How to Run in the Heat: Fall Marathon Training Considerations

Boston’s “magic window” of ideal running conditions always feels painfully short. We go from feeling elated at the cool spring temperatures of April and May, to 90’s and humid by the middle of June. Don’t even get me started on how it rains every weekend and is beautiful out during the week. 

But if you are training for a fall half marathon or marathon, running during the summer is non-negotiable. While you can’t control the weather, there are some strategies that you can implement to make summer running safer and more pleasant.

What's Actually Happening in Your Body (Heat Acclimatization)

Running in hot weather requires your body to multitask. On top of managing the strain of running, it forces your body to dump heat. As your temperature rises on a run, blood flow gets redirected to the skin to make you sweat, which pulls blood away from your muscles. (1)

This makes it even harder for your heart and lungs to keep up with demand which causes your heart rate to be higher than usual, even at easier efforts. 

The good news is that your body adapts to this stress. Regular training in higher temperatures improves sweating effectiveness and increases plasma volume which in turn makes the cardiovascular system more efficient and helps manage fluids and electrolytes better. 

This process of heat acclimatization can happen in just two weeks of consistent heat training. So how do you apply this information to your training?

Be patient with yourself in the first few hot weeks of the summer and don’t worry if your paces are slower than usual. Focus on using Rating of Perceived Exertion (0-10 subjective effort scale) as you become acclimatized.

Hydration and Fueling in the Heat

Most runners naturally drink more water when it’s hot, because their bodies are getting dehydrated faster and sending thirst signals more often. But sometimes just “drinking to thirst” leaves runners struggling. 

Part of the problem is that runners don’t always connect feeling crummy and performing poorly with a hydration or fueling problem.

Here’s what you need to know to get it right.

When you sweat, you lose water and electrolytes (mostly sodium). Drinking water replenishes the liquid, but you need to replenish the sodium too for a few reasons:

1. Sodium helps your cells hold onto water. When I eat a pastrami sandwich, my fingers swell. Why? Because my body is trying to normalize the salt content in my blood by holding onto water.

2. Reduces cramping. Sodium is essential for normal nerve and muscle function. When sodium is too low, it can cause the body to overreact and send a chaotic flood of signals to the muscles, causing cramping.

3. Avoids hyponatremia (water intoxication). Sodium is a pretty tightly regulated electrolyte in your body. If your sodium levels get severely low inducing hyponatremia, it means a visit to the ER. Not good.

The challenge is that sweat rate and sodium loss vary significantly between athletes. Some research has estimated up to a 10-fold difference between athletes. (2)

I’ve worked with some runners who were having such difficulty figuring out their hydration and electrolyte balance that it completely derailed their race performance. 

That’s why Alterra is now offering sweat testing. This takes the guesswork out of your hydration strategy to help reduce your risk of cramping up on your long runs. Whether you’re attempting to BQ or run your first marathon, sweat testing is an easy way to reduce your risk of “blowing up” on your next race. Learn out more about sweat testing at Alterra.

Sweat Testing at Alterra

Training in the heat also changes how the body processes nutrients. Training in hot conditions causes the body to burn more “pre-stored” carbohydrates in the form of glycogen in the muscles compared to carbohydrates consumed on the run. (3)

This happens because the body is shifting more blood flow to the skin to dump heat than the gut to process carbohydrates.

Basically, it’s harder to digest carbs when training in the heat. But there are two things you can do to help fuel your body properly:

1. Increase pre-exercise carb intake. Your body uses more pre-stored carbohydrates when running in hot conditions, so you need to “pre-store” more carbs when not exercising. That way, your gut can process those carbs when it’s not competing for blood flow as much with your muscles.

2. Train your gut with smaller carb doses. On workouts longer than 90 minutes, you still need to fuel on the run. But you can train your gut to work better in the heat by keeping the carb doses small. For example, maybe you eat half of your gel at minute 20 and the other half at minute 40 instead of one shotting it at minute 30. Over time, your gut should be able to handle larger doses, and more total carbs. 

Recognizing Heat Illness (and When to Stop)

This is the section most runners skim because they assume it's about other people. It's not. Heat illness doesn't sort by fitness level or experience. It happens to fast runners, experienced runners, and runners who have trained in the heat before. What changes with experience is how well you recognize the warning signs, and that matters a lot.

Heat illness exists on a spectrum. Understanding where you are on that spectrum in the moment is the difference between a bad training day and a medical emergency.

Heat cramps are the mildest end. Painful, but not dangerous on their own. They're a signal that your electrolyte balance is off, usually sodium. Stop, get out of the sun, and replace fluids with something that has sodium in it. Plain water here will help your thirst but not your cramps.

Heat exhaustion is the middle of the spectrum and the point where your body is telling you to stop. Symptoms include heavy sweating, pale or clammy skin, nausea, dizziness, headache, and a heart rate that won't come down even when you slow to a walk. You feel weak and maybe a little off, but you still know you feel weak and off. That awareness is important. It means your brain is still working correctly. Get out of the heat, lie down, apply cool water to your skin, and drink something with electrolytes. Heat exhaustion resolves with rest and cooling. Heat stroke does not.

Heat stroke is an emergency. The classic warning signs are a body temperature over 104 degrees, skin that looks red and dry rather than pale and wet, and confusion or disorientation. That last one is what matters most for runners to understand. You may not be able to recognize your own confusion. Your training partner might be the one who notices that your words aren't making sense, that you're stumbling, that you stopped responding to questions normally. This is why running alone in extreme heat is a real risk in a way that running alone in other conditions isn't.

One rule worth knowing: if someone with you seems confused, disoriented, or "not right" and it's hot out, that's a call 911 situation. Not a sit-in-the-shade situation. Not a wait-and-see. Active cooling starts immediately while you wait. Get them out of the sun, remove extra layers, and apply cold water or ice to the neck, armpits, and groin where blood vessels run close to the surface. The goal is to drop core temperature fast.

The other symptom worth flagging: sweating that stops when you've been sweating heavily. Your body uses sweat as its primary cooling mechanism. When it stops working and you're still generating heat, core temperature rises quickly. If you notice you've stopped sweating during a hard effort in the heat, that's a sign to get out of the sun immediately.

A few things that put you at higher risk on any given day: alcohol the night before, poor sleep, certain medications (antihistamines, diuretics, some blood pressure meds), starting a run already dehydrated, and training in humidity you're not acclimatized to. High humidity is often more dangerous than high temperature because it slows evaporation of sweat, which is your body's main strategy for cooling down. A 90-degree day at low humidity is a different experience than an 82-degree day at 85% humidity.

None of this is meant to scare you off summer running. The goal is the opposite: runners who understand the warning signs are the ones who can train through the summer safely, because they know when to push and when to stop. The runners who end up in trouble are usually the ones who kept going past a sign they didn't recognize.

Gear to Keep You Cool and Hydrated

There are four basic categories of products to help you train in hot conditions: cooling gear, hydration vessels, electrolyte products, and sun protection. Here are some products that I’ve either heard good things about or have used myself and found useful.

Cooling gear:

1. Ice bandana from Rabbit: There are many bandanas on the market like this one from Rabbit that has a little pocket where you can put some ice and then tie it around your neck. While all pieces of cooling gear have trade offs, this is likely the best bang for your buck. For long efforts, you would need to be able to replace the ice, but for runs < 60 minutes this is probably your best option.

There are some other pieces of cooling gear out there like the Omius headband, but I found them all to be too expensive, too fussy, not effective enough, or a combination of all three.

Hydration Vessels:

1. Insulated Water Bottle from HydraPak: It’s simple, compressible, and insulated to try to keep your liquids cold. Throwing your electrolyte mix into the blender with some ice and then pouring that slush in here should be nice and refreshing on a hot run.

2. Salomon Adv Skin 5 Set Hydration Vest: This vest has capacity for enough water to last you about 2 hours, so it should be enough for all of your summer marathon training long run needs. 

Electrolyte Products:

There are a bunch of electrolyte products on the market, and finding the right one for you often involves a lot of trial, error, and full bags of expensive electrolytes going in the trash after having a “code brown.” 

Each of these products offers something slightly different, which is actually a good thing! Remember, your sweat rate might vary wildly from another runner’s, so it’s important for you to find a product that fits your specific needs. Here’s a chart comparing some of the more popular products on the market to give you an idea of the differences:

Figuring out which product is most likely to fit your needs is as simple as booking a sweat test at Alterra. Once you know how much you sweat and how much sodium you need to replenish, choosing a hydration strategy becomes simple.

Sun Protection:

Physical sun protection

1. Path Projects Moab Arm Sleeves: These lightweight arm sleeves are built to replace sunscreen, and are especially great options for long duration runs in sunny conditions under 75 degrees. For warmer conditions, these might heat you up more than cool you off. One reviewer mentioned that getting them wet makes a big difference in keeping them cool.

2. Janji Hatchback Mesh Cap: Full disclosure, I haven’t tried this hat yet, but it’s on my wishlist. I have a big old head, so finding running hats that look okay, are light weight, and fit my head is a challenge. This hat seems promising, as well as their Trailbreaker Breaker Hat for lightweight running hats that provide some sun protection.

Sunscreen

It’s so easy to go down a rabbit hole online to figure out what kind of sunscreen is safe to use, so instead of recommending a specific one, here’s the Skin Cancer Foundation’s recommended products page. The bottom line is, you should wear sunscreen or use physical sun protection on every run.

How to Run in the Heat: Summary

Summer training is a grind. Some days, you just have to play it safe and find a treadmill. But gradual exposure to the heat, an individualized hydration plan, and the right gear can help you get through your summer training safely. 

If you want a hydration plan built around your actual sweat data, book a sweat testing session at Alterra.

Sweat Testing at Alterra

Sources

1. Racinais S, Alonso J, Coutts A, et al. Consensus Recommendations on Training and Competing in the Heat. Sports Medicine (Auckland, N.z.). 2015;45:925 - 938. doi:10.1007/s40279-015-0343-6 

2. Holmes NA, Bates G, Zhao Y, Sherriff J, Miller VS. The Effect of Exercise Intensity on Sweat Rate and Sweat Sodium and Potassium Losses in Trained Endurance Athletes. 2016;3:1-4.

3. Mougin L, Macrae HZ, Taylor L, James LJ, Mears S. The Effect of Heat Stress and Dehydration on Carbohydrate Use During Endurance Exercise: A Systematic Review and Meta-Analysis. Sports Medicine (Auckland, N.z.). 2025;55:2825 - 2847. doi:10.1007/s40279-025-02294-3