I enjoyed this article about the myth of water intake: You Don’t Need 8 Glasses Of Water A Day. This idea has been floating around for decades: you must drink more water – specifically eight glasses per day.
This threshold appears to be a long-standing medical myth. It’s not even clear where it started. The best answer I can find[…] is that the source was a 1945 publication by the National Food and Nutrition Board, a government advisory agency, that stated this: “A suitable allowance of water for adults is 2.5 liters daily in most instances. … Most of this quantity is contained in prepared foods.” The theory is that people read this, ignored the last sentence, and the eight glasses a day (about 2.5 liters)recommendation was born.
The article goes on to cite study after study which found no effect of drinking more water. A few studies suggest some value, but the threshold is much lower than eight glasses, and it’s a level most people just get accidentally.
A meta-study at Dartmouth found the same thing: "Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8x8”? (PDF; the “8x8” in the title refers to the common exhortation to consume eight 8-oz. glasses of water per day)
No scientific studies were found in support of 8 x8.
I’ve always thought this was silly. I’ve never even been close to that amount in pure water (though I used to drink eight cans of Diet Coke a day), and I’ve been fine. The idea that we’re all walking around chronically dehydrated is kind of comical, really.
How about increasing fluid intake to get over a cold? Nope. Consider: "Drink plenty of fluids”: a systematic review of evidence for this recommendation in acute respiratory infections
We found data to suggest that giving increased fluids to patients with respiratory infections may cause harm. To date there are no randomised controlled trials to provide definitive evidence […]
All is not lost, however. There may be some value in protecting the kidneys from infection: Fluid and nutrient intake and risk of chronic kidney disease
Higher intakes of fluid appear to protect against CKD. CKD may be preventable at a population level with low-cost increased fluid intake.
But what’s important to note is that we get a lot of fluids in solid food. An Australian study (What drove us to drink 2 litres of water per day?; sadly, behind a paywall) indicated that we get a lot of fluids accidentally. A baked potato, for instance, is apparently 75% water. Women get 2.4 litres of water and men get 3.2 litres, just from eating normally.