New study suggests coffee is not dehydrating after all

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I am a regular coffee drinker. In fact, I honestly cannot remember the last time I went a day without coffee. I have loved the taste of coffee since childhood, and I expect I do get something from the caffeine too. I appreciate that coffee-drinking does not have a particularly healthy reputation, though there is at least some evidence which links coffee consumption with improved health outcomes, including a reduced risk of type 2 diabetes. While these ‘epidemiological’ studies do not prove that coffee drinking is healthy (they only show that coffee-drinking is associated with improved outcomes), coffee is rich in ‘antioxidant’ substances (for example, polyphenols) that, at least in theory, might have disease-protective effects.

One common belief about coffee is that it dehydrates the body. Caffeine can act as a diuretic stimulate urine production). I, personally, have never worried about this because (even though I say so myself) my hydration status is usually good. I am pretty good about drinking water, and usually will drink enough to ensure that my urine remains yellow throughout the course of the day. In my head, I reason that as long as I ‘balance’ coffee with water, then dehydration is not a particular risk.

A study just out suggests that habitual coffee drinkers have little to worry about regarding the potential dehydrating effects of coffee. In this study, 50 men who are regular coffee-consumers ate and drank a controlled diet for three days on two occasions. On one occasion, the prescribed diet was supplemented with four, 200 ml glasses of water each day. On the other occasion, this was replaced with four, 200 ml cups of coffee, containing a total of 4 mg of caffeine per kg of body weight. The total amount of caffeine consumed was about 320 mg per day for each man (about the same amount of caffeine that would be found in four cups of instant coffee).

The men underwent a range of tests before and after each 3-day spell, including markers of hydration in the blood and urine and urine volume. The results were essentially the same, which led the authors to conclude that hydration status was unaffected by moderate coffee and caffeine consumption compared with water. In other words, in these men, coffee was essentially as hydrating as water. This is generally good news for regular coffee-consumers like me.


1. Killer SC, et al. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS ONE 2014;9(1):e84154

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