Tea and other caffeinated beverages purportedly lead to dehydration because they can have a diuretic effect. Some studies show that caffeinated drinks such as tea are just as hydrating as water when consumed in moderate amounts. Caffeine may dehydrate you if you consume 500 mg or more per day.
Caffeine is a stimulant that occurs naturally in tea, but also in coffee, cocoa beans and coffee-flavored ice creams and frozen and regular yogurts, according to the Center for Science in the Public Interest, or CSPI. Caffeine is added to many soft drinks, such as carbonated colas, and chewing gum, energy mints and even some over-the-counter pain relievers. Caffeine acts on your central nervous system to make you feel more energetic and wakeful. It can increase your ability to concentrate. Caffeinated drinks can hydrate you just as well as water, as long as you aren't a heavy caffeine user. Moderate caffeine use — between 200 and 300 mg a day — probably isn't harmful to most healthy adults.
A cup of 8-oz. black tea can has 53 mg of caffeine on average, but may contain between 40 and 120 mg of caffeine, depending on the strength of the brew. A 16-oz. serving of bottled tea, mixed tea and fruit drinks and tea drinks sold at coffee shops can have between 10 and 100 mg of caffeine. Even decaffeinated tea has some caffeine — between 2 and 10 mg per cup. Comparatively, an 8-oz. cup of brewed, black coffee has 102 to 200 mg of caffeine, depending on how strong it is, but has an average caffeine content of 133 mg. The average 12-oz. cola has around 54 mg of caffeine.
A study published in the October 2000 issue of the "Journal of the American College of Nutrition" reported the effects of caffeinated and non-caffeinated beverages on hydration. Researchers at The Center for Human Nutrition in Omaha studied 18 adult men who consumed a variety of caffeinated and decaffeinated caloric and noncaloric drinks. Researchers concluded that the results of their study "found no significant differences in the effect of various combinations of beverages on hydration status of healthy adult males" and further indicated that there was no reason to advise people to refrain from consuming caffeinated drinks. Zeratsky indicates that caffeine may act as a diuretic when you consume more than 500 to 600 mg of caffeine a day, or between 9 to 11 or more cups of brewed black tea.
Tea might not dehydrate you if you drink it in moderate quantities. However, caffeine has other side effects that you may wish to avoid, including anxiety, irritability, rapid heartbeat, upset stomach, shakiness and difficulty sleeping at night. These symptoms typically affect heavy caffeine users who consume 500 to 600 mg of caffeine a day or more, but some people are more sensitive to the effects of caffeine than others. If you're sensitive to caffeine, one cup of tea or coffee may be enough to give you the caffeine jitters. If you're concerned about proper hydration, your best choice is still water, Zeratsky states.