Ketone drink stretched time to exhaustion in the heat

Heat degrades both body and brain: hyperthermia is linked to reduced cerebral blood flow, less available substrate, lower thermal tolerance and greater cardiovascular strain, and thinking clearly under those conditions matters for anyone working hard in hot environments. Ketone monoesters raise circulating beta-hydroxybutyrate, an alternative fuel that has shown cognitive benefits in other settings, but whether that translates during exercise in the heat had not been tested. This trial asked whether adding a ketone monoester to caffeine could protect working memory, reaction time and endurance capacity when heat stress is already doing damage.

Seventeen endurance-trained men (mean age 23.8 years, VO2max 58.6 ml/kg/min) completed a randomised, double-blind, counterbalanced crossover study. On each visit they ingested 4 mg/kg caffeine plus either a ketone monoester or an energy-matched carbohydrate control, then performed 90 minutes of loaded treadmill exercise at 34 °C and 45% relative humidity, followed by a high-intensity time-to-exhaustion test. Cognitive testing covered 1-back and 2-back working memory, Dynavision reaction time and an object hit and avoid task. Blood metabolites, average heart rate and core body temperature were tracked before, during and after exercise.

On the ketone condition, 1-back total accuracy was 2.8% higher and target accuracy 8.9% higher than carbohydrate across timepoints, with better overall target discrimination (d′ +0.3). The harder 2-back task showed no between-condition differences, and neither did reaction time or the object hit and avoid task. Blood beta-hydroxybutyrate rose by 1.7 and 3.5 mmol/L at mid- and post-exercise on ketones, while blood glucose fell by 14.7 and 27.6 mg/dL at those points. Time to exhaustion was longer with ketones (8.9 ± 4.6 versus 7.0 ± 2.1 minutes). Average heart rate, maximal heart rate and core temperature did not differ between conditions.

This is the first look at ketone monoesters during exercise-heat stress, and it complicates the simple story in both directions. The cognitive benefit was selective — present on the easier memory task, absent on the harder one and on reaction-based measures — which argues against a blanket claim about mental performance in the heat. That performance improved without any change in heart rate or core temperature suggests the effect was not thermoregulatory or cardiovascular, though the mechanism is not identified here. Whether the substantial drop in blood glucose matters over longer or repeated exposures remains open.

For a hot, prolonged, load-carrying effort, this trial is a reason to think ketone monoesters plus caffeine may support some aspects of thinking and a short subsequent hard effort. It is not evidence that they make heat safer: core temperature climbed the same way in both conditions. The participants were young, highly trained men tested in a lab, so translation to recreational exercisers in real weather is speculative. Note too that the ketone was always paired with caffeine, so the two cannot be separated from these data.