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The British Medical Journal (BMJ) tackles medical myths in its current issue, including one about the quantity of water we should be drinking.

It’s impossible to find an authority for the claim, says the BMJ article, but the evidence for the need for consumption of fluids has been documented in physiological textbooks for some time. 

Little has been added to the basic physiological information since the classic studies of Passmore in 1955.  Studies on sodium and thirst summarised in Fitzsimons’ monograph on the subject in 1979 draw the same conclusions. 

Just because these are older studies doesn’t mean that they are wrong. In fact, they are particularly elegant and detailed studies that modern techniques could add little additional information to. What's more, this evidence forms the basis of the Agency’s advice to drink 6-8 glasses (about 1.2 litres) of fluid a day.

We have an elaborate array of mechanisms that make us eat and drink, not only to provide the pleasure it produces but to satisfy sensations of hunger and/or thirst. 

Based on experiments on water turnover, the average male has a total excretion of water of 2.5 litres. He gets 1.3 litres from ingested food and metabolic activity (the everyday chemical processes that occur in cells every day utilising glucose via the Krebs cycle, postulated by our first Chair's father in 1937, whereby glucose plus oxygen is converted into energy, carbon dioxide and water). So we need to make up the difference of 1.2 litres from drinking.  Depending upon the size of your glass, you can therefore calculate how many glasses of fluid you need to take in to satisfy the shortfall.

For example, some of us have 150ml drinking glasses, others have 200ml ones, while others are 250 ml. On average, you'd need 6-8 glasses to get 1.2 litres of fluid to make up the difference. So next time someone tries to tell you that it’s a myth we need to drink so much, you’ll know the truth. 

As the BMJ article states, this fluid doesn’t have to be water.  But, unfortunately, I don’t advise that you take the fluid in the form of alcohol (you probably wouldn’t want to anyway, after the excesses of the Christmas season), but because alcohol also tends to dehydrate you, you’ll have to drink even more fluid of the non-alcoholic variety.