Exogenous Ketones in Endurance Sport: Benefits, Evidence and Limits

Few supplements in endurance sport are sold as expensively as exogenous ketones. A drink that puts the body into a state otherwise produced only by fasting or a strict ketogenic diet — supposedly sparing carbohydrates, mobilising fat and raising performance. The hype is old, the evidence is thin, the prices are not. Time for a sober assessment: what ketones do in the body, what studies in real athletes actually measured, and in which narrow cases a trial is even worth considering.

What exogenous ketones actually are

Ketones are molecules the liver builds from fatty acids when carbohydrate intake is low: acetoacetate, beta-hydroxybutyrate (BHB) and acetone. They serve the brain, heart and muscles as an alternative fuel. On a ketogenic diet BHB rises to 0.5 to 3 mmol/L, similar to the fasting state. "Exogenous" means you supply these molecules from outside, without your body producing them.

There are essentially three forms on the market:

• BHB salts: bound to sodium, potassium or calcium. Cheap, but they carry a lot of salt — one serving can contain more than a gram of sodium. That hits the stomach and works against you on long efforts.
• Ketone esters: the research standard. Roughly 25 to 30 grams raise BHB to 2 to 4 mmol/L within 20 to 30 minutes — values unreachable with normal eating. The ester tastes aggressive and costs several euros per serving.
• 1,3-butanediol: an alcohol converted to BHB in the liver. Slower, milder in taste, weaker in effect and less well studied.

Important for context: exogenous ketones do not create ketosis in the sense of a metabolic switch. They flood the bloodstream with BHB for two to four hours. After that the state is gone, regardless of what you ate.

The theoretical advantage: sparing carbohydrates

The logic sounds convincing. Glycogen is the limiting factor in efforts beyond two hours; whoever uses less of it has more left at the end. Studies with ketone esters do show a measurable substrate shift: muscle cells oxidise more BHB and less glucose, glycogenolysis drops. There are also hints of less oxidative stress and — in animal work and small human studies — enhanced mitochondrial adaptation after training blocks.

The catch is in the detail. Exogenous ketones are strongly antilipolytic: they suppress fat oxidation because the body is signalled that plenty of energy is available. The effect is therefore not "more fat burning" but "less fat burning and less carbohydrate use with a simultaneously higher ketone turnover". In a 24-hour run or an ultra through the night, that can backfire — at some point the fuel the ketones cannot replace is missing.

What studies in real athletes show

The research group around Kieran Clarke in Oxford popularised the ketone ester and initially measured clear effects in their own work: in 30-minute time trials ketones extended time to exhaustion by a few percent, and in cyclists over three hours glycogen use fell. Those results were quoted loudly and widely.

Independent replication came out more sober. A series of studies in trained cyclists and runners found either no difference or worse performance, particularly in short, high-intensity efforts. In intermittent exercise — the typical mix of hard surges and recovery in road racing — more gastrointestinal symptoms appeared, and the ability to accelerate again at the end suffered. A 2022 meta-analysis of the available controlled studies concluded that the evidence for a performance-enhancing effect is weak and inconsistent.

In practice that means: there is no solid reason to use ketones as a performance booster. There is also no reason to brand them dangerous. They are an expensive supplement with an unclear effect — nothing more, nothing less.

The practical downside hardly anyone mentions

Anyone who has tried a ketone ester knows the problem: the taste is hard to tolerate and many athletes report nausea. In studies participants dropped out because they could not get the ester down. Then there is the stomach: a concentrated ketone solution is hyperosmolar and pulls water into the gut — exactly what you need least during a marathon.

Another point concerns the combination. High-dose carbohydrates (60 to 90 grams per hour) and ketones at the same time often overwhelm digestion. Testing both strategies separately and then adding them up generally does not work. Anyone trying ketones should deliberately reduce carbohydrate intake that day — which relativises the actual advantage of ketones (sparing carbohydrates).

When a trial can make sense

There are two constellations in which a structured self-experiment is defensible:

• Very long efforts at low intensity: ultra runs, brevets, 24-hour races. Here substrate supply genuinely matters, and the low intensity makes gastrointestinal problems less likely to escalate. The article on ultramarathon nutrition shows how complex this field already is.
• Carbohydrate intolerance: athletes who regularly battle nausea or diarrhoea with high carbohydrate intakes can check whether partial substitution with ketones relieves the gut. That is an individual solution, not a general recommendation.

The test itself belongs in a quiet training week, not in the dress rehearsal and never in a race. At least three sessions with the same dose and the same timing, then a comparison with an identical session without ketones. Anything else is gut feeling with a price tag.

The honest alternative: what actually improves performance

If you have 20 euros for a race day and want a measurable improvement, the order is fairly clear:

• Carbohydrates dosed correctly: 60 grams per hour is the baseline, 90 grams with glucose-fructose at a 2:1 ratio is reachable for most trained athletes. Details in glucose-fructose 2:1.
• Caffeine: 3 milligrams per kilogram of body weight 45 to 60 minutes before the start is one of the best-supported performance supplements there is. More in caffeine in endurance sport.
• Sodium: 300 to 600 milligrams per hour, depending on sweat rate and climate. See electrolytes for runners.
• Training: a cleanly periodised block with an 80/20 distribution delivers two to five percent — more than any supplement. How to build it is covered in the Peakora training approach and in the 16-week marathon plan.

Those four points together cost less than a single ketone serving and are far better supported by evidence.

Legality and anti-doping

Exogenous ketones are not on the WADA prohibited list and are permitted in competition. They fall under food and supplement law, not doping rules. In practice this creates an odd situation: a product without proven effect is allowed, while substances with clear evidence (such as caffeine in high doses) were historically monitored for years. For you as an athlete the relevant consequence is different: supplements are not subject to testing obligations, and product quality is only as good as the manufacturer. Buying ketones means buying a food without an approval process.

Common mistakes with ketones

• Confusing ketosis with ketones. A blood concentration of 3 mmol/L after a ketone ester does not mean the metabolism has switched to fat. Two to four hours later the value is gone.
• Testing for the first time in a race. The classic supplement mistake — and with ketones especially expensive because of the gastrointestinal side effects.
• Stacking carbohydrates and ketones. Both at once overwhelms digestion. Anyone taking ketones has to adjust carbohydrate intake, not add to it.
• Salt-based ketones in the heat. BHB salts bring extra sodium. Combined with electrolyte drinks this can push sodium intake unnecessarily high.
• Mistaking marketing for study data. Percentages in supplier brochures often come from small studies with unusual protocols. Independent replication is missing.

How Peakora handles supplements

The Peakora engine plans training, not shopping lists. It does show you what you actually need: the fueling calculator derives from your estimated race time, body weight and intensity how many carbohydrates, how much sodium and how much fluid per hour make sense — and whether you should plan for 60 or 90 grams per hour. Those numbers are the lever you can genuinely turn on race day. Anything beyond that belongs in a controlled self-experiment in training, not in the shopping bag before the A-race.

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