You know the drill — it's important to stay hydrated always, but especially in the summer heat. However, that message was taken to another level when the National Weather Service in Las Vegas advised residents across parts of Nevada, Arizona and California to avoid caffeine during extreme heat. The logic is that caffeine may speed up dehydration, which increases your risk for heat exhaustion or even heat stroke. The NWS advisory soon made headlines. To see if we should actually put down our iced coffees this summer, I reached out to three dietitians for the facts.
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.”
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.”
Zeitlin agrees, stating, “The kernel of truth is that yes, caffeine is a vasoconstrictor, which means it constricts your blood vessels, and preventing heat stroke (or any stroke) requires our blood running freely, unconstructed around our body.” However, for coffee to significantly affect your blood vessels, it depends on three factors: how sensitive you are to caffeine, how much caffeine you drink and your personal health and medical history.
“If you drink water throughout the day, you eat a variety of fruits and vegetables in your diet and your doctor has never directly told you to monitor your caffeine intake, then you can comfortably enjoy 1-2 cups of coffee without stressing,” concludes Zeitlin. On the other hand, if you have a cardiac health history, you should check with your doctor to figure out your daily caffeine limit.
Do experts recommend that people stop drinking coffee in the heat?
“I wouldn't recommend that most people completely stop consuming coffee just because it's hot out, as long as you're staying hydrated and not over-consuming,” says Ti. “I would recommend opting for an iced coffee, limiting extra-large or multiple cups of coffee or other caffeinated drinks, especially if you're sweating more, and watching out for signs of heat exhaustion like dizziness, rapid heart rate, nausea, headache or clammy skin.”
McAllister agrees, as long as your coffee consumption doesn't replace more hydrating fluids, like water, especially if you're sweating heavily. “I don't think there's sufficient scientific evidence to support avoiding coffee when it's hot outside,” she states. “For one, most people drink coffee for either the ‘pick-me-up' or to warm up. If you're drinking coffee, you're likely to be outside in hot weather for only a short time, so you should be able to stay cool and drink plenty of water to minimize your risk of dehydration or overheating.”
Zeitlin is on the same page. “I recommend people keep coffee to 1-2 cups daily, regardless of the weather, and to drink a full glass of water for every cup or shot of espresso they have and
enjoy it,” she says. “But if your doctor told you to avoid caffeine for any reason, then do so.”
Important precautions to take in the summer heat
Though you may not have to worry about your coffee consumption in the heat as much as headlines are saying, you should still exercise certain precautions when it's hot out this summer.
Increase fluid intake: “Drink water regularly throughout the day, and consider including electrolyte-rich beverages if you're sweating heavily or exposed to the heat for longer than 1 hour,” says McAllister. Zeitlin adds that drinking more water on hotter-than-normal days “means 2-4 glasses more than your usual baseline water intake.”
Add electrolytes if sweating heavily: “Precautions I would recommend in hotter-than-normal temperatures include hydrating with water and adding electrolytes if you're sweating heavily or outside in the heat for long periods of time,” states Ti.
Drink a glass of water before coffee: “A good general rule for anytime you consume coffee is to first have an 8-ounce glass of water,” says Zeitlin. “Your body's response to the caffeine will be more gentle and prevent jitters.”
Avoid excessive alcohol: “Drinking alcohol can increase your chances of getting dehydrated, particularly if you're spending a lot of time outdoors,” explains McAllister.
Consume foods with high water content: Ti specifically recommends watermelon, berries and greens. McAllister adds that cucumber, oranges and strawberries are great for this, especially if you don't love drinking water.
Eat raw fruits and vegetables: “Raw fruits and veggies will hydrate you with their own water content and give you essential minerals (aka electrolytes) to help prevent you from dehydrating,” states Zeitlin. “Cooked veggies are great for those minerals, but you'll be cooking out the water content.”
Skip workouts at the hottest part of the day: Unless your body is used to the heat, Ti advises scheduling your high-intensity workouts for a cooler time.
Exercise indoors: On a similar note, Zeitlin states, “Stay indoors in the air conditioning when possible and change your outdoor exercise routine to an indoor one on these super-hot days.”
Listen to your body: McAllister stresses the importance of remaining vigilant about signs of heat exhaustion, which include dizziness, headache or nausea. If you feel unwell, move to a cooler area.
