing dehydration.
- not being good at noticing you are thirsty
- not liking the taste of water
- drinking a lot of caffeine and alcohol
- environmental conditions, such as heat
The health effects of chronic mild dehydration and poor fluid intake include increased risk of:
- kidney stones
- urinary tract cancers
- some colon cancers
- heart valve disorder
- diminished physical and mental performance.
Can you have too much water?
You can have too much water as well as too little, and both can threaten life. Too much water in you body is rarer than having not enough, because your kidneys are very good at getting rid of water before it can be absorbed.
When you drink more water that you need, your body's cells swell, the thirst message is switched off and your kidneys release pee.
Food and nutrition guidelines Ministry of Health, NZ
- Jéquier E, Constant F. Water as an essential nutrient: the physiological basis of hydration European Journal of Clinical Nutrition. 2010; 64:115–123.
- Fluid Nutrition Foundation, NZ