Metabolic characteristics of keto-adapted ultra-endurance runners.
Level 4 - case-series / case-control
Cross-sectional observational comparison between two self-selected dietary groups
PubMed 26892521 · doi:10.1016/j.metabol.2015.10.028
What was done
Twenty elite ultra-marathoners and Ironman-distance triathletes completed a maximal graded exercise test and a 180-minute submaximal treadmill run at 64% VO2max. Participants were categorized by habitual diet: high-carbohydrate (HC: n=10, 59% carbohydrate, 14% protein, 25% fat) or low-carbohydrate (LC: n=10, 10% carbohydrate, 19% protein, 70% fat) consumed for an average of 20 months (range 9 to 36 months).
What was found
Peak fat oxidation was 2.3-fold higher in the LC group (1.54 ± 0.18 vs 0.67 ± 0.14 g/min; P=0.000) and occurred at a higher percentage of VO2max (70.3 ± 6.3 vs 54.9 ± 7.8%; P=0.000). Mean fat oxidation during submaximal exercise was 59% higher in the LC group (1.21 ± 0.02 vs 0.76 ± 0.11 g/min; P=0.000), accounting for 88 ± 2% versus 56 ± 8% of energy contribution (P=0.000). No significant differences were observed in resting muscle glycogen, glycogen depletion after 180 minutes of running (-64% from pre-exercise), or depletion after 120 minutes of recovery (-36% from pre-exercise).
Why it matters
The paper shows that long-term keto-adaptation permits extraordinarily high rates of fat oxidation during prolonged exercise in elite endurance athletes without compromising resting muscle glycogen levels.
Limits
The study is observational and cross-sectional with a very small sample size (n=10 per group), introducing confounding and selection bias. Participants were exclusively elite male ultra-endurance athletes, limiting generalizability, and direct competitive performance outcomes were not evaluated.
Cited by
- supports Fat oxidation rates in keto-adapted athletes exceeded previously reported medical literature, with some athletes oxidizing over 1.85 grams of fat per minute during physical activity.
- supports Athletes keto-adapted for over 4 weeks performing at above 85% VO2 max derived the majority of fuel from fat, with fat oxidation rates exceeding 1.85 grams per minute.