"Coffee dehydrates you"—it's a statement we've all heard at some point from a friend or family member.
Given the popularity of coffee and the serious effects of dehydration, knowing whether this often-repeated statement is actually true can be valuable for anyone who enjoys a cup of joe.
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The Truth about Coffee and Dehydration
When people talk about the dehydrating effects of coffee, you might hear them refer to coffee as a "diuretic," meaning it is likely to increase the production of urine. The ingestion of substances known as "diuretics" causes faster loss of fluids in greater amounts than normal, which could ultimately result in dehydration.
Countless people may have told you that coffee is a diuretic, but is there any scientific evidence behind it?
Does coffee really dehydrate you?
Most people believe that coffee depletes the body of fluids, but according to Katherine Zeratsky, R.D., L.D., "coffee and caffeinated liquids can contribute to the body's daily fluid requirement." This means that the hydrating effects of coffee outweigh any mild diuretic effect it might have. Zeratsky writes, "Drinking caffeine-containing beverages as part of a normal lifestyle doesn't cause fluid loss in excess of the volume ingested."
Zeratsky explains that normal consumption of caffeinated beverages may lead to increased urination and mild diarrhea; however, there does not appear to be a risk of dehydration. The key here is the phrase "normal consumption"—one cup of coffee per day should not dehydrate you, but if you guzzle gallons of coffee or high-caffeine soda every day, it could certainly cause issues.
A 2013 study funded by the nonprofit Institute for Scientific Information on Coffee (ISIC) sought to evaluate the statement that coffee causes dehydration. The study was funded, but not conducted, by ISIC; and ISIC played no role in its design or data collection, which centered on 50 coffee-drinking men in two separate three-day trials. In one trial, the men drink four cups of coffee per day for three consecutive days. In the other trial, the men drank an equivalent amount of water for three consecutive days. Their physical activity, fluid and food intake were controlled so as not to influence the results.
Once all the data were collected and analyzed, the researchers concluded that there was no significant difference in the men's hydration levels between the two trials. "Our data show that there were no significant differences across a wide range of hematological and urinary markers of hydration status between trials," the study's author wrote; and he went on to say that the data "suggest that coffee, when consumed in moderation by caffeine-habituated males, contributes to daily fluid requirement and does not pose a detrimental effect to fluid balance. The advice provided in the public health domain regarding coffee intake and hydration status should therefore be updated to reflect these findings."
Water should be your number one beverage, but feel free to enjoy a cup of coffee now and then without worrying about whether it will leave you dehydrated.