This article originally ran in the San Francisco Chronicle’s health section on November 14, 2013.
Myth: Caffeine causes dehydration.
Fact: For years, doctors cautioned patients, especially athletes, not to drink caffeine before a workout because, they said, it caused the body to produce more urine, increasing the risk of dehydration. A review of studies on the subject, however, show that moderate coffee and soda drinking habits won’t put anyone in harm’s way.
Many believe that caffeine is a strong diuretic – a substance that prompts the body to decrease its fluid levels through urination. Most liquids, including water, have diuretic properties, but some are stronger than others. The idea that caffeine is a strong diuretic probably stems from a 1928 study, which involved only three participants.
Researchers have tested the theory over the years. In 2002, researchers at the University of Connecticut looked at 10 studies that compared the urine output of people who drank various amounts of caffeinated beverages against subjects who drank the equivalent amounts of water or other non-caffeinated drinks. They found the urine outputs were very similar – participants who drank a caffeinated beverage retained zero to 84 percent of the initial volume consumed, while those who consumed water retained zero to 81 percent.
In 2007, the UConn scientists reviewed studies involving patients who drank two to four cups of caffeinated coffee or five to 13 cans of caffeinated soda. Only about a quarter of the experiments showed that caffeine drinkers lost more fluid than the water drinkers. The rest showed no significant difference between the two groups.