We all love a cup o’ Joe. But is this popular beverage causing dehydration?
Introduction
We have all likely been told that drinking too much coffee can cause dehydration. But is this truly the case? I mean, there is a plausible rationale for this—and we will touch upon it in this article—but as is often the case with the human body, such plausible physiological mechanisms can fail to pan out in reality. Luckily for you, I have investigated this claim thoroughly. So sit back, relax, and read on to learn what the best available evidence has to say on this topic.
Coffee consumption is associated with a range of health benefits and little risks. Learn more by reading our earlier article.
Why Coffee Might CAUSE DehydratION
The reason we’re told that drinking coffee is dehydrating is because caffeine—one of the main substances in coffee—is a diuretic. This just means that caffeine makes us pee, and if we pee more, we lose more water. In fact, urine is more than 90% water, so it doesn’t take a genius to figure out that peeing more often may cause dehydration. Further, caffeine may prevent sodium reabsorption in the kidneys, which could lead to more water ending up in our urine (1). This is because sodium attracts water (via osmosis), so if there's more sodium in the urine, there could also be more water in the urine.
However, we must keep in mind that coffee grounds are just 1–2% caffeine, so the vast majority of a brewed coffee is water (2). So, maybe any caffeine-induced increase in urine production is offset by the water we drink from the coffee. However, what if we took caffeine without water (e.g., via pills or gums), would this be more likely to cause dehydration? To get to the bottom of all of this, we need to investigate studies that instructed people to drink coffee or consume caffeine and then monitored urine production or hydration status. Thankfully, such studies exist.
Source: Wallpaper Abyss
THE EVIDENCE
In a study where 50 men were given either four cups of coffee per day for three days, or four cups of an equal amount of water per day for three days, there were no differences in total body water content from the beginning to the end of either trial, or between trials (3). In a larger analysis of 28 investigations (including a total of 379 individuals) where caffeine was consumed as the intervention, the combined result showed that while caffeine did produce a small increase in urine (+109 millilitres, or +16%) compared to non-caffeine conditions, the results were not consistent across trials, and whether this would be enough of an effect to cause dehydration is unlikely (1). However, when the authors dug a little deeper, they noticed that this effect was much more pronounced in females, but seemed to disappear altogether when caffeine was consumed during exercise (1). It must be noted that the amount of caffeine consumed in these studies was quite large, with the median dose being 300 milligrams—the amount provided by roughly four cups of coffee. So the small increase in urine must be interpreted within this context.
A more recent study showed no real difference between the amount of urine produced after drinking coffee with a caffeine content equivalent to about three cups of coffee compared to an equal quantity of water (4). However, when these participants drank coffee with a caffeine content equivalent to roughly six cups of coffee, they produced nearly twice the volume of urine compared to an equal quantity of water (4). Keep in mind that these drinks were consumed in one sitting, so these results may not translate to the real-world, where no one is drinking six cups of coffee in one go.
If you want to know all about how caffeine improves exercise performance, including how much to take, we have covered that before as well.
Summary
All in all, it seems that coffee (or caffeine) is unlikely to cause dehydration. The brand new position statement on coffee and sports performance from the International Society of Sports Nutrition agrees with this conclusion, stating that ‘...caffeine ingestion prior to exercise does not lead to dehydration’ and that any small increase in urine production when caffeine is consumed at rest is ‘minor’ (2). Further, their 2021 position statement on caffeine and sports performance stated that ‘...the majority of research has confirmed that caffeine consumption does not significantly impair hydration status, [or] exacerbate dehydration…’ (5).
To wrap things up, here are my three key takeaways:
The consumption of small-to-moderate amounts of caffeine—through coffee or otherwise—is unlikely to cause dehydration at rest
The consumption of moderate-to-high amounts of caffeine immediately before and during exercise does not seem to cause dehydration
The consumption of high amounts of caffeine at rest—through coffee or otherwise—may increase urine production slightly (and more so in females), but this is unlikely to cause dehydration (particularly for males)
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Thanks a million for your interest, and I hope you enjoyed this article.
Patrick Elliott, BSc, MPH
Health and Nutrition Science Communication Officer at Training121
Instagram: @just.health.info
Twitter: @PatrickElliott0
Health Disclaimer: this article is for informational and educational purposes only, and is not a substitute for professional advice. For health advice, speak to a physician or other qualified health-care professional, and for nutrition advice, speak to a qualified nutrition professional (e.g., registered dietitian). The use of information on this site is solely at your own risk.
References
(1) Zhang Y, Coca A, Casa DJ, Antonio J, Green JM, Bishop PA. Caffeine and diuresis during rest and exercise: A meta-analysis. J Sci Med Sport. 2015;18(5):569–74. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4725310/
(2) Lowery LM, Anderson DE, Scanlon KF, et al. International society of sports nutrition position stand: coffee and sports performance. J Int Soc Sports Nutr. 2023;20(1):2237952. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10375938/
(3) Killer SC, Blannin AK, Jeukendrup AE. 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. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3886980/
(4) Seal AD, Bardis CN, Gavrieli A, Grigorakis P, Adams JD, Arnaoutis G, Yannakoulia M, Kavouras SA. Coffee with High but Not Low Caffeine Content Augments Fluid and Electrolyte Excretion at Rest. Front Nutr. 2017;4:40. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5563313/
(5) Guest NS, VanDusseldorp TA, Nelson MT, Grgic J, Schoenfeld BJ, Jenkins NDM, Arent SM, Antonio J, Stout JR, Trexler ET, Smith-Ryan AE, Goldstein ER, Kalman DS, Campbell BI. International society of sports nutrition position stand: caffeine and exercise performance. J Int Soc Sports Nutr. 2021;18(1):1. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7777221/
Technical Terms
Caffeine: A substance found in coffee that is a central nervous system stimulant, meaning that it improves attention and cognitive performance, and also improves a range of exercise performance outcomes.
Diuretic:Â Any substance that promotes the increased production of urine (diuresis).
Osmosis:Â The movement of water molecules across a semi-permeable membrane from an area of lower concentration of solute (such as salt or sugar) to an area of higher concentration of solute, in order to equalise the concentration on both sides of the membrane. In simple terms, it describes how water tries to balance out the concentrations of salt or sugar on both sides of a barrier (or membrane).
Hydration: The process of replacing the lost fluid in something (particularly water).
Median: The median value of something is the value bang smack in the middle of all other values, which divides the numbers so that an equal amount are below, and an equal amount are above. For example, the median number between one and nine is five: 1, 2, 3, 4, 5, 6, 7, 8, 9.