Your body continually loses water: in urine and faeces, through your skin and from the lungs (when you breathe out). This means you need to top up levels on a regular basis and drink enough to replace what’s lost.
Without perspiration, the normal daily turnover of water in adults is about 4% of body weight, which is 2.8 litres for a 70 kg person. However, the amount of fluid lost will vary from person to person because we’re all different — it will depend on your own particular metabolism, your state of health, your diet and how much exercise you do, for example. And some people are at greater risk of dehydrating.
According to a review published in the Journal of the American Dietetic Association, the average sedentary man should consume at least 2.9 L of water a day, and the average sedentary woman at least 2.2 L per day. About one third of this will come from food. In terms of how much you should actually drink, the Dietitians Association of Australia recommends the average adult drinks at least eight glasses (2 L) of fluid per day — more during hot weather and physical activity.
This doesn’t mean drinking glass after glass of plain water: there are plenty of other sources of water in your diet, however, it’s recommended your daily quota is made up of plenty of water: 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 — see below.
- You’re exposed to air conditioning or heating for long stints.
- You’re pregnant or breastfeeding.
- You’ve got a fever, diarrhoea or been vomiting.
People at greater risk of dehydrating
The very young and the elderly are at greater risk of dehydration than most other people. It can result in heat stroke (extremely high body temperature) or heat exhaustion (excessive water and salt loss through sweat).
- Babies and children up to four years of age are too young to respond to their bodies’ needs, and are particularly sensitive to the effects of high temperatures. Compared with adults, daily water loss is much greater in infants, at 15% of body mass — that’s about 1 L for a 7 kg baby.
- While pregnant women require only an extra glass or so of water per day, nursing mothers need an extra 750–1000 mL above the basic recommendation.
- Elderly people lose some of the sensation of thirst, so may not drink enough water to avoid becoming dehydrated. Another issue that adds to the risk is the inconvenience and embarrassment caused by incontinence and the need for frequent urination (people have been known to drink less than they should to avoid these problems).
Water and exercise
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 only 1% of body weight can impair physical and sporting performance during exercise, while a loss of 3–5% can diminish performance by around 20%. Apart from making you feel light-headed and nauseous, dehydration also increases muscle glycogen use, so you get tired faster.
- You hydrate yourself two hours before exercising by drinking 400–600 mL — this allows time for adequate hydration and excretion of excess water.
- Drink sufficient fluid during exercise to prevent dehydration from exceeding 2% of your body weight: small amounts (150–300 mL) every 15–20 minutes should do the trick.
- To make sure you fully rehydrate after heavy exercise, weigh yourself before and after, 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 glucose and electrolytes added may be beneficial.
Not enough or too much water?
So what happens if you don't drink enough water or you drink too much?
A whole range of body processes works 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 1%, 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 1–2% of your body weight can do it. If you were 20% or more dehydrated, you’d die.
- Early signs of dehydration include headaches, 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 and persuade you to drink. And your kidneys are 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 way to 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.
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, weak and suffer convulsions.
You’d need to drink more than six litres over a short period of time for this to be a danger, and you’d probably make yourself sick in the process. But it does happen: for example, several people have died from drinking too much water after taking the drug ecstasy.