Carbohydrate Ingestion Eliminates Hypoglycemia & Improves Endurance Exercise Performance in Triathletes Adapted to Very Low & High Carbohydrate Isocaloric Diets

Very-low-carbohydrate diets (LCHF; <50g/day) have been debated for their potential to lower pre-exercise muscle and liver glycogen stores and metabolic efficiency, risking premature fatigue. It is also hypothesized that carbohydrate ingestion during prolonged exercise delays fatigue by increasing carbohydrate oxidation, thereby sparing muscle glycogen. Leveraging a randomized crossover design, we evaluated performance during strenuous time-to-exhaustion (70%⩒O 2max ) tests in trained triathletes following 6-week high-carbohydrate (HCLF, 380g/day) or very-low-carbohydrate (LCHF, 40g/day) diets to determine (i) if adoption of the LCHF diet impairs time-to-exhaustion performance, (ii) whether carbohydrate ingestion (10g/hour) 6-12x lower than current CHO fuelling recommendations during low glycogen availability (>15-hour pre-exercise overnight fast and/or LCHF diet) improves time-to-exhaustion by preventing exercise-induced hypoglycemia (EIH; <3.9mmol/L; <70mg/dL), and (iii) the “keto-adaptation” time course through continuous substrate monitoring while caloric intake, physical activity, and fat-free mass are maintained. Time-to-exhaustion performance was similar across both dietary interventions. Minimal carbohydrate supplementation prevented EIH and significantly increased time-to-exhaustion equivalently in LCHF and HCLF interventions (22%). The LCHF diet significantly lowered 24-hour glucose concentrations, which normalized after 4 weeks, at the same timepoint peak blood ketone (R-β-hydroxybutyrate) concentrations normalized. These findings (i) demonstrate that a LCHF diet sustains strenuous endurance performance, (ii) establish that minimal carbohydrate supplementation was sufficient to enhances exercise performance on LCHF and HCLF diets by mitigating EIH, and (iii) indicate that a minimum 4-week adaptation period to an LCHF diet is required to ensure normalization of metabolic homeostasis, glycemic control and exercise performance.