By Rebecca Voelker
The belief that caffeinated drinks such as coffee could cause dehydration is based on a 1928 study that demonstrated caffeine’s diuretic effect. Since then, only 2 studies have tried to show whether evidence exists to support that belief. Results were mixed, so investigators at the University of Birmingham in England devised new research to compare whether drinking coffee affected hydration differently than water consumption.
“Our research aimed to establish if regular coffee consumption, under normal living conditions, is detrimental to the drinker’s hydration status,” lead author Sophie Killer, a doctoral researcher, said in a statement. The study was published online today in the journal PLOS ONE.
Killer and her colleagues enrolled 50 men, all moderate coffee drinkers who didn’t take diuretics or caffeine-containing medication. Women weren’t included in the study because menstrual cycles may cause fluid balance fluctuations. In the study’s first phase, investigators randomly assigned the men to drink 4 cups of black coffee or an equal amount of water daily for 3 consecutive days. After a 10-day “wash-out” period, the groups switched. Coffee drinkers changed to water and vice versa.
The investigators analyzed hydration status with several established measures—body mass, total body water, and blood and urine tests. They found the hydration effects of coffee or water did not differ significantly. The study participants lost a small but significant amount of body mass each day during both study phases, 0.2%. Several factors may explain the body mass loss, the investigators wrote. One possibility is that the men simply didn’t drink enough fluids during the study. Even so, the men weren’t near the clinical dehydration level of 1% to 3% body mass loss, the investigators noted.
“Consumption of a moderate intake of coffee, 4 cups per day, in regular coffee-drinking males caused no significant difference across a wide range of hydration indicators compared to the consumption of equal amounts of water,” Killer said.
She and her colleagues noted that public health recommendations to exclude caffeinated beverages from daily fluid needs or to drink a glass of water for every cup of coffee or tea consumed “should be updated to reflect [our] findings.”