CaffeineAmericans have long accepted the idea that caffeine acts as a diuretic, causing fluid excretion to exceed fluid retention. The New York Times: Health published an article yesterday (March 4th) that suggests otherwise, citing several studies conducted in the past few years. According to the report,

… research has not confirmed that notion. Most studies have found that in moderate amounts, caffeine has only mild diuretic effects — much like water. … Investigations comparing caffeine with water or placebo seldom found a statistical difference in urine volume

While a cup of coffee might be a fine way to start your morning, remember to consume a total of 2-3 liters of fluids each day to remain truly hydrated. Be well and drink up!

In my own investigation, I found these scholarly studies supporting this claim:
(1) Caffeine, Fluid-Electrolyte Balance, Temperature Regulation, and Exercise-Heat Tolerance. Exercise and Sport Sciences Reviews. 2007. [full article - limited access]
(2) Caffeine ingestion and fluid balance: a review. Journal of Human Nutrition and Dietetics. 2003.