Exogenous ketones: performance booster or expensive trend?

Ketones as a supplement promise more energy, sharper thinking, and faster ketosis without giving up carbohydrates. What research shows — and why the reality is more nuanced than the product claims.

What ketones are and where they come from

Ketones — primarily beta-hydroxybutyrate, acetoacetate and acetone — are produced in the liver when fatty acids are used as an energy source and not enough glucose is available. This happens during extended fasting periods, very low-carbohydrate diets or intense endurance exercise. Ketones can cross the blood-brain barrier and are used by the brain as an alternative fuel to glucose.

Exogenous ketones are ketones you take from the outside as a supplement — without fasting or a ketogenic diet. They are mainly available as ketone esters or ketone salts. Ketone esters have an intense acetone-like taste, which makes taking them unpleasant for many people. Ketone salts are milder, but absorption into the blood is slower.

What happens in the body after taking them

After taking exogenous ketones, blood ketone levels rise significantly within 30 to 60 minutes and fall again after 2 to 4 hours. The body is not in a true state of ketosis — it continues to draw on glucose in parallel, if available. This fundamentally distinguishes the state from a metabolically adapted dietary ketosis.

At the same time, exogenous ketones briefly reduce insulin secretion and hepatic glucose production — which is part of the discussion around potential therapeutic applications.

What the research shows on performance

The initial excitement stemmed from a study by Kieran Clarke's group at Oxford: cyclists who drank a ketone ester before a 30-minute time trial improved their performance by an average of 2% compared to a control group. The study had 39 participants and is methodologically sound.

Later studies showed mixed results: some confirmed moderate performance benefits in elite athletes under certain conditions, others found no effect or even impairment due to gastrointestinal side effects. For recreational athletes and cognitive performance, the evidence is weaker than product claims suggest.

When exogenous ketones might be useful

The strongest evidence exists for two areas of application: first, as a short-term energy source for very intense endurance efforts lasting more than 90 minutes in trained athletes. Second, as a possible supportive measure in neurological conditions such as Alzheimer's, where the brain metabolises glucose less efficiently and might fall back on ketones — though the evidence here is still early.

For healthy individuals in everyday life — more focus, less hunger, better cognition — solid evidence is lacking. The cost of ketone esters is also considerable: a single dose typically costs between 25 and 40 Swiss francs, for an effect that lasts a few hours.

Exogenous ketones and the YUTHI app

If you try exogenous ketones, you can log the time of intake and your subjective experiences — energy, concentration, gastrointestinal response — in the YUTHI app as a diary entry. Some devices measure blood ketone levels via a finger-prick test; you can enter the result as a biomarker.

For a meaningful self-experiment, a clear protocol is recommended: what, when, how much — and what activity follows. Only then can you distinguish a subjective reaction from a real effect.

Sources

• Cox PJ et al. Nutritional ketosis alters fuel preference and thereby endurance performance in athletes. Cell Metabolism, 24(2):256–268, 2016.
• Evans M et al. No benefit of ingestion of a ketone monoester supplement on 10-km running performance. Medicine & Science in Sports & Exercise, 51(12):2506–2515, 2019.
• Stubbs BJ et al. A ketone ester drink lowers human ghrelin and appetite. Obesity, 26(2):269–273, 2018.