Carbohydrate Supplementation Mitigates Hypoglycemia And Improves Endurance Performance In Triathletes On Low/High Carbohydrate Diets

Very-low-carbohydrate diets (LCHF; <50 g/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 (CHO) ingestion during prolonged exercise delays fatigue by increasing carbohydrate oxidation, thereby sparing muscle glycogen. PURPOSE: To explore (i) if adoption to a LCHF diet impairs time-to-exhaustion (TTE) performance, (ii) whether carbohydrate ingestion 6-12x lower than current CHO fueling recommendations during low glycogen availability (>15-hour pre-exercise overnight fast and/or LCHF diet) improves TTE by preventing exercise-induced hypoglycemia (EIH; <3.9 mmol/L; <70 mg/dL), and (iii) the “keto-adaptation” time course through continuous substrate monitoring while caloric intake, physical activity, and fat-free mass are maintained. METHODS: Leveraging a randomized crossover design, we evaluated performance during strenuous TTE (70%V̇O2max) tests in trained triathletes (age 32.9 ± 4.9 years, height 179.9 ± 6.9 cm, weight 84.0 ± 11.7 kg; VO2max 56.2 ± 7.9 ml/kg/min) following 6-week high-carbohydrate (HCLF, 380 g/day) or very-low-carbohydrate (LCHF, 40 g/day) diets. During TTE, participants ingested a small quantity of carbohydrate (10.1 ± 1.4 g CHO/hour) or a non-caloric placebo (PLA). RESULTS: TTE performance was similar across both dietary interventions (LCHF: 96.4 ± 42.7 min, HCLF: 94.2 ± 29.6 min, p = 0.713). Minimal carbohydrate supplementation prevented EIH and significantly increased time-to-exhaustion equivalently in LCHF and HCLF interventions (Δ: 19 ± 7 minutes; 22%; p = 0.025). The LCHF diet significantly lowered 24-hour glucose concentrations compared to HCLF (Δ -6.4 mg/dL; p = 0.013), which normalized after 4 weeks, at the same time point peak blood ketone (R-β-hydroxybutyrate) concentrations normalized. CONCLUSION: These findings (i) demonstrate that an LCHF diet sustains strenuous endurance performance, (ii) establish that minimal carbohydrate supplementation was sufficient to enhance 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. Supported by: Supported by Grove City College Swezey Fund. Supersapiens Inc. provided continuous glucose monitors and data acquisition technology.