Does too much coffee cause dehydration?

by Mark Miller October 17, 2017

caffeine and dehydration

Drinking too much coffee doesn't appear to cause dehydration. But it's unlikely a little dehydration would stop the visitors to this café in Vienna. (Andreas Praefcke photo/Creative Commons)

Will caffeine make me dehydrated? For a while there, everybody took it as accepted wisdom. Also, it wasn't that long ago when everybody was saying you had to have eight 8-ounce glasses of water per day for a healthy body and sufficient hydration.

So if you coupled the eight glasses of water every day plus a mug or three of coffee or tea, the time spent in the bathroom must have seriously interfered with work or home life. No doubt there was a whole of urinating going on when people were ingesting a half gallon of water and a couple of mugs of coffee every day.

Urban legends

As it turns out, both claims are urban legends. Scientists have been studying whether eight 8-ounce glasses are necessary, and whether moderate amounts of caffeine make you dehydrated. The answers: no and no.

It was claimed caffeine makes you urinate so much you become dehydrated. In fact, taken in moderate amounts, caffeinated beverages can contribute to your overall daily water needs. Drink up, but beware you might have to go to the bathroom a lot.

No extra fluid loss

Mayo Clinic's website says: "Drinking caffeine-containing beverages as part of a normal lifestyle doesn't cause fluid loss in excess of the volume ingested. While caffeinated drinks may have a mild diuretic effect, meaning that they may cause the need to urinate, they don't appear to increase the risk of dehydration."

The University of Michigan Health System, in this PDF file, says you don't need eight glasses of water per day:

There is no scientific proof that, for healthy people, drinking extra water has any health benefits.Scientific research has shown that drinking large amounts of water does not:

    • Make skin look healthier or wrinkle free
    • Benefit kidney function
    • Clean out toxins
    • Make you feel more energetic.

The article says the idea of eight 8-ounce glasses arose from a U.S. Food and Nutrition Board recommendation from 1945 that 2.5 liters (84.5 ounces) of water per day were needed--even more than was recommended not that long ago! The board added that most of this water intake comes from prepared foods, not liquids.

Buying research

This idea of eight 8-ounces glasses became prevalent again in the 1980s because companies that bottle water finance research that supports the claim. A study saying nearly two-thirds of New York City and Los Angeles kids were not drinking enough was funded by the Nestle Waters subsidiary Nestec, the University of Michigan Health System article states. Part of the problem with this study was that the definition of dehydration underlying the analysis has been considered normal in healthy kids around the world for many years.

The university's article states: "Some weight-loss programs tell you to drink 8 glasses of water per day to help you lose weight. While drinking a half liter of water right before you eat may fill the stomach so you become uncomfortable if you eat large portions, there is no evidence that high fluid intake leads to weight loss."

Roots of the idea that caffeine causes dehydration

This idea that caffeine intake causes dehydration can be traced to a 1928 study. The study noted increased urination in people who drank beverages with caffeine in them. But drinking any beverage in larger volumes, with or without caffeine, causes you to urinate more. This includes water itself.

Some studies prior to 2005 studied the effects of caffeine on hydration over short periods. But in 2005, according to Live Science, Lawrence Armstrong, a professor in the Department of Kinesiology at the University of Connecticut and director of the Human Performance Laboratory, did a longer-term study.

Dr. Armstrong and his team gave 59 healthy males a controlled diet for 11 days. Each man got doses of caffeine commensurate with their body mass. The caffeine came in capsules, twice a day. The team studies what Live Science calls 20 hydration biomarkers, including urine volume and fluid-electrolyte balance to measure any possible dehydration.

Live Science reported:

The study found that the evaluated hydration indicators, including urine volume, were similar for all of the treatment groups. This finding demonstrates that caffeine does not have a dehydrating effect when compared to the control group (participants who received a placebo and did not consume any caffeine). The scientists also found that a higher dose of caffeine was no more likely to dehydrate a person than smaller doses were.

No emergency room overloads

Armstrong told Live Science: "The fact that we don't have hospital emergency rooms filled [with patients] because they [drank] caffeinated beverages is clear evidence ... If there were negative health effects, [they] certainly would have been identified."

Other studies have been conducted into the effects of caffeine on dehydration. According to a 2014 article on the BBC website:

A second study found no difference in hydration between those drinking water or coffee, leaving us with conflicting findings. Then came new research earlier this year from Sophie Killer at Birmingham University in the UK, who not only measured the volume of urine, but tested their blood for signs of kidney function as well as calculating the total amount of water in the body. The men in the study drank four cups of coffee a day, far more than the average coffee-drinker. Yet there was no evidence they were any more dehydrated than those who drank water alone. This research was funded by the Institute for Scientific Information on Coffee, whose members are coffee companies, but it has been published in a peer-reviewed journal and the authors confirm that the Institute played no role in gathering or analysing the data or writing up the research.

Some doctors recommend getting your fluids from pure water because it has no sugar or other harmful additives and does not contain caffeine. While studies have shown coffee doesn't harm people unless it's taken in large amounts, it actually helps lower the risk of some diseases and helps athletes excel.

Mark Miller
Mark Miller


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