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Nutrition

How much water do you really need to drink?

We are bombarded with messages about drinking more fluids –but is it really essential? Siân Phillips investigates.

How much should I consume?

The NHS Eatwell guide recommends that we should all consume six to eight glasses of water of day, and that low fat milk, sugar-free drinks, including tea and coffee all count towards this amount.

Why do we need this much?

“Your body is nearly two thirds water and so it is really important that you consume enough fluid to stay hydrated and healthy,” says the British Nutrition Foundation. “If you don’t get enough fluid you may feel tired, get headaches and not perform at your best.” A study into how well children performed in a visual attention test showed that those who did not drink water beforehand achieved a lower score than those who did.

But I’ve read that eight glasses a day aren’t necessary…

Yes, there has been some evidence to the contrary. An article in the British Medical Journal (1) questioned the benefits of drinking eight glasses of water a day. It said that the recommendation may have originated from a 1945 report, which stated: “A suitable allowance of water for adults is 2.5 litres daily in most instances. An ordinary standard for diverse persons is 1 millilitre for each calorie of food.” However, in reporting this figure it seems that the crucial next sentence – which said that most of our daily water could actually be obtained from food – was omitted.

Who should I believe?

For most people, the simplest way to check whether you’re drinking enough water is to check your wee. “The best advice is to keep the urine pale yellow and be guided by thirst,” says Benenden Hospital’s consultant urologist Steve Garnett. He adds: “There is no set amount to drink as it will vary according to a person's size and activity. A small 90-year-old woman doesn't need to drink as much a large rugby player running around in hot weather.”
Conversely, signs of dehydration include dizziness or light-headedness, headaches or tiredness, and a dry mouth, lips and eyes. Additionally, if you only pass a little urine and infrequently (less often than three or four times a day) then you may not be drinking enough or have an underlying health issue that needs to be explored.

Is there anyone who should take special measures to drink more?

Some people need to make a concerted effort to ensure they’re drinking enough, such as the elderly, who may not have noticed that they’re becoming dehydrated. Babies and infants can quickly become dehydrated during illness or in intense heat due to their low body weight. Pregnant or breastfeeding women need to ensure they’re drinking at sufficient levels, as do people who’ve had an illness resulting in vomiting or diarrhoea, or with other conditions such as a bladder infection. If the weather is particularly hot or you’ve been sweating a lot after exercise or manual work then your water and liquid requirement will increase too.

Is it possible to drink too much water?

Yes. Drinking too much water can lead to a condition called hyponatraemia, which can be fatal. Those who drink too much can develop the condition when their blood sodium levels fall too low. This can happen, for example, when athletes drink very large quantities of water after endurance sport, without replacing the sodium lost through perspiration.


For more information on current NHS guidelines see: http://www.nhs.uk/Livewell/Goodfood/Pages/water-drinks.aspx

Sources:
1. BMJ http://www.bmj.com/content/335/7633/1288