Under the Microscope: I look up at the tiered rows of young nursing students whom I am about to delight with a lecture on introductory biochemistry.
I am struck by one particular thing - almost every student has a plastic bottle of water. I ask why everyone is carrying a bottle of water. The general reply is - "It's good for your health to drink plenty of water". I reply "Okay, but why not drink a glass in the morning, a glass at lunch and another at dinner, rather than hauling a bottle around and sipping all day, and, why not fill your bottles with free tap-water?" I can't decipher any clear answer. Next day, another lecture with the same class, and I look up to see the forest of plastic bottles again. Do we need all this water?
Water is essential for life. It comprises about two thirds of the weight of the adult body and the basic metabolism in the trillions of cells in the body takes place in water. If you stop eating food you will live for many weeks, but if you stop taking in water you will die in days. Fortunately, it is easy for us to satisfy our need for water.
We take water into the body when we drink water, tea, coffee, soft drinks, milk, juice and so on, and when we eat solid food. We lose water from the body when we urinate, sweat, exhale and defecate. In order to maintain water balance we need to take in as much water as we lose.
How do we take in water when we eat solid food? Well, most of the food we eat - meat, fish, fruit and vegetables - is about 70 per cent water by weight. Also, we break down food in our cells to produce energy (cellular oxidation). Water is an end product of this breakdown.
On average every day we lose water from the body in the following volumes: urine (1500ml), insensible loss through lungs and skin (900ml), faeces (100ml) - total (2.5l). Of course, if you engage in prolonged heavy exercise you could lose up to three times this volume of water.
On average also every day we should take in 2.5l water to balance this loss, broken down as follows: water/beverages (1500ml), solid food (700ml), cellular oxidation (300ml). This intake maintains water balance.
The thirst mechanism ensures that we do not dehydrate. Thirst should kick in when body water is depleted by 1-2 per cent. However, thirst often does not develop until body water is significantly depleted below optimum levels. The groups most vulnerable to dehydration are infants, elderly people and athletes. Loss of 10 per cent body water impairs work performance, and will probably cause nausea, weakness and hypertension. Water loss exceeding 20 per cent of body weight is life threatening.
Caffeinated drinks are usually not counted when reckoning the 2.5l of water we should ingest each day. However, this is wrong according to research published by Grandjean and others in the Journal of the American College of Nutrition, Vol. 19(5), pp. 591-600 (2000). They measured how different combinations of water, coffee and caffeinated soft drinks affect the hydration states of healthy adults who regularly drank caffeinated drinks. The authors found no significant differences in the effect of various combinations of beverages on hydration states of the subjects studied. Caffeine is not dehydrating in healthy people who drink normal amounts of it. The only common drinks that produce a net loss of water are alcoholic drinks.
One reason why drinking plenty of water is good for you is that it "flushes out the system", assisting the body to rid itself of toxic chemicals. UCC colleague Prof James Heffron, who lectures and carries out research on the biochemistry of detoxification comments: "The human liver contains an exquisite system of some 500-600 enzymes which reduce the toxicity of any natural or synthetic chemicals in our diet, or taken in through the lungs, while simultaneously making them more soluble in water thus preparing them for excretion by the kidney. This process is known as hepatic detoxification."
There is no good evidence that drinking lots of water curbs the appetite. A litre of water will fill the stomach, but, once it moves on you tend to eat as much as you would have eaten without drinking the water. The best way to get water in so that it stays with us for a while is to ingest it in a food matrix, ie in soup, fruits and vegetables. Some fruits and vegetables are nearly 100 per cent water, eg cucumbers and watermelons.
What's the difference between bottled water and tap water? Answer: about €1 a 330ml bottle. There is no evidence that bottled water is superior health-wise to tap water. In fact, tap water by law usually has to meet more stringent quality criteria than bottled water. The great popularity of bottled water is visible proof that the country is awash with money.
Drinking too much water is dangerous for some people, eg diabetics who are taking anti-diuretic hormone to prevent the body from losing water. The excess water cannot be excreted, resulting in water intoxication with symptoms that range from headache to seizures, coma and in extreme circumstances, death.
So, in summary, it is a good idea to drink plenty of water. Several glasses of tap water plus other drinks per day is entirely adequate and our attitude to bottled water is Evian spelled backwards.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC - http://understandingscience.ucc.ie