Running in the heat: what actually helps and what doesn't
Last July I went out for a ten-mile run at 7am thinking I’d beat the heat. It was already 82 degrees with humidity somewhere around “swimming pool.” By mile four I was walking. By mile six I was sitting on a curb drinking water from a gas station. My heart rate was 20 beats higher than normal at the same pace, and my legs felt like I’d already run a half marathon.
I came home and started reading about heat and running, and a lot of what I found surprised me. Some of the standard advice is wrong. Some of it is incomplete. And some strategies that actually work well are things most runners don’t bother with.
What heat does to your body
When you run, roughly 80% of the energy your muscles produce turns into heat rather than forward motion. Your body dumps that heat through sweat evaporation and blood flow to the skin. In cool weather, this system works fine. In heat, it gets overwhelmed.
Your heart has to send blood to your skin for cooling and to your muscles for running. It can’t fully do both. Stroke volume drops, so your heart rate climbs to compensate. This is cardiac drift, and it’s why the same pace that feels easy at 55 degrees can feel brutal at 85.
The numbers are worse than most people realize. Research on marathon performance shows about a 0.3-0.4% pace decline per degree above the optimal range of 50-65F. But that’s the average. Slower recreational runners lose proportionally more than elites. Analysis of marathon finishing data found that top finishers lost around 0.9% per 9F increase, while mid-pack runners lost upward of 3.2%. If you’re a 4-hour marathoner, heat hurts you more than it hurts the person winning.
Heat also burns through glycogen faster, which means you bonk sooner on long runs. And once your core temperature passes about 102-104F, your brain starts dialing back effort whether you want it to or not. That’s not weakness. It’s a safety mechanism, and overriding it is how people end up in the medical tent.
Acclimatization is the biggest lever you have
If you only do one thing to prepare for running in heat, make it this. Heat acclimatization, spending 10-14 days gradually increasing your training in hot conditions, triggers adaptations that change how your body handles the stress in ways you can actually feel.
Within the first few days, your blood plasma volume starts expanding. By day five or six, your sweat rate increases and your body starts sweating earlier in exercise. By day ten, your heart rate at a given pace is noticeably lower and your core temperature stays lower too. Studies show acclimatized runners improve hot-weather time trial performance by 6-8%. Some research shows a 5-6% improvement even in cool conditions, which is a nice bonus.
The standard protocol is 60-90 minutes of moderate running in heat, daily, for about two weeks. That’s the gold standard, but it’s not realistic for everyone. If you don’t live somewhere hot, 20-30 minutes in a dry sauna (175-210F) after your normal run, three to six times a week for two to three weeks, triggers some of the same adaptations. It’s less effective than actual heat training but it works.
One thing to know: acclimatization fades at about 2.5% per day once you stop heat exposure. So if you acclimatize in June and then spend two weeks in air conditioning, you’ll lose most of the benefit.
Hydration: less complicated than the industry wants you to think
The sports drink industry has spent decades telling runners to “drink before you’re thirsty.” Tim Noakes, a sports science researcher, wrote an entire book (Waterlogged) arguing that this advice has done more harm than good. His main point: more athletes now suffer complications from overdrinking than from dehydration in endurance events. Exercise-associated hyponatremia, where you dilute your blood sodium by drinking too much plain water, has killed roughly a dozen athletes in documented cases.
For runs under 90 minutes in moderate heat, drinking when you’re thirsty is fine. Your thirst mechanism isn’t perfect, but it’s adequate. Where it gets more complicated is during very long efforts in serious heat, where sweat rates can hit 1-2.5 liters per hour and you may not feel thirsty fast enough to keep up.
For those longer efforts, plan to take in some sodium with your fluids. Somewhere around 300-600mg per hour is the general range, though individual variation is huge. If you finish a run with white salt crust on your skin or clothes, you’re a heavy sodium sweater and should aim toward the upper end. If not, the lower end is probably fine.
So: don’t force-drink on a schedule. Don’t avoid drinking either. Carry fluids on anything over 45-60 minutes in heat. Add electrolytes for anything over 90 minutes. And don’t drink only plain water during long hot runs. A good hydration vest makes carrying fluids on longer runs much easier than trying to plan routes around water fountains.
Cooling tricks that actually work
Before the run
Ice slurry (basically a slushy) consumed about 30 minutes before exercise has solid research behind it. One study found it lowered core temperature by about 1.2F compared to 0.5F for cold water alone, and increased submaximal running time in heat by about 19%. That’s a big number. The protocol in the research is 7-14 grams per kilogram of body weight, but in practical terms, drinking a large Slurpee before your run works.
Ice vests worn during warm-up can also lower core temperature, though they’re mostly practical for race day warm-ups rather than daily training.
During the run
Dumping water on your head and neck works, especially in dry conditions where the water evaporates quickly. In humid weather it’s less effective because evaporation is already impaired.
Ice in your hat or a bandana around your neck extends your body’s ability to shed heat. Some runners fill their hydration vest pockets with ice and let it melt during the run. This works.
Here’s an odd one: menthol mouth rinse. One study found that swishing a menthol solution improved running performance and thermal comfort in heat without actually changing core temperature. You feel cooler, so you pace less conservatively. The placebo effect is real and useful.
Clothing: what matters less than you think
Light colors vs dark
Everyone says wear white in the heat. The reality is more nuanced. A frequently cited study of Bedouin robes found that black robes absorbed 2.5 times more solar radiation than white ones, but the heat reaching the wearer’s skin was the same. The extra absorbed heat was lost through convection before it reached the body, because the robes were loose.
For tight-fitting running gear, though, lighter colors do help because there’s minimal air gap for convective cooling. So the advice is right for running clothes, just for slightly different reasons than people think.
Hat vs visor
Visors shade your face while letting heat escape from the top of your head, which is where a lot of heat dissipation happens. Hats protect your scalp from direct sun but trap heat. In humid conditions, a visor is usually the better call because maximizing evaporative cooling from your head matters more. In dry, intense sun, a hat might win because blocking radiation is the bigger concern. Either way, pick something you can soak with water.
Sunscreen doesn’t impair performance
I’ve heard runners say they skip sunscreen because it “blocks the pores” and interferes with sweating. A 2024 study from Penn State’s HEAT project tested this directly and found that sunscreen, both mineral and chemical, did not alter sweating rate, evaporative heat loss, or thermoregulation in any measurable way. Wear sunscreen. Getting sunburned is far more likely to impair your thermoregulation than the sunscreen itself.
Fabric
Moisture-wicking synthetics move sweat away from your skin, which supports evaporative cooling. Cotton holds moisture and gets heavy, but for a short easy run it honestly doesn’t matter as much as the fabric companies want you to believe. Loose fit matters more than the material for convective cooling. The tightest compression shirt in the best fabric will trap more heat than a loose cotton t-shirt.
Myths worth letting go
“Muscle cramps are from sodium depletion.” A study of Ironman athletes found no difference in blood sodium levels between those who cramped and those who didn’t. Current thinking is that cramps are neuromuscular, driven by fatigue rather than electrolyte imbalance. Taking salt won’t prevent them.
“You need to replace all your sweat losses.” You don’t. Mild dehydration of 1-2% body weight during exercise is normal and doesn’t meaningfully impair performance. Trying to fully replace all fluid losses during a run is how you end up overdrinking.
“If you’re fit, heat won’t affect you.” Fitness helps with thermoregulation, but it doesn’t replace acclimatization. A very fit runner who trains exclusively in air-conditioned gyms will suffer more in sudden heat than a moderately fit runner who’s been training outside all summer.
“Salt tablets prevent heat illness.” Multiple studies have found no significant impact of sodium supplementation on thermoregulation or cardiovascular drift during exercise. One study even found that training volume and avoiding overhydration mattered more than electrolyte intake. Salt tablets aren’t harmful in normal doses, but they’re not the insurance policy people treat them as.
When to stop
Heat cramps (involuntary muscle spasms during exercise) are the earliest warning sign. They’re annoying but not dangerous. Slow down, get some electrolytes, move to shade.
Heat exhaustion is more serious: heavy sweating, headache, dizziness, nausea, fast heart rate, and sometimes feeling cold or clammy despite the heat. That last one is counterintuitive but it’s a red flag. If this happens, stop running. Get to shade. Take off extra clothing. Cool down with water. If you don’t improve within 15-20 minutes, get medical help.
Heat stroke is an emergency. The key difference from heat exhaustion: your sweating stops, your skin feels hot and dry, and your brain stops working right. Confusion, slurred speech, aggression, disorientation. If you see this in another runner, call 911 immediately and start aggressive cooling. Don’t wait. Every minute matters.
The best prevention is simple. Slow down more than you think you need to. Run earlier in the morning. Watch the weather on your GPS watch before you head out. And give yourself permission to cut the run short. No training run is worth a trip to the hospital.
