Metabolic characteristics of keto-adapted ultra-endurance runners

Volek JS, Freidenreich DJ, Saenz C, Kunces LJ, Creighton BC, Bartley JM, Davitt PM, Munoz CX, Anderson JM, Maresh CM, Lee EC, Schuenke MD, Aerni G, Kraemer WJ, Phinney SD · 2016 · Metabolism

DOI: 10.1016/j.metabol.2015.10.028View source ↗

“Peak fat oxidation was 2.3-fold higher in the LC group (1.54 ± 0.18 vs 0.67 ± 0.14 g/min).”

Summary

The FASTER (Fat-Adapted Substrate utilization in Trained Elite Runners) study compared 20 elite ultra-endurance athletes — 10 habitually consuming a high-carbohydrate diet (59 percent carbs) and 10 long-term keto-adapted (10 percent carbs, 70 percent fat, average 20 months on the diet) — across maximal and submaximal exercise testing. The headline finding was record-setting: peak fat oxidation in the keto-adapted athletes was 2.3-fold higher than in the carb-adapted group (1.54 vs 0.67 grams per minute), the highest fat-oxidation rates ever recorded in humans during exercise. During submaximal exercise (3-hour run at 64 percent VO2max), fat contributed 88 percent of the energy in keto-adapted athletes versus 56 percent in carb-adapted athletes. Notably, muscle glycogen utilization and post-exercise glycogen repletion were similar between groups despite the dramatic substrate-source shift — meaning keto-adapted athletes used proportionally less carbohydrate from glycogen stores during the run, so their glycogen actually lasted longer. The paper transformed how the field thinks about athletic substrate use: humans can adapt to fat as their dominant fuel without losing the ability to use carbohydrate when it matters.

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References cited by this entry

ExtendsThe human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidationPhinney SD et al. · 1983
Phinney 1983 first demonstrated that keto-adapted athletes can preserve submaximal exercise capacity; the FASTER study (Volek/Phinney 2016) extended that finding to elite ultra-endurance athletes adapted for an average of 20 months and documented record-high fat oxidation rates.
ContradictsLow carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkersBurke LM et al. · 2017
FASTER reports favorable metabolic adaptations in keto-adapted ultra-endurance runners; Burke 2017 found that LCHF impaired race performance in elite race walkers despite increased fat oxidation. Both studies are valid; the contradiction reflects population (ultra-endurance vs short-distance speed), adaptation duration (20+ months vs 3 weeks), and outcome (metabolism vs race time).

Entries that reference this one

ContradictsLow carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkersBurke LM et al. · 2017
FASTER (Volek/Phinney 2016) documented favorable metabolic adaptations in long-term keto-adapted ultra-endurance runners; Burke 2017 found that 3-week ketogenic adaptation impaired race performance in elite race walkers despite increased fat oxidation. The contradiction reflects different sport types (endurance ultra vs race walking speed), adaptation durations, and outcome measures.