Because caffeine is a mild diuretic, there is a common assumption that caffeinated beverages, such as coffee, also have this effect.

The problem is that a kernel of scientific knowledge can be misconstrued in news outlets. As we discussed on Thanksgiving, everything in a Thanksgiving dinner contains chemicals found by someone somewhere to be a carcinogen in rats and could therefore be banned if they did not occur naturally. 

The dose matters, though telling that to people who insist they are allergic to genetically modified corn is probably a waste of time. Conflating pure caffeine with coffee is also difficult but if a researcher wants to find something, they can. Impartial investigations about the effects of caffeine in the form of coffee on hydration status have been inconclusive. 

A new paper finds no evidence for a link between coffee consumption and dehydration - at least when it comes to normal coffee consumption. Coffee is mostly water so a moderate amount of coffee not only doesn't result in dehydration,  it contributes to daily fluid requirements just as other fluids do. 

Mean total body water estimates from Day 1–Day 3. n = 25. 
doi:10.1371/journal.pone.0084154

The authors say this is the first study to directly assess the effects of coffee consumption compared to equal volumes of water. Sophie Killer, a  University of Birmingham School of Sport and Exercise Sciences researcher and lead author of the study commented, "Despite a lack of scientific evidence, it is a common belief that coffee consumption can lead to dehydration and should be avoided, or reduced, in order to maintain a healthy fluid balance. Our research aimed to establish if regular coffee consumption, under normal living conditions, is detrimental to the drinker's hydration status."

In a sample of regular coffee drinkers, the authors measured the effects of moderate consumption of black coffee compared to the consumption of equal volumes of water on fluid balance and hydration status. Fifty male participants were tested in two phases, where they were required to drink four mugs (200ml) of either black coffee or water per day for three days. In the second phase, those who had initially drunk coffee switched to water and vice versa.

The two phases were separated by a ten day 'wash out' period. Females were excluded from the trial to control against possible fluctuations in fluid balance resulting from menstrual cycles.

To assess hydration status, the researchers used a variety of well-established hydration measures including body mass and total body water, as well as blood and urine analyses. The researchers found no significant differences in total body water or any of the blood measures of hydration status between those who drank coffee and those who drank water. Furthermore, no differences in 24-hour urine volume or urine concentration were observed between the two groups.

"We found that consumption of a moderate intake of coffee, four cups per day, in regular coffee drinking males, caused no significant differences across a wide range of hydration indicators compared to the consumption of equal amounts of water," said Killer. "We conclude that advice provided in the public health domain, regarding coffee and dehydration, should be updated to reflect these findings."