As the summer sun beats down, we should all stay hydrated, as dehydration can increase our risk of heat exhaustion and heat stroke. However, when I saw the National Weather Service in Las Vegas advising residents across parts of California, Nevada and Arizona to stop drinking caffeine in extreme heat to prevent dehydration, I wondered if that was taking things too far.
To find out whether caffeine actually causes dehydration to the point where you need to put your iced coffee down, I consulted three experts.
Does coffee cause dehydration?
"Caffeine, the main active ingredient in coffee, does have a mild diuretic effect," explains Jessica McAllister, MS, RDN, a registered dietitian based in Honolulu, HI. "However, the common idea that drinking coffee automatically causes significant dehydration, especially in hot weather, is an oversimplification that deserves a closer look."
A diuretic is a substance that increases the production of urine. Though coffee mildly has this effect, McAllister states that for healthy individuals, moderate coffee intake doesn't cause dehydration, but can instead contribute to daily fluid intake.
Zeitlin adds that what is being talked about in NWS Las Vegas's post is caffeine, which isn't just in coffee. It can also be found in sodas, energy drinks and chocolate. She advises that people keep their caffeine intake to about 300mg daily. This is about 1-2 cups (8-16 ounces of drip coffee) or 1-2 shots of espresso daily. If you stick to this and are otherwise healthy, Zeitlin states that you don't need to be concerned about the outside temperature.
Amelia Ti, a registered dietitian and diabetes educator in New York City, who is also part of CNET's medical review board, adds, "It's not a major concern unless you're already at risk (e.g., high blood pressure or other cardiac issues) or heat-sensitive. The main concern with drinking coffee when it's hot out is that caffeine may mask early signs of dehydration or overheating, especially if you're sweating a lot or not replenishing fluids adequately."
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Is coffee a vasoconstrictor?
In response to these caffeine-dehydration headlines, one Bluesky user stated that the issue isn't dehydration. It's actually vasoconstriction, which is the narrowing of the blood vessels.
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