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New Study Debunks Myth That Caffeine Causes Dehydration
Released: June 30, 2005

Release #05051
Larry Armstrong
(860) 486-2647 (office)

Beth Krane (
860) 486-4656
(UConn Media Relations)

STORRS, Conn.— For decades, health and exercise experts have cautioned that caffeine causes dehydration – despite a dearth of scientific evidence to support that notion. A new study conducted at the University of Connecticut’s Human Performance Laboratory, however, shows that consuming the equivalent of two to four cups of coffee a day causes no more dehydration than drinking water.

“Think about how much caffeine Americans consume. If the myth is true, why aren’t hospitals filled with severely dehydrated people?” asked Larry Armstrong, a professor of exercise science and environmental physiology in UConn’s Neag School of Education, who led the study.

Although several studies have shown that caffeine acts as a mild diuretic over the course of one to three hours, none of them yielded any evidence that caffeine consumption coupled with exercise produces chronic dehydration, as the medical establishment has warned, Armstrong said.

“A short-term diuretic effect does not equal dehydration,” Armstrong explained.   

Armstrong’s study is the first laboratory investigation of caffeine consumption and hydration status, electrolyte balance and renal function to last longer than 24 hours.

Researchers at UConn’s Human Performance Lab gave 59 moderately active men between the ages of 18 and 34 controlled amounts of caffeine for 11 consecutive days. All of the men ingested capsules with 226 milligrams of caffeine – the equivalent of two cups of coffee – daily for the first six days of the study.

For the final five days, one third of the group was given placebos or pills without any caffeine; one third of the men maintained the same level of caffeine (226 milligrams daily) and the remaining third took capsules with 452 milligrams of caffeine – or the equivalent of four cups of coffee – daily.

Among Armstrong’s findings:                 

  • Men in all three groups showed similar levels of hydration for the entire duration of the study (20 measures of hydration status were used);
  • There were no discernable differences in whole-body electrolyte balance (sodium and potassium) and renal function in any of the men, based on diet intake records and excreted electrolyte measurements; and
  • All of the men maintained their regular regimens of moderate exercise, including running, biking and resistance training, during the 11-day study and still none of them showed increased signs of dehydration, electrolyte imbalance or impaired renal function.

“These findings show that the health and performance of athletes and active adults will not be impaired if they consume caffeine or caffeinated beverages in moderation,” Armstrong said. “This is contrary to the advice of most nutritionists, athletic trainers and physicians.”

 Armstrong, an avid runner and well-respected scientist who has been conducting fluid balance and body temperature regulation research for 25 years, began studying caffeine consumption and hydration status after observing that his own experiences did not conform to conventional wisdom on the subject.

In 2002, he published a research review study that concluded no scientific evidence existed to suggest that moderate caffeine consumption contributed to chronic dehydration.

Armstrong’s study, funded by the Washington-based International Life Sciences Institute, used a controlled, randomized, double blind design.

 His findings were published in the International Journal of Sport Nutrition and Exercise Metabolism earlier this month.

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