Ketone esters: not race fuel, but an adaptation tool?

Ketone esters have been through a full cycle of sporting fashion. First they were sold as a secret race fuel, then study after study failed to find any gain in time trials, and the topic nearly died out. Now it is back from a different angle: maybe we were simply drinking them at the wrong moment. Not before the start, but after training — not as fuel, but as a signal to adapt.

What the first wave got wrong

The logic of “ketones = extra fuel” looked neat: during fasting the liver produces ketone bodies, muscle and brain can oxidise them, so drink an ester and you get one more energy source alongside carbohydrate. In practice it ran into a simple fact — at high intensity the body already works beautifully on carbohydrate, and ketones add little under those conditions while sometimes competing with glycolysis.

The new hypothesis is fundamentally different. Ketone bodies are also signalling molecules. They influence gene expression, the inflammatory response and recovery. If that is the case, the point of taking them is not out on the road but in the recovery window, when the body is deciding which adaptations to lock in.

What the eight-week study showed

Work by Robberechts et al. (The Journal of Physiology, 2026) tested exactly this version. 28 trained male cyclists (VO₂peak around 50–55 ml/kg/min) completed 8 weeks of structured training — two four-week blocks following a “3 weeks of rising load + 1 recovery week” pattern. Half received 25 g of a ketone monoester after every session and another 25 g 30 minutes before bed on training days; the other half got an isocaloric placebo.

The gap that opened up was substantial:

Mean power in the ester group was roughly 4% higher by week seven, and the advantage held after the taper.
Peak power rose by 52 W versus 25 W in the placebo group.
VO₂peak gained 6.4 versus 3.3 ml/kg/min.
Citrate synthase — the classic marker of mitochondrial density — increased more than twice as much.
Complex II protein content in the respiratory chain rose by 25%, while in the control group it did not change at all.

On top of that, the authors recorded enhanced angiogenesis in skeletal muscle and elevated circulating erythropoietin (EPO). So this is not about “pedalling feeling easier” — it is about remodelling the muscle and the oxygen transport system.

Why the rest of the data says something else

An honest caveat is mandatory here, otherwise this turns into an advert.

A systematic review of ketone supplementation (10 studies, 112 participants) examined 16 performance outcomes: 3 positive results, 10 null and 3 negative. In other words, the modal outcome is “no difference”. And a separate paper in the Journal of Applied Physiology (2025) showed that under acute hypoxia a ketone ester actually worsened performance, while affecting neither cognitive function nor EPO levels.

How does that square with the eight-week experiment? Most likely like this: almost all the older literature measured the acute effect on performance — drink it and ride a time trial. The new study measured chronic adaptation to a training block. Those are two different questions, and a “no” to the first says nothing about the second. But the reverse holds too: one study in 28 men is not a verdict in favour of ketones, it is a reason to replicate.

Who this is (not) for

The practical layer, without illusions:

This is not a pre-race supplement. If you are counting on drinking an ester an hour before the start and running faster, the data will not back you. What worked was taking it after training and before bed.
It is expensive. 50 g of ester a day on training days is a line of spending on a different scale from ordinary sports nutrition. On a limited budget, sleep, food and a sensible plan will give you more.
Taste and gut. Ketone esters are notorious for their unpleasant taste and their ability to cause nausea and stomach upset. Trying them for the first time ahead of an important block is a bad idea.
The sample is narrow. Trained male cyclists of moderate fitness. How this works in women, in runners and in genuinely elite athletes, who have less adaptive headroom, is unknown.
Ketone esters are not on the WADA prohibited list, but with any supplement it is worth checking batch certification.

The key thing is not to confuse cause and effect. In this study ketones did not replace training, they amplified the response to it. Without the training block itself there would have been nothing to amplify.

Key points

The old idea of “ketones as race fuel” has not been confirmed by the evidence — null results dominate the reviews.
The new hypothesis is ketones as recovery signalling molecules: take them after training and before bed, not before the start.
In an 8-week RCT (28 cyclists, 25 g ×2 on training days) the ester group gained considerably more: peak power +52 versus +25 W, VO₂peak +6.4 versus +3.3 ml/kg/min, twice the rise in citrate synthase.
This is one study in a narrow sample — trained male cyclists. Do not transfer the conclusion to yourself automatically.
Under hypoxia a ketone ester worsened performance — context matters.
Expensive, unpleasant, potential gut problems. Sleep, nutrition and a plan first; exotica later.

Sources: Robberechts R., Bekhuis Y., Stalmans M. et al. “Post-exercise ketone supplementation improves endurance performance and mitochondrial adaptations during an 8-week endurance training intervention”, The Journal of Physiology, 2026. https://doi.org/10.1113/JP290315. “Utility of Ketone Supplementation to Enhance Physical Performance: A Systematic Review”, Advances in Nutrition. https://www.sciencedirect.com/science/article/pii/S2161831322002654. “Ketone ester ingestion impairs exercise performance without impacting cognitive function or circulating EPO during acute hypoxic exposure”, Journal of Applied Physiology, 2025. https://doi.org/10.1152/japplphysiol.00097.2025