A supplement that can spare muscle glycogen, improve cognition under fatigue, and fuel both the brain and the muscles simultaneously — without requiring dietary carbohydrate restriction. If that sounds too good to be true, the exogenous ketone literature will both confirm your scepticism and surprise you.
Exogenous ketone supplements (EKS) deliver beta-hydroxybutyrate (BHB) — the primary circulating ketone body — directly into the bloodstream without requiring a ketogenic diet. Two forms dominate the market: ketone esters (BHB directly bonded to 1,3-butanediol) and ketone salts (BHB bound to sodium, calcium or magnesium). Esters raise blood BHB to 3–5 mmol/L within 30–45 minutes; salts raise it to approximately 1–1.5 mmol/L — physiologically meaningful but substantially different.
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The headline study that generated mass attention was Cox and colleagues (2016) in Cell Metabolism. Elite rowers consuming a ketone ester drink before performance testing showed a 2% improvement in time trial output compared to isocaloric carbohydrate. The proposed mechanism was glycogen sparing — ketones were being preferentially oxidised, reducing the rate of muscle glycogen depletion at matched power outputs.
Holdsworth and colleagues (2017) followed up in a well-designed crossover trial showing that ketone ester ingestion before a 30-minute cycle time trial improved performance by approximately 2%, with elevated blood BHB (3.5 mmol/L) and reduced muscle lactate accumulation.
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However, subsequent research introduced important caveats. Training status appears to be a critical moderator. Dearlove and colleagues (2019), publishing in the British Journal of Sports Medicine, tested elite-level cyclists and found no performance benefit from ketone ester ingestion at doses that clearly elevated BHB. Their proposed explanation: highly trained athletes have already optimised their substrate utilisation to the degree that the added ketone oxidation offers no incremental advantage.
Mujika (2018) summarised the state of the literature critically, noting significant non-responder rates in trained populations and the difficulty of blinding trials adequately (ketone esters have a distinctive taste that complicates placebo control).
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Where the evidence is more consistently positive for exogenous ketones is cognitive performance under physical fatigue. The brain preferentially uses BHB over glucose when BHB is available in adequate concentrations — a phenomenon originally studied in the context of starvation and epilepsy. During prolonged exercise where cognitive demands are high (navigation sports, team sport decision-making, ultra-endurance events), ketone availability may maintain prefrontal cortex function as blood glucose begins to fluctuate.
MCT oil (specifically C8 caprylic acid) provides a more accessible route to mild ketosis without the cost and gastric distress of ketone esters. C8 is rapidly absorbed, transported to the liver via the portal vein, and converted to acetoacetate and BHB within 90–120 minutes. Blood BHB concentrations achieved (0.3–0.7 mmol/L) are lower than ester ingestion but sufficient for meaningful brain ketone uptake.
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GI tolerance is a significant practical barrier. Ketone esters cause nausea, bloating and gastric discomfort in a substantial proportion of users — particularly at performance-relevant doses (25–35ml per serving). Carbohydrate co-ingestion, which blunts BHB rise, reduces GI effects but undermines the glycogen-sparing mechanism. This trade-off creates a practical paradox for race day use.
For endurance athletes in keto-adaptation phases who want to understand how ketone-available substrates interact with their carbohydrate intake, the macro planning tool at winsport.uk/tools/nutrition/keto-macro-calculator calculates personalised macronutrient targets that account for fat adaptation stage, training intensity, and competitive fuelling strategy — the nutritional context in which exogenous ketones are most likely to show a measurable effect.
Have you experimented with ketone esters or MCT oil supplementation? Did you find a performance or cognitive benefit?