8 Glasses a day?

Do I really need to drink eight glasses of water a day?

The short answer no.

The myth we need to drink eight glasses of water a day has been around for a long time now and is actually based on part of a report by the Food and Nutrition Board of the National Research Council in 1945, where it recommended 2.5 litres of water as a “suitable allowance” of water for most adults. They, however, pointed out that “most of this quantity is contained in prepared foods,” but it seems that people interpreted this to mean that 2.5 litres of water is the right amount to drink each day. The advice was repeated again in 1948, without a scientific backing. The upshot is that we are now told to drink eight glasses of water.

The human body is very good at regulating itself and will let you know when it needs more fluids by making you thirsty or hungry. The best way to gauge if you are drinking enough is to pay attention to the colour of your pee. If you are dehydrated then your urine is darker.

Do I have to drink water?  

The short answer no. 

Water is a good thing to drink, but so is fruit juice, fruit squash, tea, coffee, pop, wine and beer. The thing to bear in mind that everything except water contains calories. Some research indicates that caffeine and alcohol can make some individuals urinate more frequently. This has led to claims that we can't include tea, coffee, pop and booze as part of our daily fluid intake. What this really means is that you should be aware that this may effect you and to drink more as a result. If you pee regularly and its pale then keep up the good work.

The reason this myth is still so prevalent, might have something to do with the bottled water industry. In a recent study by the university of Nottingham, it is estimated that the UK bottled water industry is worth over 2 billion pounds.

In conclusion, drink whatever satisfies your thirst, whilst being aware of the calories you are drinking and pay attention to your pee. 

The Science:
Food and Nutrition Board, National Academy of Sciences. Recommended Dietary Allowances, revised 1945. National Research Council, Reprint and Circular Series, No. 122, 1945 (Aug), p. 3-18
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Refuting the Aqua-Dogma: An Interview with Dr. Heinz Valtin