With heat advisories popping up all across the US, the Las Vegas National Weather Service recently advised residents in parts of California, Nevada and Arizona to stop drinking caffeine when the heat is extreme. This is to prevent dehydration, which can lead to heat stroke. However, curious if we should all be putting our iced coffees down this summer, I reached out to three dietitians for the tea on caffeine, dehydration and heat safety.
Does coffee cause dehydration?
"Caffeine, the main active ingredient in coffee, does have a mild diuretic effect," explains Jessica McAllister, a registered dietitian based in Honolulu. "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.
Brigitte Zeitlin, registered dietitian and founder of BZ Nutrition, 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 to 2 cups (8 to 16 ounces of drip coffee) or one or two 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.
When I asked McAllister about this, she said, "Studies have found that while caffeine can cause some blood vessel narrowing, or vasoconstriction, this doesn't necessarily translate to reduced heat dissipation or increased body temperature in real life."
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