Ketone Esters Boost Performance in Heat

A ketone monoester supplement, taken with caffeine, allowed endurance-trained men to exercise for 27% longer in a high-intensity time-to-exhaustion test following 90 minutes of work in the heat. This performance boost came without a corresponding increase in heart rate or core temperature, according to a new study published in the *Journal of the International Society of Sports Nutrition*.

Testing Ketones Under Heat Stress

Researchers from the University of South Carolina, led by Blaine S. Lints, Riccardo F. Romersi, and Jenica N. Earl, designed the study to address a gap in the evidence. While ketone supplements have shown promise for cognitive support, their effects during the combined strain of exercise and environ (iron bisglycinate)mental heat were unknown. Hyperthermia can impair brain function by reducing cerebral blood flow and increasing cardiovascular strain, creating a challenging scenario for both mental and physical performance.

The team recruited seventeen endurance-trained males with an average VO2 max of 58.6 ml·kg⁻¹·min⁻¹. In a randomized, double-blind crossover design, each participant completed two brutal experimental trials. They ingested 4 mg·kg⁻¹ of caffeine combined with either a ketone monoester (KET) or an energy-matched carbohydrate control (CHO). The trial itself consisted of 90 minutes of loaded treadmill walking in hot conditions (34°C, 45% humidity), immediately followed by a high-intensity time-to-exhaustion (TTE) run. Cognitive tests, blood draws, heart rate, and core temperature were tracked throughout.

Selective Cognitive Boost and Clear Metabolic Shift

The cognitive results were nuanced. The ketone-caffeine combination provided a clear benefit for the simpler 1-back working memory test. Total accuracy was 2.8% higher and target accuracy was 8.9% higher with KET compared to CHO across all time points. The supplement also improved overall signal detection (d’) in the task. However, for the more difficult 2-back test, there was no difference between the ketone and carbohydrate conditions. Reaction time and visual motor performance were also unaffected.

Biochemically, the supplement worked exactly as intended. Blood beta-hydroxybutyrate (BHB) levels—the primary ketone body—were massively elevated in the KET condition, rising by 1.7 mmol·L⁻¹ mid-exercise and 3.5 mmol·L⁻¹ post-exercise. Concurrently, blood glucose was lower by 14.7 and 27.6 mg·dL⁻¹ at those same time points. This confirms the supplement successfully provided an alternative fuel substrate to glucose during the endurance activity.

A 27% Longer Burn-Out with No Extra Strain

The most striking finding was in the performance test. After 90 minutes of grueling work in the heat, the participants who took the ketone monoester and caffeine lasted significantly longer in the all-out time-to-exhaustion run. The TTE duration was 8.9 minutes for KET versus 7.0 minutes for CHO, a 27% improvement.

What makes this result notable is what didn’t change. Despite exercising longer at a high intensity, the athletes’ average heart rate during the 90-minute bout, their maximal heart rate during the TTE test, and their core body temperature were not different between the two conditions. The performance gain did not come at the cost of added cardiovascular or thermal strain. This aligns with other research on how alternative fuels like ketones may influence lactate and metabolic clearance during stress.

Practical Implications for Endurance Athletes

This study offers a specific, evidence-based scenario where a ketone monoester supplement could be beneficial. For endurance athletes facing long training sessions or competitions in hot environments, the combination with caffeine may offer a dual advantage: a modest preservation of certain cognitive skills and a potentially meaningful extension of high-intensity capacity at the end of a race or hard effort.

It is important to note the context. The participants were already highly fit, the heat stress was extreme, and the cognitive benefits were not universal. The supplement is not a magic bullet for all aspects of performance. Furthermore, the protocol used both ketones and caffeine together, making it difficult to separate their individual effects. Caffeine is a well-established performance enhancer on its own, often used in strategies like those discussed in our article on sodium bicarbonate for anaerobic power.

For the general fitness enthusiast focused on Zone 2 training, the direct application may be limited. However, the research contributes to a broader understanding of fuel flexibility and how supporting brain function during physical stress can impact outcomes. It connects to the growing interest in how metabolites like lactate and ketones interact with brain health and endurance.

The Bottom Line

The 2026 study provides strong evidence that ingesting a ketone monoester with caffeine can extend high-intensity endurance following prolonged exercise in the heat, likely by providing an efficient alternative fuel to glucose. The cognitive benefits were real but selective. For athletes operating at the extremes, this could be a valuable tool in a carefully managed nutrition strategy, one that appears to improve output without increasing perceived physiological strain.

Source: Lints BS, Romersi RF, Earl JN, et al. Ketone monoester ingestion and cognitive and physical performance during exercise-heat stress. J Int Soc Sports Nutr. 2026;23(1):2695136. doi:10.1080/15502783.2026.2695136. PMID: 42460838.