Ketones Boost Exercise Performance in Heat

Ketone monoester and caffeine supplementation improved working memory accuracy and extended high-intensity endurance by nearly two minutes during exercise in extreme heat. This new finding comes from a study published in the *Journal of the International Society of Sports Nutrition* by researchers at the University of South Carolina [PMID: 42460838](https://pubmed.ncbi.nlm.nih.gov/42460838). The work examines whether ketone bodies, an alternative fuel source, can protect cognitive and physical performance under the dual stressors of heat and exertion.

How the Study Was Conducted

Seventeen endurance-trained men completed a randomized, double-blind crossover trial. Each participant underwent two experimental sessions in a hot environ (iron bisglycinate)ment (34°C, 45% relative humidity). Before each session, they ingested 4 mg/kg of caffeine combined with either a ketone monoester (KET) or an energy-matched carbohydrate control (CHO).

The protocol was demanding: 90 minutes of loaded treadmill exercise in the heat, followed immediately by a high-intensity time-to-exhaustion (TTE) test. Researchers measured cognitive performance using tasks like the 1-back and 2-back (tests of working memory), Dynavision reaction time, and an object hit-and-avoid task. They also tracked blood metabolites (beta-hydroxybutyrate [BHB] and glucose), average and maximal heart rate, and core body temperature at multiple points before, during, and after exercise.

A Clear Boost in Simple Working Memory

The cognitive results were specific. For the simpler 1-back task, ketone ingestion led to significantly greater total accuracy (+2.8%, p=0.001) and target accuracy (+8.9%, p<0.001) across all timepoints compared to the carbohydrate drink. It also improved overall target discrimination. “This suggests ketones provided a more stable substrate for basic cognitive processing under stress,” said lead author Blaine S. Lints.

However, this benefit did not extend to more complex tasks. No differences were seen in performance on the harder 2-back task, nor in general reaction time or the object hit-and-avoid outcomes. The cognitive boost appears confined to fundamental working memory operations.

Metabolic Shift and Performance Extension

The blood data confirmed a dramatic metabolic shift. At mid- and post-exercise, circulating BHB (ketone) levels were +1.7 and +3.5 mmol/L higher in the KET condition, while blood glucose was correspondingly lower by 14.7 and 27.6 mg/dL. The body was clearly utilizing the supplemental ketones as fuel.

This shift translated into a direct physical performance advantage. The time-to-exhaustion in the final high-intensity test was significantly longer following ketone ingestion: 8.9 ± 4.6 minutes versus 7.0 ± 2.1 minutes with carbohydrate (p=0.04). This represents a 27% extension in endurance capacity at the end of a grueling heat-stress protocol.

A notable finding was that this performance gain occurred without changes in cardiovascular or thermal strain. Average heart rate, maximal heart rate during the TTE, and core body temperature did not differ between conditions. The benefit likely originated from improved central (brain) function or enhanced muscular fuel efficiency.

Practical Implications for Endurance Athletes

This study offers a targeted application for endurance athletes facing extreme conditions. Ketone monoester supplementation, combined with a moderate dose of caffeine, may help preserve simple decision-making accuracy and extend high-intensity endurance when exercising in the heat. This could be relevant for marathon runners, cyclists, or military personnel operating in hot environments.

The selective cognitive benefit is important. It suggests ketones aid in maintaining baseline focus, but won’t necessarily improve complex problem-solving under stress. For athletes, this could mean better adherence to a race plan or more consistent pacing decisions, rather than improved tactical analysis.

The performance extension is compelling, but the context is specific: it followed 90 minutes of loaded exercise in the heat. It remains to be seen if similar benefits occur in cooler environments or during different exercise durations. The study also aligns with broader research on how fuel availability affects performance. For instance, our article on Elite Rowers’ Glucose Patterns explores how metabolic fuel management is critical in endurance sports.

Connections to Metabolic Fitness

The study reinforces the principle that metabolic flexibility—the ability to efficiently use multiple fuel sources—is a cornerstone of endurance. Ketones represent an alternative pathway that, in this case, spared glucose and supported performance. This concept is central to Zone 2 Training Boosts Mitochondrial Health, which details how low-intensity aerobic work enhances the cellular machinery that processes fuels.

Furthermore, the brain-performance link observed here resonates with other research on how exercise metabolites affect cognition. Our article on Lactate and Brain Health in Zone 2 Training examines a different metabolite’s role in cognitive function, highlighting the multifaceted relationship between endurance physiology and the brain.

A Note on Specificity and Future Research

The authors, including Riccardo F. Romersi and Jenica N. Earl, caution that the effects were not universal. Benefits were seen in simple working memory and time-to-exhaustion, but not in other cognitive metrics. This specificity is key for practical application. The combination also included caffeine, a known performance enhancer, so the exact contribution of ketones alone requires further study.

For the informed athlete, this research provides a data point suggesting ketone monoesters could be a useful tool for extreme heat-stress endurance scenarios. It underscores that performance optimization often involves fine-tuning fuel availability, especially when the brain and body are under environmental pressure.