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Nutrition

Ketone Drinks in the Heat: Longer Endurance and Sharper Memory?

Dr. Gymbro · · 4 min read
#supplements #endurance #caffeine #cognitive performance #ketones #ketone monoester #heat stress

What if the thing that helps you push through a brutal hot-weather session isn't another carb gel, but a ketone drink? A new crossover trial in the Journal of the International Society of Sports Nutrition suggests ketone monoester may help in exactly that setting. The study is small, and the headline numbers come with caveats worth knowing before you buy anything.

What the researchers did

A team led by Lints and Arent at the University of South Carolina tested 17 endurance-trained men (average age 23.8 years, VO2max 58.6 ml/kg/min). The design was randomized, double-blind, counterbalanced and crossover, so every participant did both conditions.

In each visit, athletes took 4 mg/kg of caffeine combined with one of two things:

  • KET: a ketone monoester, (R)-3-hydroxybutyl-(R)-3-hydroxybutyrate.
  • CHO: an energy-matched carbohydrate control (Cluster Dextrin).

They then completed 90 minutes of loaded treadmill exercise at 34 °C and 45% relative humidity, followed by a high-intensity time-to-exhaustion (TTE) test. The motivation was military: special operations forces must stay mentally sharp and physically capable in extreme heat, when cerebral blood flow, fuel availability and thermal tolerance all suffer.

The headline: nearly two extra minutes to exhaustion

Time to exhaustion was longer with ketones: 8.9 ± 4.6 minutes versus 7.0 ± 2.1 minutes (p = 0.04). That is roughly 1.9 minutes, or about a quarter longer, at the end of a hard, hot session. For a final push or a sprint to the finish, that is a meaningful gap.

The result is also statistically modest. The p-value sits just under 0.05, and the ketone condition had a wide spread (a standard deviation of 4.6 minutes), which suggests individuals responded very differently. Treat it as a promising signal rather than a guarantee.

The brain finding: better 1-back, but not everything

The cognitive results were selective. Compared with carbohydrate, ketones produced:

  • Higher 1-back total accuracy: +2.8% (p = 0.001).
  • Higher 1-back target accuracy: +8.9% (p < 0.001).
  • Better target discrimination (d'): +0.3 (p = 0.001).

These effects held across all timepoints. But the benefit did not extend to the harder 2-back task, Dynavision reaction time, or the object hit and avoid task, where no between-condition differences appeared. So ketones did not make participants uniformly sharper. They improved certain aspects of working memory.

The metabolic fingerprint

The blood data show the ketones did what ketones do. Beta-hydroxybutyrate (BHB) was higher by 1.7 mmol/L mid-exercise and 3.5 mmol/L post-exercise (both p < 0.001). Blood glucose was lower in the ketone condition, by 14.7 mg/dL mid-exercise and 27.6 mg/dL post-exercise.

That means the performance and memory edge appeared despite lower circulating glucose, which is a notable observation, since carbohydrate is usually considered the go-to fuel for hard efforts.

What didn't change: heart rate and core temperature

The authors highlight what ketones did not do. Average heart rate showed a main effect of time (lower in the first 45-minute bout than the second) but no effect of condition (p = 0.453). Maximal heart rate during the TTE test did not differ (p = 0.112). Core body temperature rose over time in both conditions and did not differ significantly between them (p = 0.065).

The core temperature p-value is close enough to the threshold that a larger study could clarify it. For now, the takeaway is that ketones did not clearly add to cardiovascular or thermal strain, and the performance gains did not come with a measurable heat penalty.

Limitations you should know about

  • Small sample: only 17 participants.
  • Men only: young, endurance-trained males, so the results may not transfer to women, older adults or less-trained people.
  • Single session per condition: there was no repeated exposure, so we don't know how consistent the effect is.
  • Caffeine in both arms: the study compares ketone plus caffeine against carbohydrate plus caffeine. It cannot say what ketones do on their own or without caffeine.
  • Mixed cognitive results: benefits appeared in 1-back only, not 2-back, reaction time or object hit and avoid.
  • Variable TTE response: the wide spread in the ketone group indicates uneven individual responses.

The authors reported no conflicts of interest.

Practical takeaways

Dose: The abstract confirms caffeine at 4 mg/kg in both conditions, but it does not give a ketone monoester dose, so we won't guess one. Check the full paper (PMCID PMC13378723) for the protocol, and follow the product's labeled serving.

Timing: The abstract does not detail ingestion timing either. The intervention was co-ingested with caffeine before a 90-minute heat session and a hard finishing effort.

Who it might suit: Trained athletes and tactical personnel who work hard in the heat and want to preserve working memory and a finishing kick. Treat it as something to trial in training, not on race day.

Who should skip it, or be cautious:

  • People sensitive to caffeine, since 4 mg/kg is a substantial dose.
  • Anyone with a medical condition, anyone pregnant, or anyone on medication affecting glucose. Ask a clinician first, particularly because ketones lowered blood glucose in this study.
  • Anyone expecting a cognitive boost across the board. The data don't support that.
  • Athletes prone to stomach upset. Gastrointestinal side effects of ketone products are commonly reported, though this abstract doesn't address them.

Bottom line

In this small, well-controlled trial, ketone monoester plus caffeine beat carbohydrate plus caffeine for time to exhaustion and one working-memory task after prolonged hot exercise, without measurable differences in heart rate or core temperature. It's an intriguing signal, particularly for heat-stressed performance, but it needs replication in larger and more diverse groups before it is more than a promising experiment.

Original study

Read the original research on pubmed.ncbi.nlm.nih.gov →

This article summarizes published research for general information. It is not medical advice.

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Dr. Gymbro

Dr. Gymbro writes for Dr. Gymbro: science-backed fitness, nutrition & performance.

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