Can Ketones Help You Perform Better in the Heat?

Heat changes everything.

When you exercise in hot conditions, your body has to do two jobs at once. It has to continue delivering oxygen and fuel to working muscles while also sending more blood toward the skin to help get rid of heat. As core temperature rises, cardiovascular strain increases, cerebral blood flow can fall, and cognitive performance can begin to deteriorate.

That makes prolonged exercise in the heat especially difficult for endurance athletes, military personnel, firefighters, and anyone expected to make good decisions while physically exhausted.

A new 2026 study published in the Journal of the International Society of Sports Nutrition looked at whether ketone monoester supplementation combined with caffeine could help preserve both cognitive and physical performance during prolonged exercise in the heat.

The results were surprisingly interesting.

After 90 minutes of loaded treadmill exercise at 34°C, or about 93°F, participants who consumed ketone monoester plus caffeine lasted significantly longer during a high intensity time to exhaustion test than when they consumed an energy matched carbohydrate drink plus caffeine. They also performed better on selected measures of working memory.

But there is an important catch.

The ketones did not lower heart rate or core temperature.

So this doesn't appear to be a cooling supplement.

It may instead help the brain and body continue performing despite the heat stress.

TLDR: Do Ketones Improve Performance in the Heat?

This small but well controlled study suggests ketone monoesters combined with caffeine may help preserve certain aspects of cognitive performance and high intensity endurance after prolonged exercise in hot conditions. The results are promising, but they are preliminary and should not be interpreted as proof that ketones are a general purpose heat performance supplement.

The main points:

• Researchers studied 17 endurance trained men with an average VO₂ max of approximately 58.6 mL/kg/min.
• Participants performed 90 minutes of loaded treadmill exercise at 34°C and 45% humidity.
• Both trials included 4 mg/kg of caffeine.
• Participants received either ketone monoester or an energy matched carbohydrate control.
• The ketone condition used two 25 gram doses, for a total of 50 grams of ketone monoester.
• Time to exhaustion after the heat protocol averaged 8.9 minutes with ketones versus 7.0 minutes with carbohydrate, an improvement of roughly 27%.
• About 53% of participants showed a worthwhile improvement in time to exhaustion with ketones.
• Ketones improved selected measures of working memory and target discrimination.
• They did not improve every cognitive outcome.
• Core temperature and heart rate were not lower with ketones.
• The study therefore suggests ketones may help preserve performance under severe physiological stress rather than reduce the heat strain itself.

 

Why Exercise in the Heat Is So Difficult

When you're exercising in cool conditions, the cardiovascular system primarily has to supply working muscles with enough oxygen and nutrients to maintain the workload.

Heat adds another major demand.

As body temperature rises, blood flow to the skin increases so heat can be transferred away from the core. That creates competition for blood flow between working muscles, the skin, and the brain.

As the authors explain, increasing heat strain can make it harder to maintain blood pressure, muscle blood flow, cerebral blood flow, and cooling at the same time. Previous research cited in the paper has reported roughly an 18% reduction in global cerebral blood flow during exercise heat stress compared with similar exercise in normal temperatures.

That can contribute to reductions in short term memory, information processing, attention, and decision making.

For an endurance athlete, that might mean poor pacing decisions, forgetting to fuel, missing course markings, or simply losing the mental drive to continue pushing.

For military personnel, the consequences can obviously be much more serious.

Why Ketones Might Help

Most of us think about glucose as the brain's primary fuel.

That's generally true.

But the brain can also use ketone bodies for energy.

Ketones are normally produced by the liver when carbohydrate availability is low. Exogenous ketone supplements allow blood ketone concentrations to increase rapidly without fasting or following a ketogenic diet.

The ketone used in this study was a ketone monoester, specifically R 3 hydroxybutyl R 3 hydroxybutyrate.

Once consumed, it dramatically increases circulating beta hydroxybutyrate, commonly abbreviated BHB.

The authors point out that ketones may be particularly interesting under conditions where glucose availability, cerebral blood flow, or metabolic stress is compromised. Previous human studies cited in the paper have also shown increases in cerebral blood flow alongside elevated BHB concentrations.

In theory, ketones may provide the brain with another usable energy substrate at exactly the time heat and prolonged exercise are making normal energy delivery more difficult.

What Did the Researchers Actually Do?

The study included 17 healthy, endurance trained men between 18 and 35 years old. Participants had to have a VO₂ max above 55 mL/kg/min, and the group's average VO₂ max was approximately 58.6.

This wasn't a sedentary population thrown onto a treadmill.

These were legitimately fit participants accustomed to regular endurance and resistance training.

The researchers used a randomized, double blind, counterbalanced crossover design. That means every participant completed both experimental conditions, allowing the researchers to compare each athlete against himself.

During both trials, participants consumed 4 mg/kg of caffeine. The difference was what they consumed alongside it: ketone monoester or an energy matched carbohydrate control.

This point matters.

The study did not compare ketones with nothing.

It compared:

Ketones + caffeine

versus

Carbohydrate + caffeine.

The Heat Protocol Was No Joke

Participants first consumed a standardized breakfast and 4 mg/kg of caffeine.

About 45 minutes later, they consumed their first 25 gram dose of ketone monoester or carbohydrate.

Then the real fun started.

They completed two 45 minute treadmill bouts inside a heat chamber set to:

34°C, approximately 93°F

and

45% relative humidity.

They walked or ran at 50% of their individual velocity at VO₂ max while wearing a weighted vest equal to 20% of their body weight.

After the first 45 minutes, they completed cognitive testing and consumed another 25 gram dose of ketone monoester or carbohydrate. Then they went back into the heat for another 45 minutes.

After the full 90 minutes, they removed the weighted vest and performed a high intensity time to exhaustion test at 90% of their velocity at VO₂ max.

That is a pretty serious amount of physiological stress.

And that's what makes the study interesting.

This wasn't testing whether ketones helped someone run a fresh 5K in an air conditioned lab.

The researchers deliberately tried to create a state of heat stress, fatigue, and cognitive strain.

The Biggest Finding: Time to Exhaustion Improved

After 90 minutes in the heat, participants lasted:

8.9 ± 4.6 minutes with ketones

versus

7.0 ± 2.1 minutes with carbohydrate.

That difference was statistically significant.

The authors describe this as approximately a 27% longer time to exhaustion in the ketone condition.

That sounds huge.

And percentage wise, it is.

But some context is important.

The final test lasted only several minutes. We're talking about roughly 1.9 additional minutes of high intensity running after 90 minutes of loaded exercise in the heat.

That's still meaningful.

But don't turn a 27% improvement in a short laboratory time to exhaustion test into a claim that ketones will make you 27% faster in a marathon.

That's not what this study showed.

Not Everyone Responded the Same Way

The individual data are particularly interesting.

The paper's Figure 5 shows every participant's time to exhaustion in both conditions, and the responses are all over the place.

Nine of the 17 participants, or about 53%, improved by more than the researchers' threshold for a meaningful change.

Four participants got meaningfully worse.

And four had no meaningful difference.

This is a great reminder that average study results don't mean everyone responds the same way.

Some people appeared to respond very well to ketones.

Others didn't.

And a few actually performed worse.

That variability is important if you're considering ketones for real world use.

Ketones Also Helped Some Cognitive Measures

The researchers didn't only look at physical performance.

They used several different cognitive tests before, during, and after the exercise protocol.

The clearest benefit appeared in a working memory task called the 1 back test.

Compared with carbohydrate, ketones increased:

Total 1 back accuracy by about 2.8 percentage points

and

Target accuracy by about 8.9 percentage points.

They also improved a measure called d prime, which reflects how well participants could distinguish targets from non targets.

The figure on page 8 makes this easy to see. Participants generally maintained higher 1 back accuracy and target accuracy in the ketone condition compared with carbohydrate.

That suggests ketones may have helped preserve attention and working memory as fatigue and thermal stress accumulated.

But Ketones Did Not Make Everyone Smarter

Again, we need to keep this in perspective.

There was no significant improvement in the more difficult 2 back working memory task.

Ketones also did not improve:

Reaction time.

Word recall.

Dynavision performance.

Or the object hit and avoid task.

 

So this wasn't a global improvement in cognition.

The authors describe the results as selective.

That's the right word.

Certain aspects of working memory and target discrimination improved.

Others didn't.

The Difference May Have Been Preservation, Not Enhancement

There is another subtle point here.

The authors note that ketones may have helped preserve cognitive function rather than actually improve it above normal levels.

That's an important distinction.

Under normal resting conditions, ketones might not make your brain noticeably better.

But under significant stress from heat, exercise, fatigue, reduced cerebral blood flow, and changing substrate availability, an additional energy source might help prevent performance from deteriorating.

The authors specifically suggest that the findings may reflect preservation of sustained attention and working memory rather than a generalized cognitive enhancement effect.

I think that is probably the more useful way to think about ketones in this context.

Blood Ketones Went Through the Roof

The supplement clearly did what it was supposed to do metabolically.

After the first dose, blood beta hydroxybutyrate concentrations were approximately:

2.3 mmol/L with ketones

versus

0.6 mmol/L with carbohydrate.

After the second dose and later exercise, concentrations increased to approximately:

4.0 mmol/L with ketones

versus

0.5 mmol/L with carbohydrate.

 

Those are large differences.

For context, you normally need substantial fasting or carbohydrate restriction to naturally achieve comparable levels of nutritional ketosis.

The ketone monoester did it within hours.

Blood Glucose Dropped

Another interesting metabolic finding was that blood glucose was lower in the ketone condition.

At the midpoint, glucose was approximately 14.7 mg/dL lower with ketones.

By the later measurement, glucose was approximately 27.6 mg/dL lower.

That is consistent with previous ketone research.

But it also complicates the performance story.

The ketone condition had substantially less circulating glucose than the carbohydrate condition and yet still produced better average time to exhaustion.

That supports the possibility that ketones were serving as an alternative energy substrate during a period of significant metabolic stress.

Here's What Ketones Did Not Do: Cool the Body

This part really matters.

Core body temperature rose substantially during the protocol.

By the second 45 minute bout, average core temperature approached roughly 39°C, or around 102°F.

But ketones did not significantly lower it.

Heart rate also increased as exercise continued, which you would expect in the heat.

Again, there was no meaningful difference between ketones and carbohydrate.

So if ketones helped performance, they did not appear to do it by reducing cardiovascular strain or lowering core temperature.

That's why I would not describe ketones as a heat protection supplement.

A better description might be:

A potential performance support strategy during heat stress.

Those are very different claims.

Ketones Didn't Make the Heat Less Stressful

This is probably one of my favorite findings in the paper because it helps clarify the mechanism.

Participants still got hot.

Their heart rates still increased.

They still experienced substantial thermal and cardiovascular strain.

The ketones didn't prevent any of that.

Yet some cognitive measures and time to exhaustion improved anyway.

The authors suggest possible explanations involving brain substrate availability, cerebral blood flow, energetic efficiency, neuroprotection, or the ability to maintain central drive despite hyperthermia.

Those mechanisms are plausible.

But this study did not directly measure cerebral blood flow or brain metabolism.

So for now, they're explanations rather than proof.

The Caffeine Piece Is Important

There is a big detail here that you could easily miss if you only looked at the headline.

Everybody took caffeine.

The dose was:

4 mg/kg.

For a 160 pound athlete, that's roughly 290 mg of caffeine.

For a 180 pound athlete, about 327 mg.

For a 200 pound athlete, about 363 mg.

So the researchers did not demonstrate that ketones by themselves improve heat performance.

They demonstrated that:

Ketones plus caffeine outperformed carbohydrate plus caffeine under these specific conditions.

That matters because there could be an interaction between caffeine and ketones.

The authors even suggest future research should examine whether there is some context dependent synergy between the two during combined metabolic and heat stress.

What About Carbohydrates?

There's another wrinkle.

The control condition used an energy matched carbohydrate supplement.

That means the ketone condition wasn't getting more calories.

But it also means researchers weren't comparing ketones with a typical endurance fueling strategy where athletes might consume substantial carbohydrate throughout the entire exercise bout.

This is important if you're thinking about marathons, ultramarathons, or cycling events.

I would not look at this study and decide:

"Ketones are better than carbohydrates, so I should stop fueling with carbs."

Absolutely not.

The study wasn't designed to answer that question.

In fact, the authors specifically note that future research should explore combined strategies involving ketones, carbohydrate, and caffeine together.

For endurance sport, that's probably the more interesting question.

Could Ketones Be Useful Late in a Hot Race?

This is where I think the research becomes practically interesting.

Imagine you've been running an ultramarathon for several hours in high temperatures.

Your core temperature is elevated.

Your heart rate has drifted upward.

You're mentally tired.

You have been making pacing, fueling, navigation, and hydration decisions for hours.

And eventually, your brain becomes part of the performance limitation.

Under those circumstances, providing another rapidly available cerebral substrate could theoretically be useful.

This study doesn't prove that strategy works in ultramarathons.

But the physiological context is surprisingly similar:

Heat.

Prolonged exercise.

Fatigue.

High cognitive demand.

A final hard effort.

That makes this a study I'd like to see replicated in actual endurance racing.

There Are Major Practical Problems With Ketone Esters

Before everyone starts adding ketone monoesters to their race bag, there are a few realities worth mentioning.

First:

They're expensive.

The authors explicitly mention cost as a limitation to widespread use.

Second, this study used 50 grams of ketone monoester across two doses.

That's a substantial amount.

Third, ketone esters have a notoriously unpleasant taste.

And fourth, gastrointestinal tolerance can vary between athletes.

None of those things make them useless.

But they're not nearly as simple or inexpensive as caffeine, carbohydrates, electrolytes, or many other established performance strategies.

The Study Has Some Important Limitations

The biggest limitation is the sample size.

There were only 17 participants.

The crossover design helps because each participant served as his own control, but 17 people is still a small study.

Every participant was also male.

That makes sense given the original military special operations focus of the research, but it means we shouldn't assume women would respond identically.

Some of the cognitive measures also showed fairly high variability, which makes detecting real differences harder.

And several cognitive outcomes did not actually deteriorate very much during the heat protocol. If the heat isn't significantly impairing a particular task, there is less opportunity for a supplement to preserve it.

Another limitation is that most cognitive testing occurred outside the heat chamber, although participants wore space blankets to reduce cooling during those periods.

Finally, the study did not isolate:

Ketones alone.

Caffeine alone.

Carbohydrate alone.

Or ketones plus carbohydrate.

That makes it impossible to determine exactly how much of the response was driven by ketones themselves versus the combination with caffeine.

This Study Is Actually Different From Much of the Previous Ketone Research

Another reason the findings are interesting is that ketones haven't consistently improved endurance performance in previous studies.

In fact, the authors acknowledge that much of the existing literature has found neutral or even negative effects of exogenous ketones on high intensity endurance performance.

This trial was different because the final performance test occurred after 90 minutes of loaded exercise in significant heat stress.

The authors suggest ketones may be more useful for preserving performance under substantial physiological strain than for making a rested athlete faster.

That distinction makes a lot of sense to me.

Maybe ketones aren't primarily an "exercise fuel" in the traditional sense.

Maybe their most interesting application is when the brain and body are beginning to struggle because of heat, hypoxia, sleep deprivation, or prolonged fatigue.

That is a very different use case.

Does This Mean Endurance Athletes Should Start Taking Ketones?

Not yet.

This study moves ketones into the category of:

Interesting enough to keep watching.

It doesn't move them into:

Everyone should use this.

The established hierarchy for hot endurance events still looks pretty familiar.

Heat acclimation matters.

Hydration matters.

Sodium matters.

Carbohydrate fueling matters.

Pacing matters.

Cooling strategies matter.

Fitness matters.

Ketone esters would sit much farther down that list.

But for athletes who already have the fundamentals dialed in and are looking for small advantages in extreme conditions, this research is genuinely interesting.

The Most Interesting Application May Be Mental Performance

I actually think the cognitive findings might ultimately prove more important than the physical ones.

Fatigue during long endurance events isn't purely muscular.

The brain is making decisions constantly.

How hard should I push this climb?

Should I eat now?

Am I getting dehydrated?

Can I maintain this pace?

Where is the next aid station?

Do I back off or keep pushing?

Heat makes all of those decisions harder.

If ketones can help maintain attention, working memory, and stimulus discrimination during prolonged heat stress, that might have value even if they never become a huge direct ergogenic aid.

That's something worth studying.

The Bottom Line

This is one of the more interesting ketone studies I've seen because the researchers tested them under circumstances where an alternative brain fuel might actually matter.

Seventeen endurance trained men completed 90 minutes of loaded treadmill exercise in 93°F heat, followed by a high intensity run to exhaustion. Everyone took 4 mg/kg of caffeine, while participants received either ketone monoester or an energy matched carbohydrate control.

With ketones, average time to exhaustion increased from 7.0 to 8.9 minutes, roughly a 27% improvement. Selected measures of working memory and stimulus discrimination also improved.

But ketones did not lower core temperature.

They did not lower heart rate.

And they did not improve every cognitive outcome.

That's why I wouldn't call ketones a heat shield.

Instead, the evidence suggests something more subtle and arguably more interesting:

Ketones may help you continue performing when heat and prolonged exercise are already pushing your brain and body toward their limits.

It's early research.

But it's definitely research worth watching.

Reference

Lints BS, Romersi RF, Earl JN, et al. Ketone Monoester Ingestion and Cognitive and Physical Performance During Exercise Heat Stress. Journal of the International Society of Sports Nutrition. 2026;23(1):2695136.