If It's Hot Outside, Should You Eat More Carbs?

Summer racing season is here.

Whether you're lining up for a marathon, ultramarathon, gravel race, triathlon, or long training run, you've probably heard the same advice:

"Take in plenty of carbohydrates."

It's good advice...most of the time.

But a brand-new systematic review published in the Journal of the International Society of Sports Nutrition asked an important question:

Does carbohydrate supplementation actually improve endurance performance when you're exercising in the heat?

The answer?

Sometimes but not nearly as consistently as most people think.

Here's what the science says.

Why Is Exercising in the Heat So Much Harder?

Before talking about carbohydrates, it's important to understand why hot weather hurts performance in the first place.

When temperatures rise, your body has to balance two competing priorities:

• Deliver blood to your working muscles
• Deliver blood to your skin to help cool you

At the same time, you lose fluids through sweat, your heart works harder, and your core temperature continues climbing.

The result?

• Earlier fatigue
• Higher perceived effort
• Faster glycogen use
• Reduced endurance performance

In fact, marathon performance has been shown to decline by roughly 15% at temperatures around 26°C (79°F) compared to ideal racing temperatures near 6–10°C (43–50°F).

Heat is simply one of the biggest performance limiters endurance athletes face.

The Big Question

Researchers wondered whether consuming carbohydrates during exercise could offset some of these negative effects.

After all, carbohydrates:

• maintain blood glucose
• preserve liver glycogen
• improve endurance performance in normal temperatures
• reduce fatigue during prolonged exercise

So shouldn't they help even more in the heat?

That's exactly what researchers investigated.

What This New Review Looked At

Researchers analyzed:

• 9 randomized crossover studies
• trained endurance athletes
• exercise lasting roughly 50 to 152 minutes
• temperatures ranging from 27–35°C (81–95°F)

Carbohydrate intake ranged from:

14 grams/hour all the way to 140 grams/hour.

The Results Were Surprisingly Mixed

Five studies found carbohydrates significantly improved performance.

Four studies found no significant benefit at all.

Overall, researchers concluded:

Carbohydrate supplementation during endurance exercise in the heat does not consistently improve exercise performance.

That doesn't mean carbohydrates don't work.

It means heat changes the equation.

Why Aren't Carbs as Effective in the Heat?

Several factors appear to be involved.

1. Heat Changes What Causes Fatigue

During normal conditions, glycogen depletion is one of the primary reasons athletes slow down.

In hot conditions, however, fatigue becomes much more complex.

Performance can now be limited by:

• rising core temperature
• dehydration
• cardiovascular strain
• central nervous system fatigue
• gastrointestinal distress

Carbohydrates only address one piece of that puzzle.

2. Your Gut Doesn't Like Heat

One of the biggest takeaways from this review wasn't about muscles.

It was about your digestive system.

Heat reduces blood flow to the intestines, slows gastric emptying, and increases the risk of gastrointestinal distress.

That means consuming large amounts of carbohydrate becomes harder to tolerate.

Ironically, taking in too many carbohydrates may actually hurt performance if your gut can't handle them.

3. More Isn't Better

One study used an extremely concentrated carbohydrate drink providing about 140 grams per hour.

Instead of improving performance...

Performance actually got worse.

Researchers suspect excessive carbohydrate intake overwhelmed the gut, increasing gastrointestinal discomfort.

This reinforces what sports nutrition research has shown for years:

The maximum amount you can absorb isn't necessarily the maximum amount you should consume.

What About Hydration?

Here's another surprising finding.

Researchers found carbohydrate drinks generally did not improve hydration status compared to placebo.

Most athletes assume sports drinks hydrate better simply because they contain carbohydrates.

In reality, hydration depended much more on:

• drinking enough fluid
• replacing sodium losses
• managing sweat rate

Carbohydrates weren't the deciding factor.

Carbs Didn't Lower Core Temperature Either

Another common misconception is that fueling helps athletes stay cooler.

It doesn't.

Across nearly every study, carbohydrate intake had no meaningful effect on core body temperature during exercise.

Your cooling strategy still matters far more than your fueling strategy.

So Should You Still Take Carbs?

Absolutely.

This review isn't saying carbohydrates are useless.

Far from it.

Instead, it suggests athletes should have realistic expectations.

Carbohydrates remain one of the most effective performance-enhancing nutritional strategies available.

However...

When temperatures rise, they become only one piece of a much larger performance puzzle.

Practical Recommendations for Endurance Athletes

Instead of simply increasing carbohydrate intake because it's hot outside, consider this approach.

Before Exercise

• Begin well hydrated.
• Eat a carbohydrate-rich meal 2–4 hours before training or racing.
• Avoid trying anything new on race day.

During Exercise

For sessions lasting longer than about 90 minutes:

• Aim for roughly 30–60 g/hour of carbohydrate for most athletes.
• For highly trained athletes who have practiced gut training, higher intakes (up to ~90 g/hour using multiple transportable carbohydrates) may be appropriate based on established sports nutrition guidelines.
• Avoid dramatically increasing carbohydrate intake simply because temperatures are higher.
• Pair carbohydrate intake with adequate fluids and sodium replacement.

Train Your Gut

One of the strongest practical messages from this review is the importance of gut training.

Practicing your race-day fueling strategy during long training sessions may improve carbohydrate absorption while reducing gastrointestinal distress during competition.

The Bottom Line

Carbohydrates remain an essential fuel source for endurance athletes.

But the latest evidence suggests they aren't a magic solution for exercising in the heat.

When temperatures climb, successful performance depends on much more than fueling alone.

The biggest priorities become:

• staying hydrated
• managing core temperature
• acclimating to the heat
• practicing your race-day nutrition
• training your gut to tolerate carbohydrates

Carbohydrates still matter.

They just aren't enough by themselves to overcome the physiological stress of hot-weather endurance exercise.

References

1. Journal of the International Society of Sports Nutrition. Salamea A, Brown D, Oueijan K, McCullough D. Carbohydrate supplementation for endurance exercise in the heat: a systematic review with practical recommendations. 2026.
2. American College of Sports Medicine. Thomas DT, Erdman KA, Burke LM. Nutrition and Athletic Performance. 2016.
3. International Society of Sports Nutrition. Jeukendrup AE, Moseley L. Multiple transportable carbohydrates enhance gastric emptying and fluid delivery. 2010.
4. Costa RJS et al. Gut-training: the impact of repetitive gut challenge during exercise on gastrointestinal status and running performance. 2017.