How much should we drink? Do we really need 8 glasses of water a day?

Image: TF28 via Flickr
Image: TF28 via Flickr

What prevents you from collapsing into a pile of goo on the floor? As you read this, you are supported, upright in front of your screen, by a rigid scaffold of bones, and a network of fibrous muscles and tendons. However, you’re not as solid as you think. Your body is mainly water (~65%), and the cells in your body are bathed in a watery environment.

It may seem strange that so much of us is water, but all this liquid is essential to maintain our normal bodily functions; practically every life-giving chemical reaction within our body occurs in water. Too much or too little water, however, and we can become very ill. Thankfully, drinking sensible amounts is normally straightforward. It only really gets confusing when we start worrying about counting glasses.

How much to drink? 8 glasses?

Like a leaky bucket, our bodies lose water through several routes. Dr Doyle from the British Nutrition Foundation estimates that adults lose about 3 litres of fluid each day – 1-1.5 litres as urine, 0.5-1 litres as sweat, 300 ml in our faeces, and 400 ml in our breath. To keep our buckets topped up (so to speak), we take in liquid in our diet.

3 litres a day sounds like a lot, but we don’t have to drink all of that, because our body extracts a surprising amount of our daily water intake from food (around 1 litre). Most foods naturally contain lots of water – fruits and vegetables are chock full of the stuff. (Believe it or not, there is more water in lettuce than there is in orange juice – 95% vs 88%!) Even something seemingly dry like bread has a lot of moisture in (about 40%). Breaking down and ‘burning’ the food that we eat within the body also generates water – in much the same way that burning petrol in a car produces water (you often see it dripping out of the exhaust first thing in the morning). Through a chemical process called respiration, another 300-500 mls of water is generated in the body per day.

Based on these estimates, that leaves just 1.5 litres to be replaced by drink. And if you eat a lot of fruit and veg, then it may be significantly less than that.

fluid in with labelsfluid out with labels

The ‘8 glasses a day’ myth and ‘Big Water’

A common mantra is to drink “eight glasses of water a day” – about 2.5 litres. It is an idea that is still bandied about a lot today, but has fallen out of favour with many experts who claim it has very little scientific grounding. Critics say it is a myth peddled by ‘Big Water’ – the bottled water industry. Hydration for Health, for example, is an organisation that promotes drinking more water that was created by and is financed by Danone – the company that bottles and sells Evian and Volvic water.

Contrary to what sites such as Hydration for Health may imply, water is not harmless and over-hydration is potentially problematic. Drinking too much water dilutes the salts (‘electrolytes’) in the blood, potentially leading to hyponatraemia and – in extreme circumstances – death. Claims of improved skin tone, reduced risk of urinary tract infections, and other purported health benefits of drinking extra water have also been criticised as having very little science to back them up.

The truth is that our bodies are remarkably good at maintaining the balance of water. The colour of our urine is a good example of this: if we are dehydrated, our kidneys produce less urine, resulting in more concentrated, darker wee. Healthy adults are encouraged to allow their thirst, rather than the ‘eight glasses’ rule, to guide them on when to drink. Extra attention is needed for children and the elderly who may be less mindful of their thirst. Similarly, people who have kidney or heart problems may also need to follow strict fluid regimens.

Article by Michael Mckenna

April 23, 2015

Mike is currently doing a PhD in biochemistry at the University of Manchester. When not talking about proteins, he watches an obscene amount of films and enjoys the odd pub quiz.

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