Exogenous ketones have emerged as one of the most debated ergogenic aids in endurance sport. The premise is straightforward: by elevating blood beta-hydroxybutyrate (BHB) to 1–3 mmol/L, athletes can theoretically spare muscle glycogen and tap into an alternative fuel source. Professional cycling teams reportedly experimented with ketone esters as early as 2018, and the supplement industry has since rushed to bring consumer-grade products to market. But the gap between theoretical promise and practical performance gains remains substantial.
Understanding the distinction between ketone esters and ketone salts — and their vastly different pharmacokinetics — is essential before committing to a protocol. The research landscape is evolving rapidly, with studies published in the Journal of Physiology, Medicine & Science in Sports & Exercise, and Frontiers in Physiology offering conflicting conclusions. Let's break down what the current evidence actually supports.
Ketone Esters vs Ketone Salts: Key Differences
Ketone esters (typically R-1,3-butanediol acetoacetate diester) are the form used in most positive research trials. A standard dose of 573 mg/kg body weight elevates blood BHB to 2.5–3.0 mmol/L within 30 minutes, peaking at approximately 60 minutes post-ingestion. For a 70 kg athlete, that translates to roughly 40 g of ketone ester per serving. These compounds taste notoriously bitter — often described as a mix of nail polish remover and rubbing alcohol — and cost $30–50 per single serving.
Ketone salts pair BHB with sodium, potassium, calcium, or magnesium ions. They are significantly cheaper ($2–5 per serving) but far less potent, raising blood BHB to only 0.5–1.0 mmol/L — well below the threshold most researchers consider metabolically meaningful. Ketone salts also deliver a substantial mineral load: a dose providing 12 g of BHB may contain 1,300 mg of sodium, which complicates electrolyte planning during exercise.
The Glycogen Sparing Hypothesis
The central performance claim for exogenous ketones is glycogen sparing: by providing an alternative oxidative fuel, ketones reduce the rate of muscle glycogen depletion, extending time to exhaustion. A landmark 2016 study by Cox et al. in Cell Metabolism reported a 15% reduction in glycogen utilization during cycling at 75% VO2max when athletes consumed ketone esters alongside their normal carbohydrate intake.
However, subsequent studies have failed to replicate this magnitude of effect. A 2020 meta-analysis in Sports Medicine covering 13 studies found the glycogen sparing effect ranged from negligible to approximately 50%, depending on exercise intensity, duration, and co-ingestion of carbohydrates. Critically, several studies showed that ketone ingestion actually impaired high-intensity performance above lactate threshold, possibly due to a slight reduction in carbohydrate oxidation rate when it was most needed.
Performance Outcomes: Mixed at Best
When researchers measure actual performance — time trials, peak power, time to exhaustion — the results are underwhelming. Only 3 of 13 controlled trials in the 2020 meta-analysis showed statistically significant performance improvements. Most found no benefit, and two reported performance decrements of 2–4% during high-intensity efforts lasting 15–40 minutes. The scenario most likely to benefit is ultra-endurance exercise exceeding 3 hours where glycogen depletion is the primary limiter, though even here the evidence remains preliminary.
A 2023 study in the International Journal of Sport Nutrition and Exercise Metabolism found that ketone ester ingestion during a 180 km simulated cycling stage produced no performance difference compared to carbohydrate-only fueling, despite achieving blood BHB levels of 2.1 mmol/L.
GI Tolerance: A Significant Barrier
Gastrointestinal distress is the most consistent finding across ketone studies. Between 30% and 50% of participants report nausea, reflux, or abdominal cramping following ketone ester ingestion. A 2021 dose-response study found that GI symptoms scaled linearly with dose: at 573 mg/kg, 47% of subjects experienced moderate to severe nausea. Splitting the dose into two servings 30 minutes apart reduced but did not eliminate symptoms. Ketone salts generally cause fewer upper-GI issues but may provoke osmotic diarrhea at higher doses due to their mineral content.
For athletes already pushing gut tolerance limits with 80–120 g/hour of carbohydrate during racing, adding a ketone supplement introduces meaningful GI risk. This is a non-trivial consideration in events where a single episode of vomiting can end a competitive effort.
Dosing Protocols and Practical Considerations
If you choose to experiment with exogenous ketones, start with a reduced dose of 250–300 mg/kg body weight in training to assess GI tolerance. Gradually increase toward the research-standard dose of 573 mg/kg over 3–4 sessions. Always co-ingest with carbohydrates — the evidence for ketones as a standalone fuel without carbohydrate is poor. Time ingestion 30–45 minutes before the period where you expect glycogen depletion to become performance-limiting.
- Ketone esters: 573 mg/kg body weight, 30–45 min pre-exercise or at the onset of prolonged sub-threshold effort
- Ketone salts: Not recommended for performance — blood BHB levels insufficient for meaningful metabolic shift
- Cost per Ironman race (2–3 servings): $60–150 for ketone esters
- Always trial in training — never debut in competition
Who Might Benefit — and Who Should Skip
The athlete profile most likely to see marginal gains is an ultra-endurance competitor racing for 5+ hours at sub-threshold intensity, who has already optimized carbohydrate fueling to 80–90 g/hour and has demonstrated GI tolerance to ketone esters in training. For most competitive athletes in events lasting under 3 hours, optimizing carbohydrate periodization, hydration, and electrolyte strategy will deliver far greater performance returns per dollar spent.
Before investing in ketone supplements, ensure your foundational fueling is dialed in. Use our Race Day Nutrition Planner to build a carbohydrate and electrolyte strategy matched to your sweat rate and event duration. For a comprehensive guide to fueling fundamentals, see our breakdown of electrolyte strategies for endurance athletes.