Does Drinking Coffee Really Make You Dehydrated?

It is commonly thought that drinking coffee makes you more dehydrated, but does the evidence support this claim?

Caffeine is one of the most widely consumed substances in the world, and is found in coffee, tea, and chocolate. Many people, including both health care practitioners and the public, believe that caffeine increased urine output leading to dehydration. 

However, a number of studies have looked at the question in close detail. In one study from 2014, researchers looked at 50 male coffee drinkers habitually consuming 3–6 cups per day. The participants consumed either four 200mL cups of coffee containing 4mg/kg caffeine or the equivalent amount of water for three days. The results showed no change in total body water, 24-hour urine output, or blood markers of hydration between the coffee and water drinkers. ​

Other studies tell a different story. In a study from 1997, twelve regular coffee drinkers abstained from coffee consumption for 5 days, and then drank 6 cups of coffee. This led to an increase in 24-hour urinary output and a decrease in total body water, as well as increased excretion of sodium and potassium. 
Another study found that coffee with high caffeine content (6mg/kg) caused an acute diuretic effect, but low caffeine coffee (3mg/kg) did not. 

Lastly, a literature review from 2003 concluded that "acute ingestion of caffeine in large doses results in a short-term stimulation of urine output in individuals who have been deprived of caffeine for a period of days or weeks. A profound tolerance to the diuretic and other effects of caffeine develops, however, and the actions are much diminished in individuals who regularly consume tea or coffee. Doses of caffeine equivalent to the amount normally found in standard servings of tea, coffee and carbonated soft drinks appear to have no diuretic action."

Take Home Message

The available evidence suggests that high caffeine consumption (3-6 cups of coffee) causes an acute increase in urinary output, but the effect is diminished in habitual coffee drinkers. 
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Bottom Line: If you drink coffee regularly, it is not likely to cause dehydration.