Claire Bugos is a New York City-based health and science reporter and writer.

Heather Mercer is native to Northwest Ohio (go Walleye!) and graduated from Loma Linda University with two doctorate degrees (psychology and public health). She is currently a professor at Owens Community College, as well as a fact-checker for Verywell Health. She has gained experience in a variety of settings, ranging from corporate wellness and preventive medicine, to mental health, chronic disease, and end-of-life care. 

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Heatwaves are rippling across the U.S., setting record high temperatures in some regions.

Hot, sweaty days make for just the right condition for dehydration. A typical healthy person might sweat out 1.5 liters in an hour in the heat. Someone who is used to the heat will sweat even more, and can lose as much as 3 liters per hour while exercising.

This water loss can interfere with how well the body functions, and may impact mood and concentration even after just 1% dehydration.

Drinking water is the most straightforward way to replenish hydration, Stella Volpe, PhD, RDN, ACSM-CEP, FACSM, a nutritionist and exercise physiologist at Vermont Tech, told Verywell.

But many beverages can be hydrating. Eating a balanced diet, getting your electrolytes in when needed, and sipping regularly if you spend a day in the heat are also key.

The most important thing, Volpe said, is to listen to your body, and drink when you’re thirsty.

Water makes up about 60% of the average adult body and roughly 80% of the brain. It keeps blood flowing to deliver nutrients to important organ systems, like the brain, heart, kidneys, and muscles. Hydration helps to flush toxins from the body, lubricate joints, keep the brain healthy, and regulate sleep quality and mood.

The amount of water each individual needs will change based on their personal needs each day. There’s no easy way to know just how much liquid you lose each day from breathing, sweating, peeing, or pooping.  

Still, there are many guidelines to help people gauge their water consumption. The Institute of Medicine recommends consuming between 2.7 and 3.7 liters of total water daily from foods and beverages. Health providers have long used the 8x8 rule—a person should drink eight 8-ounce glasses of plain water daily—though some experts disagree on the validity of this rule-of-thumb.

But water drinking guidelines can be hugely variable and imprecise, Mitchell Rosner, MD, a nephrologist and professor with a focus on fluid and electrolyte disorders at the University of Virginia, told Verywell.

“Probably the best way that we know of [to stay hydrated] is to drink fluids based upon your thirst,” Rosner said. “Humans have an incredibly good ability to use their thirst sensation to basically give us a clue that we're getting behind on our fluids and we need to drink more.”

Getting the right amount of fluids, Rosner said, requires a bit of common sense. It’s best to adjust your drinking based on your personal needs, the environment, and your level of physical activity.

Many factors will influence how much water an individual should drink—how hot it is, their activity level, how acclimatized they are to the heat, and how much they tend to sweat. Those who are used to the heat—say a construction worker in Texas—will sweat more to quickly cool off.  

Some people don’t feel they can trust their thirst, Rosner said. In that case, he recommends weighing yourself periodically. If you gain weight after a period of time when you’re mostly drinking fluids, after a gym session for instance, it may be a sign you’re drinking too much water. If lose a significant bit of weight pretty quickly, it’s likely water weight.

Pale yellow urine—as opposed to clear urine—is a sign of good hydration.

The best way to hydrate, Volpe said, is by drinking water—it’s calorie-free, relatively accessible, and provides the best source of hydration. But water is by no means the only way to boost hydration.

“You can get water from sources of food as well as drink, and pretty much any liquid drink will count toward your hydration needs,” Volpe said. She helped set the Institute of Medicine guidelines, which say that hydration can come from a variety of dietary sources.

In fact, milk, tea, and orange juice appear to be more hydrating than plain water when consumed on a temperate day, researchers concluded in a 2015 study comparing a dozen different drinks to a special “rehydration solution.” The researchers measured what portion of the water content of each drink the volunteer retained. As it turns out, the sports drinks, coffee, and lager beer they tested had about the same hydrating effect as water.

Volpe said she used to teach the rule that a cup of coffee should be followed by a cup of water. That’s because caffeine is often considered a diuretic, meaning it increases the amount of urine a person produces. She's since changed her tune. While coffee may flush fluid from the body more quickly than water would, Volpe said the drink still adds more fluid to the body than is lost, boosting hydration.

Of course, drinking too many alcoholic, caffeinated, and sugary beverages can be damaging to health in other ways. Plus, the higher the caffeine or alcohol levels in a beverage, the less likely they are to be hydrating.

During a heatwave, it's best to drink water. Heat can exacerbate some of the intoxicating effects of alcohol, like confusion and impairment of coordination.

Foods can also be a key source of moisture. Juicy fruits and vegetables—think cucumbers, watermelon, and lettuce—all contribute to your daily fluid intake.

The reason other beverages may help someone hydrate more than water alone is that certain ingredients, like amino acids, fats, and minerals may help the body retain more water. The most vital electrolytes for replenishing hydration are potassium, sodium, and chloride. These minerals are important to balance out water content in cells and keep organs functioning well.

For athletes or others who sweat a lot daily, it’s important to eat plenty of salt to replenish the electrolytes lost through sweat. They might supplement their beverages or diet with potassium and salt.  

Most people in the U.S. consume plenty of salt in their food. Potassium is found in many fruits and vegetables, and most people won’t need to use a potassium supplement if they eat a balanced diet, Volpe said.

“The person, say, who exercises 30 minutes or even an hour a day is going to be fine if they drink enough fluids and if they have salt in their diet,” Volpe said.

Rosner agrees, adding that for the average person, there’s “not a lot of evidence” that drinks with electrolytes are better than plain water.

Unless you're aggressively working out, you probably don't need an electrolyte-enhanced beverage.

While dehydration tends to be a more pressing concern, overhydration is also possible.  

This tends to happen when a person is drinking water faster than their kidney can process it. An influx of water can cause the body to flush out too much sodium in the urine, causing an imbalance.

The severe form of overhydration is called hyponatremia. The symptoms can be severe, and sometimes lead to seizures and, very rarely, death.

The goldilocks zone for hydration, Volpe said, is for urine to look pale yellow. Clear urine could be an indicator of over-hydration. Bright or darker yellow urine lets you know it’s time to drink some more.  

Most people who do moderate exercise on a day with mild or cool weather can stay hydrated by drinking when they feel thirsty.

But staying ahead of your hydration can be particularly important on hot days and when doing more intensive activity to avoid dehydration. Research shows that in this context, people's thirst does not always align with the amount of fluid they need to replenish.

Drinking a glass or two of water before heading outside and sipping throughout the day can be beneficial for keeping the body healthy, as well as keeping you hydrated.

“If someone has a good habit of drinking water throughout the day, that's excellent for their body,” Volpe said.

If, after a day in the heat, you feel thirsty, dizzy, or develop a headache, it’s your cue to drink some water. You may be severely dehydrated if you experience confusion, muscle cramping, confusion, an increase in heart rate or blood pressure, or an inability to sweat or urinate. In that case, seek medical attention.

To rehydrate, drink water that’s close to room temperature, as your body will process it quicker. Volpe recommends against taking salt tablets, though drinking a beverage with some sugar and electrolytes should help.

“People should pay attention to their bodies and not get to the point where you’re starting to have headaches and feel dizzy,” Volpe said. “If you're out in the sun…don't wait for the point of getting thirsty.”

Listen to your body and drink when you're thirsty. If you're unsure about how many fluids to consume in a day, you can try weighing yourself to measure your water weight change. Talk to your doctor about how to avoid overhydrating if you are taking a medication that may cause your body to retain water.

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