The idea that an adult needs to drink eight glasses of water a day is one of the more persistent health concepts you'll encounter. However, as we've pointed out before, it's almost entirely bogus.
On analysis site the Conversation, Deakin university nutrition professor Tim Crowe thoroughly debunks this idea, pointing out that the amount of water each individual needs will vary depending on "body size, physical activity levels, climate and what types of food [you]are eating" He also emphasises something that lots-of-water proponents often miss: the water that you get from food and from other drinks does contribute to your daily liquid needs. In this context, coffee is nowhere as evil as many people suggest:
The “coffee makes you dehydrated” mantra is another myth that needs to be busted. Drinks such as coffee, tea and cola do have a mild diuretic effect from the caffeine but the water loss caused by this is far less than the amount of fluid consumed in the drink in the first place.
None of which means that drinking water isn't a good idea, simply that setting an arbitrary target doesn't make sense. As Crowe points out, the colour of your urine remains the simplest indicator of whether you're getting enough liquid. If it's dark, you need more. Hit the link for all the details.
Monday’s medical myth: drink eight glasses of water a day [The Conversation]