When Jac Folkert prepares for a run, she goes through a mental checklist of her gear: Running shoes, check. Colorful outfit, check. Watch, check.
But when the weather is warm—which often is the case for Folkert, 54, who lives in Los Angeles—that list also includes some specific items to keep her safe: a lightweight backpack with a 1.8-liter water bladder, an 18-ounce soft flask for electrolyte drinks, and a lightweight hat to protect her from the sun. And those colorful running shorts and tops? They all have an Ultraviolet Protection Factor rating to protect her from the sun's ultraviolet rays.
“I don't fool around in the heat,” says Folkert, who was diagnosed with relapsing-remitting multiple sclerosis (MS) in 2006. “Overheating and MS are not friends.”
Her carefulness in managing her hydration and exposure to heat and the sun has helped keep her in the fast lane. This year, she competed in her 18th Boston Marathon. And in 2024, she completed two 50-kilometer (31-mile) ultramarathons in Arizona. (Her husband Daryl, also an ultra-distance runner, often accompanies her.)
Regardless of whether you have a neurologic condition like Folkert, the biggest challenge with managing heat is the risk of dehydration—the absence of sufficient water in the body. If you keep exerting yourself for a sustained period without addressing your hydration, it could lead to a progression of heat illnesses as your body temperature rises.
How Dehydration Works
People become dehydrated when they lose enough bodily fluid—2 percent—that their body can't function properly. One of the earliest signs is your frame of mind, as it initially affects cognition. At that point, “you would be grumpier, and things will feel harder,” says Douglas J. Casa, PhD, CEO of the University of Connecticut's Korey Stringer Institute, named for an NFL player who died of heat stroke after a 2001 practice.
Other signs include dark yellow urine, weakness, low blood pressure, dizziness, and confusion, according to guidelines from Harvard Health Publishing. Dehydration also can negatively impact short-term memory, attention, and vigor, a 2019 study in the International Journal of Environmental Research and Public Health found.
People can get dehydrated in all conditions, but it's more likely to happen during the heat and physical exertion “because you're losing fluids through sweat as your body tries to cool itself, and blood gets shunted toward the skin,” says Frank Conidi, DO, MS, FAAN, FAHS, director of the Florida Center for Headache and Sports Neurology in Port St. Lucie. You don't need to run a marathon to feel the effects, either. Changes in mood and your sense of how hard you're working could be evident even in a 30-minute walk, Dr. Casa says.
Sometimes people have a tendency to forge ahead, though, and that's where problems can arise. Prolonged exposure to heat and humidity without relief or adequate fluid intake can cause heat-related illnesses, including heat cramps, the most mild form of these illnesses; heat exhaustion, in which the body cannot properly cool itself; and heat stroke, a life-threatening condition that occurs when the body's core temperature reaches at least 104 degrees F. In heat stroke, the body's heat regulatory system essentially breaks down, potentially damaging vital organs.
Exacerbating Conditions
Cognitive changes from dehydration often are more severe in vulnerable populations, according to a 2017 study published in Annals of Nutrition and Metabolism. The brains of children and older adults may not have as many resources to manage the effects of dehydration, so cognitive tasks may be more demanding for them, and their performance is more likely to be impaired, the study notes.
For people with neurologic conditions that already compromise some aspect of the brain or its functions, the risks of dehydration and heat illness can be magnified. Those who fail to take the kind of precautions Folkert does—even if they are not nearly as active as she is—risk a number of problems. “You can exacerbate existing conditions,” Dr. Conidi says. “Patients who had a stroke could see some of their stroke symptoms come back. Patients with Alzheimer's and Parkinson's diseases often lose the drive to drink.”
People with MS should do what they can to avoid heat illness. If they develop a high core temperature, “they could be at greater risk of either developing increased symptoms from their underlying condition or potentially even relapse,” says Brian Hainline, MD, FAAN, clinical professor of neurology at NYU Grossman School of Medicine.
Besides staying hydrated, patients with MS can combat the heat by applying cold cloths, wearing cooling garments, drinking cold or icy liquids, having a fan blow on them while exercising, and following other cooling strategies. Swimming can be an excellent option for them “because the water will dissipate heat, and they can often move weakened muscles in the water that they are unable to move on land,” says Barbara Giesser, MD, FAAN, director of the MS clinic at the Pacific Neuroscience Institute in Santa Monica, CA; co-author of Navigating Life With Multiple Sclerosis, part of the Brain & Life book series; and a member of the Brain & Life editorial board. They should avoid heated pools and hot tubs, however.
What to Drink
Kirk Skinner, 65, doesn't look at his daily outdoor activity as exercise. “It's a responsibility,” says the Montgomery, AL, resident. Diagnosed with Parkinson's in 2018, Skinner has 17 acres of land that need constant care. “I work in the shade as much as possible,” he says. “And I know when to back off.”
Once you're dehydrated, catching up can be tough—especially if you're active on a summer day. “Once you're down a couple of liters, it's a spiral,” Dr. Hainline says. Most people should drink 4 to 6 cups of water daily, but that can vary depending factors such as health conditions, medications, and what else they drink, the Harvard guidelines note.
To reduce your risk of dehydration, Dr. Hainline recommends avoiding exercise during the hottest parts of the day and wearing lightweight clothing that protects you from the sun's rays. You also can quickly nip dehydration in the bud by stopping or slowing down what you're doing, Dr. Casa says. “Get into shade and get access to fluid,” he adds. “If it's mild dehydration like that, it's easily correctable.”
Folkert sets her watch to remind her to take a drink at certain intervals when she runs. Skinner has learned to monitor his body while raking or trudging through the soil. “Some days, I'll work a little and stop,” he says. “I have two big swings by my barn in the shade, and I have a fan down there.”
Water works well for moderate bouts of activity, but the American Council on Exercise says sports drinks that include carbohydrates and electrolytes—to replace minerals lost through sweat—are best for exercise sessions lasting longer than 60 to 90 minutes. Skinner nibbles on juicy fruits when working outdoors—a smart way to stay hydrated, says guidance from the Cleveland Clinic, which lists watermelons, strawberries and grapes as good fruits to help with summer hydration.
Patients who take medications for neurologic conditions should check with their doctor about how to best stay hydrated, because fluid needs are highly individualized. Pushing yourself to drink is important, though, “especially if you have underlying risk factors because you're taking medications,” Dr. Conidi says. “A lot of neurological patients are prescribed diuretics, and these can deplete fluids.”
Stay Active
Fear of heat-related illnesses shouldn't prevent you from working out, though. “I think we've erred too much on the side of telling our patients, ‘Don't exercise in the heat,’” Dr. Hainline says. “Yes, we need to take some precautions, but we also need to be physically active.”
If you don't like the idea of working out in tough conditions, head to the air-conditioned safety of your local gym instead. Just remember to keep hydrating there, too.
Whether it's his outdoor work or the exercise classes he takes at the YMCA, Skinner knows the activity is good for him—mentally and physically: “I always feel a lot better after I've done it.”
On the rare occasion that Folkert has felt the heat affect her despite her precautions, she simply shortens her run or stops in the shade and hydrates. “It's the smart, safe choice,” she says. “And as they say, I live to run another day.”
And perhaps that's what's most important: Folkert gets to keep doing what she enjoys. “I believe in living big and doing what you love with people you love,” she says. “So I choose to do that as safely as possible and adjust when needed.”
