"Coffee is 'as hydrating' as drinking water," is the claim in the Daily Express. It reports on a new study suggesting that moderate coffee consumption does not dehydrate the body, as some had previously thought...
“Coffee is 'as hydrating' as drinking water,” is the claim in the Daily Express. It reports on a new study suggesting that moderate coffee consumption does not dehydrate the body, as some had previously thought.
The research behind the headline was a small experimental study including 50 healthy male volunteers who were used to drinking three to six cups of coffee a day. On two separate occasions, each man drank either four cups of coffee a day for three days, or drank an equivalent amount of water for three days.
They compared the body’s overall hydration levels using blood samples and looking at urine output. They found no significant difference in measures of hydration between those drinking water and those drinking coffee.
However, there are important limitations to this research. The study participants were a small group of healthy men, all of whom were used to drinking the amount of coffee tested. So they are not necessarily representative of other groups.
In particular, the research may not apply to people who are at risk of dehydration, such as those with diarrhoea and vomiting, or health conditions such as kidney disease. This may also apply to those taking medications that affect their body's ability to regulate fluids.
The study also only looked at the hydrating effects of coffee over a short period (three days) so does not provide information on any longer term effects.
So it cannot be concluded from this study alone that coffee is broadly as hydrating as water. And unlike water, coffee can cause side effects such as insomnia and irritability.
Where did the story come from?
The study was carried out by researchers from the School of Sport and Exercise Sciences, University of Birmingham, and was funded by the Institute for Scientific Information on Coffee (ISIC).
The ISIC website describes the organisation as non-profit and devoted to the study and disclosure of science related to coffee and health. Its members are seven major European coffee companies representing a potential conflict of interest. However, the publication states “the funders [ISIC] had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.”
The study was published in the peer reviewed open access medical journal Plos One and can be read online or downloaded for free.
Overall both the Express and the Daily Mail failed to consider the limitations of this research when interpreting the results and considering their implications.
What kind of research was this?
This was an experimental study that used a crossover design to directly compare the effects of coffee consumption and water consumption upon hydration in a group of men.
The researchers explain how it is often suggested that coffee causes dehydration and coffee consumption should be avoided or significantly reduced to maintain fluid balance.
They also report that previous research has suggested there is wide variation in the amount of fluid that adults drink each day, ranging from half a litre to over four litres per day.
This suggests there is no clear consensus on the correct amount of fluid to consume. Meanwhile published guidelines are said to vary in recommended fluid intake from 1.5l/day to 3.7l/day in men. The researchers point out that some guidelines say caffeinated beverages should not be included in daily fluid requirement guidelines, and that a glass of water should be consumed with every cup of coffee or tea.
Given the uncertainty around the amount of fluid needed, and whether coffee should be included or excluded, the researchers set out to answer the question of whether coffee did indeed have a dehydrating effect compared to water.
What did the research involve?
The study included 50 healthy men (aged 18 to 46) with stable weight, diet and fluid intake, selected from a potential screened group of 100. Women were said to be excluded due to possible fluid balance changes caused by the menstrual cycle. All participants were described as moderate coffee drinkers consuming between three and six cups per day (300 to 600mg/day caffeine) as assessed by a three-day food diary.
Then on two separate occasions, each man drank four, 200ml cups of black coffee per day for three days (providing 4mg/kg of caffeine per day), or three cups of water per day for four days. During each trial period the participants drank a regulated amount of water provided in bottles, the amount of which was determined for each of them based on their three-day food diary. Therefore during the water-only trial, they were drinking four extra cups of water a day. During each trial period the participants also did no physical activity, did not drink alcohol and ate a controlled diet as provided by the researchers. This was to try to limit the effects of these factors on their overall hydration.
The two trial periods were separated by a 10 day wash-out period when the person resumed their normal caffeine intake, diet and activity.
Before and after each trial, the researchers measured total body water. Every day they also recorded body mass and blood and urinary markers of hydration (such as sodium, potassium and creatinine levels). During the coffee trial, blood was also analysed for caffeine levels to confirm compliance.
What were the basic results?
The researchers found no significant difference in total body water from before and after each trial. There was also no significant difference in total body weight between the two trials.
Neither were there any differences between the two trials in blood markers or urinary markers of hydration, or 24 hour urine volume.
Urinary sodium levels were found to be higher during the coffee days. The authors say that previous research has suggested that caffeine increases kidney excretion of sodium.
However, the researchers found no difference in other measures of hydration or urine output.
There were no significant differences in body mass between the two trials, though there was a small gradual daily fall in body mass in both trials.
How did the researchers interpret the results?
The researchers say their findings of no significant difference in blood and urinary markers of hydration status between trials “suggest that coffee, when consumed in moderation by caffeine habituated males[,] provides similar hydrating qualities to water”.
There are several important points to bear in mind when interpreting the meaning of this research:
- The trial is very small, including a specific group of only 50 healthy men who were all used to drinking three to six cups of coffee per day. In this study, the men drank the same amount of coffee as they were used to drinking. It could be that their bodies have adapted to this level of caffeine over time and it therefore has less effect on hydration. Different findings may be seen in another group of men – particularly those not used to drinking coffee.
- Different findings could also be seen in women; in children; and perhaps most importantly, in people who are dehydrated or at risk of dehydration (for example with vomiting and diarrhoea), or with certain health problems or taking medications that affect their fluid balance (such as heart or kidney problems).
- In this group of men they also only tested the effects of drinking four coffees a day for three days, or drinking water only for three days. We don’t know whether continuing with either pattern – water-only drinking, or continued coffee drinking – would have different effects on hydration if measured over the longer term.
- The study is only measuring blood and urinary measures of hydration but does not examine the other effects of caffeine on the body, for instance, its stimulant properties.
Coffee contains caffeine, which is known to be a diuretic. As many people who drink several cups of coffee will know, it makes you pass urine. Also, as many people may have noticed themselves, when feeling particularly thirsty, a glass of water is more likely to quench your thirst than a cup of coffee.
Taking all of these things into account, it cannot be concluded from this study alone that coffee is as hydrating as water as the news headlines state.
If you are in good health then a moderate amount of coffee is not going to cause you any problems. But it is not recommended as your sole source of hydration as caffeine, unlike water, can cause side effects.
For example, a study we discussed in 2013 found that people who drank coffee during the afternoon had impaired sleep quality compared to those who went caffeine-free.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.