Have you ever been told that for every cup of coffee you drink, you should chase it with an extra glass of water? This belief is so widespread that many people feel a pang of guilt enjoying their espresso, convinced they are harming their body’s hydration. The good news is that modern science tells a very different story. Coffee, when consumed in moderation, does not dehydrate you. In fact, it contributes to your daily fluid balance.
Let us dive into the origins of this persistent myth and explore what current research actually shows.
The origin of the myth: a 1928 study
The idea that coffee causes dehydration traces back to a study published in 1928. In that research, participants consumed caffeine in elevated doses and researchers observed an increase in urine output. The conclusion at the time was straightforward: caffeine has a diuretic effect, therefore coffee dehydrates.
The problem is that this study had significant limitations. The number of participants was small, the caffeine doses were considerably higher than what a normal coffee drinker would consume, and the study failed to account for the volume of liquid ingested along with the caffeine. The fact that coffee is more than 98 percent water was entirely overlooked.
How the myth spread
For decades, the simplified conclusion that caffeine equals dehydration was repeated in nutrition textbooks, medical offices, and public health materials. The logic seemed intuitive: if you urinate more, you lose more water, therefore you are dehydrating. But science is rarely that linear.
What current science says
Coffee counts as fluid
A landmark study published in 2014 in the journal PLOS ONE, conducted by researchers at the University of Birmingham, directly compared the effects of coffee and water consumption on hydration. Fifty healthy men consumed four cups of coffee per day (equivalent to 800 ml) for three days, and in a separate period, the same volume of plain water. Researchers measured hydration markers in blood and urine.
The result was clear: there was no significant difference in hydration indicators between the two groups. Coffee, when consumed in habitual amounts by regular drinkers, contributes to body hydration in a manner comparable to water.
The diuretic effect: real but compensated
Caffeine does have a mild diuretic effect. It partially inhibits sodium reabsorption in the kidneys, slightly increasing urine production. However, this effect is modest at the doses found in regular coffee consumption, generally between 3 and 4 cups per day, corresponding to approximately 300 to 400 mg of caffeine.
Furthermore, the body develops tolerance to caffeine’s diuretic effect. Research shows that regular coffee consumers exhibit a significantly reduced diuretic response compared to people who rarely consume caffeine. If you drink coffee daily, your body has already adapted and the effect on urine production is practically negligible.
The balance is what matters
The key concept here is fluid balance. A standard cup of coffee contains between 200 and 250 ml of water. Even accounting for the slight increase in diuresis caused by caffeine, the volume of water ingested far exceeds the extra volume eliminated. The net balance is positive: you gain more water than you lose.
The European Food Safety Authority (EFSA) and the United States Institute of Medicine both include caffeinated beverages in the calculation of total daily fluid intake. Your cup of coffee counts.
Moderate consumption: the sweet spot
It is worth emphasizing that these benefits apply to moderate consumption. Most health organizations define moderate intake as 3 to 4 cups of coffee per day, or up to 400 mg of caffeine for healthy adults.
When dehydration might be a concern
At very high caffeine doses, above 500 mg at once, the diuretic effect becomes more pronounced and can indeed contribute to greater net fluid loss. However, this scenario is uncommon in habitual coffee drinking.
Situations that deserve special attention include:
- Intense physical exercise in hot weather, when fluid loss through sweat is already significant
- Drinking coffee during prolonged fasting without consuming other liquids
- People who are not accustomed to regular caffeine consumption
In these cases, supplementing coffee with water is sensible advice, not because coffee dehydrates, but because the body requires more fluids than usual.
Coffee and summer: a perfect pairing
Cold brew as a hydration ally
In hot weather, cold brew becomes a particularly appealing option. Beyond being naturally refreshing, cold brew is prepared with a generous ratio of water and tends to have a smoother, sweeter flavor profile that invites consumption in larger volumes.
A practical summer recipe is to prepare a concentrated cold brew the night before and dilute it with iced water and ice cubes at serving time. This way, you multiply the volume of liquid consumed while maintaining the flavor and pleasure of coffee.
Combining coffee and water throughout the day
A smart summer strategy is to alternate between coffee and water throughout the day. Start your morning with your favorite brew, keep a water bottle accessible during the day, and during the hottest afternoon hours, opt for a diluted cold brew or an iced coffee.
This alternation is not necessary because coffee dehydrates, but simply because varying your sources of hydration makes it easier to reach the daily recommended fluid intake, which ranges between 2 and 3 liters for most adults.
Other related myths
Dark roast has more caffeine than light roast
Another common misconception. Light roasts actually tend to preserve slightly more caffeine than dark roasts, since the roasting process degrades a small portion of the molecule. The difference, however, is minimal and rarely significant in practice.
Espresso has more caffeine than filtered coffee
A single espresso shot contains about 63 mg of caffeine, while a large cup of filtered coffee can contain between 95 and 200 mg. Espresso is more concentrated by volume, but because it is served in smaller doses, the total caffeine content is lower.
Coffee is addictive
Caffeine can cause mild physical dependence, with withdrawal symptoms such as headaches when consumption stops abruptly. However, this dependence is considered benign by the medical community and does not compare to the dependence caused by truly addictive substances.
The barista’s role in education
We believe a specialty coffee shop has a responsibility to educate as well as to serve. When a customer asks whether they should order water alongside their coffee, we can explain that there is no obligation, but that water complements the sensory experience: cleansing the palate before tasting allows you to perceive flavor nuances more clearly.
The water served alongside espresso in Italian cafes did not originate from fear of dehydration. It arose as a palate-cleansing ritual, a gesture of hospitality that prepares the drinker to appreciate every note in the cup.
Conclusion: drink your coffee with peace of mind
The science is clear: moderate coffee consumption does not cause dehydration. On the contrary, each cup contributes positively to your daily fluid balance. The myth was born from an oversimplified interpretation of early research and was perpetuated for decades without critical review.
This does not mean abandoning water, of course. Good hydration comes from a variety of fluid sources throughout the day: water, coffee, teas, juices, and water-rich foods like fruits. Coffee is simply another ally in that balance.
Visit Consciencia Cafe in Foz do Iguacu and discover your favorite cup with complete peace of mind. Whether you are exploring the breathtaking Iguazu Falls or working remotely in the Triple Border region, our baristas are ready to recommend the ideal coffee for your moment, from a bold espresso to a refreshing summer cold brew.