Running in the Heat: How to Fuel and Hydrate in Hot Weather

Planning on running in the heat this summer? Your usual fueling strategy might not cut it.

Running in hot weather changes the game when it comes to hydration, fueling, and performance. Maybe your stomach suddenly can’t tolerate the gels you normally use or you feel drained halfway through a run that usually feels manageable. Or maybe you finish a long run completely wiped out, despite doing “everything right.”

That’s because running in the heat puts extra stress on your body and impacts both your sweat rate and your ability to tolerate fuel, which can lead to a whole slew of issues with hydration, carb intake, and recovery. However, with a few strategic adjustments, you can stay well fueled and hydrated as the temps rise. 

In this post, I’ll walk you through the essentials of fueling and hydration for running in the heat, including fluid, sodium and carb needs, cooling strategies, and how to adapt your nutrition plan for different environments and types of running. 

Note: While this blog focuses on running, these principles apply to other endurance sports too.

Want an even deeper dive into heat fueling strategies for endurance athletes? I’m teaching a masterclass inside my Fuel for Life Crew membership this month called Fueling in the Heat: Don’t Let Temperature Tank Your Performance. Join us live on May 28th at 4pm PT or catch the replay afterwards!  

Why Running in the Heat Makes Fueling Harder

Running in and of itself creates challenges for digestion and fueling, and when you add heat on top of that, your body has even more competing demands. 

This happens because during exercise, blood flow gets redirected away from your digestive system and toward your working muscles. When you’re running in hot weather, your body also needs to send more blood toward the skin to help cool you down through sweating and heat dissipation.

In short, your gut gets less support.

This reduced blood flow and oxygen delivery to the gut can slow digestion and increase intestinal permeability, which helps explain why GI symptoms like nausea, cramping, bloating, or “runner’s stomach” tend to show up more often during hot runs.

For more information on troubleshooting GI issues, check out this podcast episode. 

At the same time, elevated core temperature changes how your body uses fuel. You burn through glycogen faster in the heat than you would in cooler conditions. Unfortunately, heat also tends to suppress appetite and reduce gut tolerance, so the last thing most runners want to do is eat or drink more.

On top of all that, dehydration becomes more of an issue in hot weather (especially when exercising for longer periods of time), because your sweat rate increases, resulting in higher fluid and electrolyte losses. 

The margin for error becomes much smaller in hot conditions, which is why proactive fueling and hydration matter so much more during summer running.

The Three Pillars of Heat Performance for Runners

When it comes to fueling and hydration in hot weather, you have three main priorities:

1. Start Well Fueled and Hydrated

Starting a run even slightly dehydrated can accelerate fatigue and make it harder to stay on top of fluids once you’re already sweating heavily. The same goes for your nutrition. Don’t start already behind! Read this blog for more info.

2. Fuel and Hydrate Consistently While Running

During hot weather long runs, you need a steady intake of carbs, fluids, and sodium to support performance and maintain hydration status. The details of this will of course vary based on the duration and intensity of the run, as well as individual needs. Learn more here.

3. Recover Intentionally

Rehydration after a hot run isn’t just about drinking water. You need to replace fluid and electrolyte losses, and also consume carbs and protein to recover faster and prepare for your next training session. This blog deep dives into recovery post-run.

Alright, now let’s break each of these down.

Pre-Run Hydration: Setting Yourself Up for Success

Good hydration habits start long before your run begins. A simple baseline hydration target for many athletes is: 

• Body weight (lbs) ÷ 2 = ounces of fluid per day

That’s just a rough starting point, though, and does not account for exercise losses. Your total daily fluid needs may increase significantly with hot weather running, high altitude, high training volume, and high sweat rate.

For pre-run hydration, here are some general guidelines:

• Drink 16–24 oz of fluids if you have 2+ hours before you run

• Drink 8-16 oz of fluids if you only have 1 hour before you run

If you’re doing a longer run, running in extreme heat, or you know you’re a particularly salty sweater, you may also benefit from consuming extra sodium and fluids prior to exercise. 

Morning runners especially need to think ahead. If you’re heading out at 6am into rising temperatures, your hydration status the night before matters just as much as whatever you chug while tying your shoes half awake.

Remember, not all hydration has to come from water. It can also come from seltzer, juice, tea, coffee, milk, or whatever beverages you enjoy (except alcohol). 

If you want more detailed hydration guidance, including how to calculate your sweat rate and individual fluid needs, check out my hydration blog post for endurance athletes. And if you want to learn more about what to eat before a run, read this blog.

Dry Heat vs Humid Heat

Not all heat affects the body the same way.

Dry heat like what you may experience in a desert or high altitude environment feels a lot different than trying to run (or even just survive) in a humid climate. I remember the massive struggle of summer running back when I lived in NYC, training for my fall marathons in the oppressive humidity. But both types of heat present challenges.

Dry Heat (Low Humidity)

In dry environments, sweat evaporates quickly, which helps cool the body effectively.

But you may be sweating far more than you realize because it evaporates so fast. Many runners underestimate fluid losses in dry climates because they never actually feel drenched in sweat.

This means dehydration can sneak up on you… fast.

Key strategies for dry heat running:

• Do a sweat test in similar conditions, if possible, to estimate your fluid losses

• Stay proactive with hydration instead of relying on thirst alone

• Watch for dehydration signs like headache, dark urine, dizziness, or unusual fatigue

• Make sure you're taking in appropriate amounts of sodium during longer efforts

• Practice cooling strategies, as needed (see below)

Training tip: If you know you’re going to be running or racing in a hot climate, heat training is an effective strategy. This can mean sauna use, as well as practicing running, fueling, and hydrating in the heat to acclimate your body and figure out what works best for you. 

Humid Heat

Humidity is where things get tricky and extra uncomfortable. When the air is already saturated with moisture, sweat can’t evaporate effectively. Your body keeps producing sweat in an attempt to cool itself, but the cooling mechanism just doesn’t work nearly as well.

That’s why the heat index (“feels like” temperature) matters more than the actual air temperature.

In humid conditions:

• Overheating risk increases significantly

• GI issues tend to become more common

• Pacing adjustments (aka… slowing down) are absolutely necessary

• Cooling strategies become critical

External cooling can really help, including:

• Dumping water over your head/body

• Ice bandanas

• Putting ice in handheld bottles or hydration vests

• Using ice slurries before or during races

An ice slurry (basically a sports drink slushy) can help lower core temperature more effectively than cold liquids alone. Small sips usually work best unless you enjoy fighting a brain freeze. You may also tolerate more liquid calories than solids or thick gels in humid conditions, especially during longer efforts.

Always watch for signs of heat illness closely, especially in hot, humid conditions. 

During Your Run: The Non-Negotiables

Fluids

Fluid needs while running in the heat vary widely, but many runners can aim for 12-28 oz per hour (heavy sweaters may need even more).

Running generally comes with lower fluid tolerance than cycling because of the mechanical impact and GI stress involved. Slamming a huge bottle all at once while running rarely ends well. Smaller, more frequent sips tend to work much better.

Your goal during longer runs is to replace 50–90% of fluid losses, depending on duration, intensity, and conditions. The longer the run, the higher percentage of losses you need to replace (but never 100%).

And yes, this is where sweat testing becomes very helpful because it gives you a better idea of your actual hydration needs. 

Sodium

Sodium plays a critical role in helping your body maintain fluid balance. It also helps prevent hyponatremia during exercise, which is when you have abnormally low sodium concentration in your blood. 

Consuming sodium becomes particularly important when it's hot out and when you're running for longer periods of time, both of which lead to increased sodium losses in sweat. A general starting point for endurance athletes is ~500–800mg sodium per liter of fluid. Some athletes need more, some need less.

Note that this range refers to sodium concentration not an hourly goal, because you always need to think of sodium relative to fluid.

Signs you may be a salty sweater include:

• White residue on clothes or hats

• Sweat stinging your eyes

• Frequent cramping

• Struggling excessively in the heat

At the same time, more sodium isn’t always better. Popping salt pills like they’re candy without a clear plan can also cause problems, specificaly upper GI symptoms like nausea and vomiting.

Aim for intentional, individualized sodium intake based on your unique sweat losses and experience.

Carbohydrates

Heat increases carbohydrate utilization, which means your glycogen stores get depleted faster.

But here’s the catch… increased carb needs don’t necessarily mean increased carb tolerance.

General carbohydrate guidelines during running are:

• 30–60g per hour for runs lasting 1-2.5 hours

• 60–90g+ per hour for runs > 2.5 hours

Sports drinks can work especially well in the heat because they provide carbohydrates, fluids, and sodium all in one system. Many runners also find that liquid calories are easier to tolerate than chews or solid foods in very hot conditions.

Not all sports drinks are created equal, and understanding the difference matters more in the heat (more on that below).

The key is practicing your fueling strategy in the heat during training, not just during comfortable spring runs when everything feels easy and your stomach is behaving.

Understanding Osmolality: Why Product Concentration Matters

When choosing what to drink when running in the heat, it helps to understand a concept called osmolality, or how concentrated a drink is relative to your blood. This affects how quickly fluid is absorbed and how well your gut tolerates it, which matters even more when heat is already making digestion less cooperative.

Sports drinks generally fall into three categories:

• Hypotonic drinks are less concentrated than blood, so they’re absorbed quickly and work best when hydration is the priority. These are typically lower in carbs and sodium, like electrolyte tabs mixed into water. They pair well with separate carb sources like gels or real food.

  
  

• Isotonic drinks are similar in concentration to blood and provide a balance of fluids, carbs, and sodium. Products like Skratch Labs Sport Hydration mixes, Gatorade Endurance, and Tailwind Endurance Fuel are common examples. How you mix these products matters, because it impacts the tonicity of the drink.

  
  

• Hypertonic drinks are more concentrated than blood. Examples include high-carb mixes from Maurten, Tailwind, or Skratch. These can be very useful for maximizing carb intake, but your body has to pull water into the intestine to dilute them. That can worsen dehydration and increase the risk of GI distress, especially if you’re not drinking enough fluid alongside them.

One more important thing to know: sodium doesn’t just replace sweat losses. Sodium and glucose work together to help move fluid from the gut into the bloodstream, which improves fluid absorption.

The practical takeaway: isotonic drinks are often the most versatile starting point because they provide hydration, sodium, and carbs together. If you’re using a more concentrated high-carb drink, make sure you’re also getting enough fluids. And if GI distress is already an issue in the heat, dialing back to a hypotonic drink and getting carbs separately may help settle things down.

For more guidance on what to eat and drink during long runs, check out this blog post. 

Road vs Trail Running in the Heat

Road Running in Hot Weather

Road running often means:

• More consistent pacing

• Easier prediction of sweat rate and fuel needs

• Greater heat radiating off pavement = core temp rises faster

• Potentially less shade = higher heat exposure

• Higher sustained effort with fewer natural breaks compared to trail running

Road runs can still mean 3–5+ hours on your feet, especially during summer training cycles or hot marathons. Even though aid stations and water access may be more predictable, the environmental heat load can still be relentless.

Another challenge is that runners often try to maintain pace targets despite rising temperatures. This makes overheating happen faster if you don’t adjust appropriately.

Hydration logistics can sometimes be easier if you’re running loops or have regular access to fluids at aid stations, but carrying enough fluid, sodium, and carbs still requires planning, especially during long runs in the heat.

Key Strategy: Know your hourly fluid, sodium, and carb needs ahead of time, plan hydration access points, and adjust pace expectations based on conditions, not just your goal pace.

Trail and Ultra Running in Hot Weather

Trail running is a completely different beast. Terrain changes, climbing, altitude, technical footing, and variable weather can all affect your effort level, sweat rate, and fueling tolerance.

Depending on what you’re doing, you may encounter:

• Longer stretches without water and fuel access

• Rapid changes in temperature

• More variable pacing

• Greater overall energy demands

If you’re in the mountains, you have the extra challenge of elevation changes, which affect effort, gut tolerance, and temperature. Depending on the terrain, you may potentially have more shade, which can help relieve you from the intense heat and sun for a bit.

Key Strategy: Carry more than you think you need. Fuel and hydrate based on effort and conditions, not just elapsed time.

Sea Level vs Altitude

Altitude adds another layer of complexity.

At higher elevations (>8,000ft):

• Baseline fluid needs increase

• Breathing rate increases due to less oxygen being available, so you have more respiratory fluid loss

• Urine production often increases

• Recovery may take longer

Combine altitude and heat, and the risk of dehydration skyrockets. 

Key Strategy: Trail runners racing or training at altitude in hot weather should be especially diligent about fueling and hydration. Here are some tips:

• Take frequent sips and bites to help get in adequate fluids and carbs while balancing potentially reduced gut tolerance 

• Adjust pacing as needed

• Include extra recovery hydration

• Plan for long stretches between aid stations/water sources

Even if the temp drops, you can expect your sweat rate to increase with elevation, so keep that in mind. 

Running vs Other Sports: Why Runners Have It Harder

Runners often compare fueling strategies with cyclists and wonder why their stomach can’t tolerate the same intake.

Well… when you’re on a bike, the ride is smoother. When you’re running, your gut is getting jostled around like a washing machine.

Running creates significantly more mechanical stress on the digestive system than cycling or swimming. Add heat-related blood flow shifts on top of that, and it’s no surprise that runners often tolerate less fluid and carbohydrate intake overall.

Liquid calories also tend to work better for runners in extreme heat because they empty from the stomach more efficiently and reduce chewing/solid food burden.

Bottom line: Don't compare your fueling capacity to cyclists. Start conservatively and adjust based on what your body can handle while running in hot conditions.

Common Heat Running Mistakes & How to Fix Them

Mistake: Drinking only plain water without electrolytes on long hot runs

Fix: Add sodium, starting with a concentration of ~500-800mg sodium per liter of fluid, to help replace sodium losses and retain the water you consume.

Mistake: Popping salt pills like they’re candy

Fix: Be intentional with your sodium intake, and remember, too little and too much sodium can lead to problems.

Mistake: Waiting until you feel thirsty to drink

Fix: Drink on a schedule during runs. Thirst means you’re already behind. 

Mistake: Trying new fueling strategies on race day

Fix: Practice your heat fueling plan in training runs.

Mistake: Skipping post-run rehydration

Fix: Aim to replace about 125–150% of fluid losses within 4 hours after your run (roughly 20–24 oz per lb lost).

Mistake: Running the same pace in heat as in cool weather

Fix: Your GPS watch doesn't know it's 90 degrees out. You do. Slow down.

Your Hot Weather Running Action Plan

Before your next hot run, run through this checklist. Your future self will thank you!

• Know your sweat rate (do a test on a hot run) 

• Calculate your hourly fluid and carb goals

• Determine your sodium strategy

• Practice fueling in hot conditions during training runs

• Develop a pre-run hydration and fueling routine

• Practice cooling strategies (e.g. ice bandana, ice in bottles, dousing with water)

• Plan your post-run recovery (fluid + sodium + carbs + protein)

• Adjust pacing expectations for heat

Need Help Fueling In the Heat?

Running in the heat doesn’t have to derail your training or performance, but it does require a more intentional approach to hydration and fueling.

Small adjustments, like improving pre-run hydration, increasing sodium intake, switching to more liquid calories, or using cooling strategies, can make a huge difference in how you feel and perform.

And this post is really just scratching the surface.

If you’re looking for more personalized fueling and hydration strategies for hot weather running, I’d love to work with you!

Here’s what others are saying: 

"Claire is amazing! We worked together for 4 months during the build up to a race and I couldn’t be happier with how it all went. Claire helped me with all of my nutrition issues and needs, coached me through how to approach my diet in a more positive and healthy way, and was always available to chat and answer my questions. I would 100% recommend folks do the 3-month 1:1 program. It’s definitely worth it! Thank you, Claire!!" - João C.

"My initial goals included prepping for a half marathon, and adjusting my general nutrition. I was super well prepared for the half I ran, and have run much longer races (and long runs) with total confidence in how I'm fueling and hydrating. Learning about sodium in particular was a complete game changer and something I would have never thought about.” - MB

Ways to Get Support

Here are a few ways I can help, depending on where you're at:

• Join Fuel for Life Crew and get access to my full masterclass on this topic where I break down exactly how to build your individualized plan. You'll also get direct access to me for questions in our private Slack community.

• Download my Mini Guide Bundle, which includes a Hydration mini guide with a sweat test worksheet to help you start building your individualized hydration plan.

• Fill out a new client inquiry form for in-depth 1:1 nutrition coaching.