Carbohydrate ingestion eliminates hypoglycemia and improves endurance exercise performance in triathletes adapted to very low- and high-carbohydrate isocaloric diets.
Level 2 - randomized trial
Randomized crossover trial evaluating dietary and intra-exercise carbohydrate interventions.
PubMed 39786965 · doi:10.1152/ajpcell.00583.2024
What was done
Trained triathletes completed a randomized crossover trial comparing 6-week isocaloric high-carbohydrate (HCLF, 380 g/day) versus very-low-carbohydrate (LCHF, 40 g/day) diets. Participants underwent strenuous time-to-exhaustion tests at 70% VO2max with continuous substrate monitoring after a >15-hour overnight fast. The study evaluated whether minimal carbohydrate ingestion during exercise (10 g/h, 6–12× lower than standard recommendations) prevented exercise-induced hypoglycemia (<3.9 mmol/L or <70 mg/dL) and extended time to exhaustion, while tracking keto-adaptation kinetics over the 6-week period.
What was found
Time-to-exhaustion performance was similar between the LCHF and HCLF dietary interventions. Minimal carbohydrate supplementation (10 g/h) during exercise prevented exercise-induced hypoglycemia and increased time to exhaustion by 22% equally in both diet groups. The LCHF diet significantly reduced 24-hour glucose concentrations initially, which normalized after 4 weeks concurrently with the normalization of peak blood ketone (R-beta-hydroxybutyrate) concentrations.
Why it matters
This study indicates that at least 4 weeks of adaptation to a very-low-carbohydrate diet normalizes metabolic homeostasis and preserves strenuous endurance performance, and demonstrates that minimal intra-exercise carbohydrate supplementation is sufficient to alleviate hypoglycemia and boost endurance regardless of chronic baseline diet.
Limits
The abstract does not report the sample size, sex distribution, or baseline characteristics of the triathletes. Performance was measured using open-ended time-to-exhaustion tests under fixed workloads after a prolonged >15-hour fast, which may not fully reflect real-world race time-trial conditions or typical pre-race fueling strategies.
Cited by
- supports Ironman competitors on a 6-week ketogenic diet sustained 70% VO2 max cycling performance without deterioration compared to a high-carb diet despite experiencing higher rates of hypoglycemia.
- supports Administering 10 grams of carbohydrates per hour during prolonged strenuous exercise improved endurance performance by 22% in both high-carbohydrate and ketogenic athletes.