There is a popular belief that drinking lots of water is good for one's health. One classic recommendation from doctors to patients--nobody knows where it comes from--is to drink 8 glasses of 8 ounces of liquid beverages daily.
Is this really true? Certainly we are confident that drinking copious amounts of water is a good thing for those who have a history of nephrolithiasis, but are there any medical conditions for which this is true? A review of the literature in a recent issue of JASN by Negoianu and Goldfarb concludes that there is no proven benefit--or at least, there is a dearth of clinical evidence looking at how much water is optimal to prevent various health problems.
What about phrasing the question differently: How much water is too much water? Due to our amazing kidney's impressive power to dilute the urine (in health young folks up to 50 mosm/kg/L), you have to try pretty hard to became hyponatremic as a result of excess free water intake. In 1998, a 28-year-old woman entered in a California radio contest in which individuals were judged based on their ability to ingest large quantities of water without urinating--termed "Hold your wee for a [Nintendo] Wii"--was found dead after ingesting several liters of water over a short period of time.