Additionally, the idea that one must specifically drink water because the diuretic effects of caffeinated drinks such as coffee, tea, and soda actually produce a net loss of fluid appears to be erroneous. The average person retains about half to two-thirds the amount of fluid taken in by consuming these types of beverages, and those who regularly consume caffeinated drinks retain even more:
Regular coffee and tea drinkers become accustomed to caffeine and lose little, if any, fluid. In a study published in the October issue of the Journal of the American College of Nutrition, researchers at the Center for Human Nutrition in Omaha measured how different combinations of water, coffee and caffeinated sodas affected the hydration status of 18 healthy adults who drink caffeinated beverages routinely.
"We found no significant differences at all," says nutritionist Ann Grandjean, the study's lead author. "The purpose of the study was to find out if caffeine is dehydrating in healthy people who are drinking normal amounts of it. It is not."
The same goes for tea, juice, milk and caffeinated sodas: One glass provides about the same amount of hydrating fluid as a glass of water. The only common drinks that produce a net loss of fluids are those containing alcohol — and usually it takes more than one of those to cause noticeable dehydration, doctors say.
The best general advice (keeping in mind that there are always exceptions) is to rely upon your normal senses. If you feel thirsty, drink; if you don't feel thirsty, don't drink unless you want to. The exhortation that we all need to satisfy an arbitrarily rigid rule about how much water we must drink every day was aptly skewered in a letter by a Los Angeles Times reader:
Although not trained in medicine or nutrition, I intuitively knew that the advice to drink eight glasses of water per day was nonsense. The advice fully meets three important criteria for being an American health urban legend: excess, public virtue, and the search for a cheap "magic bullet."
