Fueling the Modern Endurance Athlete: The High-Carbohydrate Revolution, Exogenous Ketones, and an Evidence-Based Approach for the Sports Medicine Clinician

CONTEXT: Carbohydrate is the principal limiting fuel for prolonged, high-intensity endurance exercise, and consensus guidelines have long capped recommended intake during exercise at approximately 90 g/h. Two developments have challenged and extended contemporary fueling practice: a shift toward carbohydrate intakes of 100 to 150 g/h in elite endurance sport, and widespread commercial availability of exogenous ketone supplements marketed as ergogenic aids. Both have outpaced the clinical evidence and have reached the competitive and recreational athletes who present to sports medicine clinicians. EVIDENCE ACQUISITION: Narrative review of primary metabolic and performance studies, systematic reviews, meta-analyses, and consensus statements published through mid-2026, identified through PubMed and reference tracing. STUDY DESIGN: Narrative review. LEVEL OF EVIDENCE: Level 5. RESULTS: Multiple-transportable-carbohydrate formulations (glucose plus fructose) raise exogenous carbohydrate oxidation above the roughly 1 g/minute ceiling imposed by intestinal glucose transport, providing the mechanistic basis for fueling rates of ≥90 g/h. Ingesting 120 g/h increases exogenous carbohydrate oxidation, but the best-controlled studies have not demonstrated additional endogenous (liver and muscle) glycogen sparing or a consistent performance advantage, and gastrointestinal tolerability requires deliberate gut training. For exogenous ketones, an influential 2016 study reported altered substrate use and improved time-trial performance. Subsequent trials and multiple systematic reviews and meta-analyses have not confirmed an acute ergogenic effect, and several have documented gastrointestinal distress or impaired performance. The more promising and reproducible signal for ketones lies in postexercise recovery and attenuation of overreaching. Finally, athletes who restrict in-competition and training carbohydrate to pursue leanness risk low energy and carbohydrate availability, the substrate of Relative Energy Deficiency in Sport (REDs). CONCLUSION: Carbohydrate intake of ≤90 g/h, built through progressive gut training, is supported by consensus guidelines and mechanistic evidence for prolonged, high-intensity endurance events; progression toward 120 g/h is reasonable in selected, well-adapted athletes competing in the longest events, with the understanding that a performance benefit >90 g/h remains unproven. Routine exogenous ketone supplementation for acute performance is not supported by current evidence; a recovery-focused application is plausible and increasingly supported but requires further study. Clinicians should frame both strategies within adequate energy availability to protect long-term athlete health.Strength-of-Recommendation Taxonomy (SORT):B.