Extreme heat is sending unprecedented numbers of Americans to emergency departments—and to untimely deaths. Scientists, clinicians and public health officials know that people need to stay hydrated to prevent heat-related illnesses. But it can be hard for an individual to figure out how much liquid is enough; needs can vary a lot depending on circumstances and body type.
In search of some actionable guidance, Scientific American spoke with physicians and researchers about how to hydrate during a heat wave (or any other weather condition).
Why is it especially important to hydrate in extreme heat?
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Heat-related illness can occur whenever people are exposed to hot outside temperatures, particularly those above 90 degrees Fahrenheit (32 degrees Celsius). Humid climates make sweating less effective and increase the threat, says Stavros Kavouras, director of the Hydration Science Lab at Arizona State University.
High temperatures leach water from a person’s body, which reduces blood volume and forces the heart to work harder to support other essential organs. Dehydration, or severe water loss, in turn limits sweating, feeding a cycle of overheating. At its most innocuous, a spike in core body temperature can cause painful muscle cramps in people exercising outdoors. But heat-related illness can take the form of a more serious condition known as heat exhaustion, which can progress to a potentially lethal one called heat stroke.
Knowing the various signs of heat-related illness can save lives. “When I’ve seen people who progressed into heat stroke, it was because they weren’t aware what was happening to them, and the people around them didn’t know they were unwell,” says Sheri Belafsky, an occupational and environmental medicine physician at UC Davis Health.
Common symptoms of heat exhaustion are profuse sweating, fatigue and nausea. The condition itself isn’t fatal, but repeated cases can cause long-term health effects such as chronic kidney disease. Heat stroke occurs at body temperatures of 104 degrees F (40 degrees C) and above, can cause confusion, loss of consciousness and seizures. People who don’t receive emergency medical attention will undergo major organ failure.
What heightens the risk of dehydration?
People who spend long periods of time outside or are physically active, such as outdoor laborers and athletes, are most at risk. Age and medical status can also make dehydration more likely. Older adults sweat less efficiently, and the bodies of pregnant people and children have greater difficulty regulating body temperature. A broad swath of medications, including beta-blockers, stimulants, antipsychotics and antidepressants, make it harder for the body to detect or defend against dehydration. People with heart or kidney issues also risk their conditions worsening.
How can if you tell if you’re properly hydrated?
Thirst is a delayed and often shaky indicator of dehydration, Kavouras says. Our ability to detect thirst decreases as we age, and certain medications such as antidepressants and weight-loss drugs can dampen the signal. Experts suggest that people keep water bottles nearby and track how much they drink. Major health organizations advise women to take in 1.6 to 2.1 liters of water per day and men to take in two to three liters—but those recommendations increase in the heat. A well-hydrated person should urinate every two to three hours, and their urine should generally be pale yellow (though an individual’s diet and medication can sometimes alter the baseline color).
For people who are outdoors in extreme heat, it helps to drink a glass of water every half hour, says Brandee Waite, a sports medicine doctor at UC Davis Health. Overhydration is rare and usually only occurs if someone drinks more than a liter of fluid per hour.
Does only water count for hydration?
Nope! Many foods, particularly fruits and vegetables, contain water. But people do need to consume liquids of some kind, in addition to their meals, to meet health guidelines. Beverages such as milk and some fruit juices are mostly water. People who dislike the taste of plain water can try flavoring their drinks—or even making at-home popsicles, says Sarah Eby, a sports medicine specialist at Mass General Brigham and Spaulding Rehabilitation. Drinks that contain caffeine or alcohol, however, cause people’s body to lose more water and should be consumed in moderation when the weather is extremely hot. Waite recommends, for instance, that people at picnics or outdoor parties alternate a glass of water with each alcoholic drink.
When should a person replenish their electrolytes?
As we sweat, we lose small amounts of important minerals such as sodium, potassium and magnesium. Such electrolytes help our muscles and nerve cells function. Most people’s diet contains enough electrolytes to make up for what they lose by perspiring. But those who sweat a lot—either from prolonged heat exposure or intense physical activity that lasts more than an hour—should consider consuming electrolyte products, such as sports drinks or gummies.
The method of delivery, be it powder, tablet or gel, doesn’t affect the body’s absorption of electrolytes. The ingredient list is more important; effective products are rich in salt and contain small amounts of sugar. For a sports drink, that might look like 450 milligrams of sodium and less than 30 grams of sugar per 24 ounces (0.7 liter) of liquid, Eby says. Small amounts of sugar speed up fluid absorption, but larger amounts have the opposite effect: too much sugar can cause diarrhea and thereby dehydrate the body.
People who decide to make their own electrolyte drinks at home should follow recipes that are simple and cheap, Waite suggests. A mixture of apple juice, water and salt can suffice—though “you don’t want to make it so [salty] that your face puckers as you drink it,” she adds.
The kidneys often filter out extra electrolytes, but drinking too many can cause muscle cramps, nausea, diarrhea and a fast heart rate. It’s safest to stick to existing products that offer serving size recommendations and to reach out to physicians for more precise guidance when necessary.
How much does hydration matter outside of a heat wave?
Water plays a vital role in “every single physiological function of the body,” Kavouras says. Despite this, more than half the world’s population fails to meet normal daily hydration guidelines. The kidneys can stave off dehydration by secreting less liquid, which keeps symptoms minor for most people. But the consequences of even mild water deficits, or “underhydration,” accumulate over time. Inadequate hydration can worsen day-to-day cognitive and physical functions—and some research has linked chronically high sodium levels to faster aging and a higher risk of chronic diseases, such as diabetes, dementia and heart failure.