Water and your health

Water is essential to life. But how much should we drink each day to stay healthy?

And does it have to be water, or do other fluids count? What happens if we don't get enough - or too much?

In this report, we explain how much water we need daily, list good food sources of water, and tell you how to look out for signs of dehydration.

Why we need water

Your body consists mainly of water - about 70 percent of your total body weight is water. Women typically have lower average body-water content than men because they tend to have more body fat, which contains less water than muscle tissue. It's possible to survive for more than 50 days without eating, but you can live only a few days without water.

Water is vital to all sorts of bodily functions. One key fluid - your blood - helps transport essential nutrients and oxygen around your body. Other fluids act as lubricants for joints and eyes, help you swallow, act as cushions for your nervous system, and help with waste disposal. And water serves to regulate your body temperature - you sweat when you get too hot.

There are other ways in which water contributes to good health.

  • It maintains the health of your kidneys by helping flush toxins and body waste through them.
  • It reduces the risk of stones in your urinary tract (kidney stones or bladder stones).
  • Some research has suggested drinking plenty of fluid (particularly plain water) may reduce the risk of cancers of the colon and urinary tract (including kidneys and bladder) and breast cancer. However, it's too early for definitive conclusions to be drawn.
  • It's thought drinking more water (but not fatty or sugary drinks) could help prevent childhood and adolescent obesity, partly because it creates a feeling of fullness and also because we can sometimes mistake feeling thirsty for feeling hungry.

How much?

Your body continually loses water through urine and faeces, through your skin, and from your lungs when you breathe out. So you need to top up your water levels regularly by drinking enough to replace what's lost.

Without perspiration, the normal daily turnover of water in adults is about four percent of body weight, which is 2.8L for a 70kg person. However, the amount of fluid lost will vary from person to person because we're all different - it will depend on your metabolism, your state of health, your diet and how much exercise you do.

According to the Ministry of Health, the average man needs about 3L of water each day; for women it's about 2.2L. Allowing that about one third comes from food, dietitians recommend the average adult drinks at least eight glasses (2L) of fluid per day - more during hot weather and physical activity.

You don't need to drink glass after glass of plain water. There are several other sources of water in your diet. But plenty of water is recommended. Apart from anything else, it's best for quenching your thirst, is cheap (or free) and contains no calories.

Drink more water if any of the following applies:

  • You're exposed to high temperatures.
  • You're carrying out strenuous work or physical activity.
  • You're exposed to air conditioning or heating for long stints.
  • You're pregnant or breastfeeding.
  • You've got a fever or diarrhoea, or have been vomiting

Too much and not enough

Not getting enough?

A whole range of body processes work to keep your fluid levels relatively stable - the thirst mechanism and kidney function are just two. Generally, your total body fluid levels vary by less than one percent, regardless of fluctuations in what you drink. But there are dangers if you persistently drink too little.

It takes only small changes in your overall fluid levels to destabilise your system and bring about the symptoms of dehydration - as little as one to two percent of your body weight can do it. If you were 20 percent dehydrated or more, you'd die.

Early signs of dehydration include headache, fatigue, loss of appetite, flushed skin, heat intolerance, light-headedness, dry mouth and eyes, a burning sensation in the stomach and dark urine with a strong odour.

If dehydration becomes more advanced, you may get symptoms such as difficulty swallowing, clumsiness, shrivelled skin, sunken eyes and dim vision, painful urination, numb skin, muscle spasms and delirium.

Of course, long before these things start happening a powerful thirst should kick in. Your kidneys are also super-efficient at regulating water - if things get desperate, you'll probably stop urinating.

If you're not sure whether you're getting enough water, a simple check is to keep an eye on the colour of your urine. It will be yellow first thing in the morning, but should become paler by mid-morning. If it doesn't, you're not drinking enough.

Don't wait till you get thirsty to drink. Thirst is actually an emergency response. You don't become thirsty until you're already dehydrated. The typical symptoms, such as a horrible taste, dry throat and cravings for cool wet liquid, are physiological responses your body uses to signal its dehydration.

Too much water

It's possible to drink too much water. Your body's fluid balance can be dangerously upset if you drink more water than your kidneys can excrete. Your body cells swell, and you may feel drowsy and weak and suffer convulsions.

You'd need to drink more than six litres over a short period for this to be a danger, and you'd probably make yourself sick in the process. But it does happen. Several people have died from drinking too much water after taking the recreational drug ecstasy.

Exercising

When you're losing water rapidly, as you do when carrying out strenuous physical activity, you could already be dangerously dehydrated by the time your thirst mechanism kicks in.

A water loss of one percent of body weight can impair physical and sporting performance during exercise, while a loss of three to five percent can diminish performance by around 20 percent. Apart from making you feel light-headed and nauseous, dehydration also increases muscle glycogen use - so you get tired faster.

Experts say you should:

  • Hydrate yourself two hours before exercising by drinking 400 to 600mL. This allows time for adequate hydration and excretion of excess water.
  • Drink sufficient fluid during exercise to prevent dehydration from exceeding two percent of your body weight. Small amounts (150 to 300mL) every 15 to 20 minutes should do the trick.
  • Weigh yourself before and after exercise, and make up the weight loss in water.
  • For exercise that lasts less than 90 minutes, plain water is fine. For longer periods of exercise, a sports drink with added glucose and electrolytes may be beneficial.