Moderate amounts of coffee do not cause dehydration, confirms a recent British study in PLOS ONE. Because caffeine is a mild diuretic, many people assume that caffeinated beverages increase urination and cause a net loss of fluid from the body. Not true.
In the new study, 50 young men, all habitual coffee drinkers, consumed four cups of either black coffee or water a day for three days (the rest of the diet was controlled); after ten days they switched beverages for another three days. The researchers found no differences in body water, urination or other measures of hydration between the periods when the men drank coffee and when they drank water.
This bears out what the Institute of Medicine said in 2005: “Caffeinated beverages appear to contribute to the daily total water intake similar to that contributed by noncaffeinated beverages.”