Author(s): CHARRIER, D., DE LIMA-JUNIOR, D., MONTANARI, A., CERULLO, G., AGARAJ, P., BASTIN, A., LIBARDONI, L., STIRCU, V., SPINELLO, G., GIVRALLI, J., CANATO, M., BASSETTO, F., VINDIGNI, V., MARCORA, S., MORO, T., PAOLI, A., Institution: UNIVERSITY OF PADOVA, Country: ITALY, Abstract-ID: 2399

INTRODUCTION:
Ketogenic diets (KD) have been proposed to enhance endurance performance by increasing fat oxidation and reducing carbohydrate dependence. However, their effects on muscle glycogen availability and high-intensity performance remain controversial, particularly after short-term adaptation. Therefore, the aim of this study was to investigate the effects of 10-day KD adaptation on (1) metabolic and physiological adaptation during moderate-intensity endurance exercise and (2) subsequent high-intensity endurance performance.
METHODS:
Twenty-four trained endurance athletes (29.7 ± 3.8 years; VO₂max 59.1 ± 6.3 mL·kg⁻¹·min⁻¹) completed a 7-day standardized diet before being randomized to a ketogenic diet (KD; <5% CHO, 2.0 g/kg/day protein) or control diet (CD; >55% CHO, 2.0 g/kg/day protein) for 10 days, while maintaining habitual training. Body composition was assessed by DXA and BIA pre- and post-intervention. After an overnight fast, participants performed a 2-h cycling bout at 55% peak power output. VO₂, respiratory quotient, and substrate oxidation were measured intermittently (5 min every 15 min) via indirect calorimetry. Vastus lateralis biopsies were obtained pre- and post-exercise to quantify muscle glycogen. Time to exhaustion (TTE) was subsequently assessed using an incremental protocol (starting at 75% VO₂ peak, +5% every 5 min). Data were analyzed using two-way RM ANOVA.
RESULTS:
KD significantly reduced body mass (−2.1 ± 1.9 kg, p<0.001), primarily due to decreases in total (−1.15 ± 0.79 kg, p<0.001) and intracellular water (−0.64 ± 0.14 kg, p<0.001). Pre-exercise muscle glycogen was markedly lower following KD (254 ± 68 vs 530 ± 140 mmol·kg⁻¹ d.w., p< 0.001). Although net glycogen utilization during exercise was reduced in KD, post-exercise glycogen concentrations remained significantly lower than pre-diet values (142 ± 71 vs 250 ± 120 mmol·kg⁻¹ d.w., p<0.001). KD increased mean fat oxidation during exercise compared to pre-diet values (1.07 ± 0.19 vs 0.55 ± 0.15 g·min⁻¹, p<0.001). However, this shift in substrate utilization was accompanied by a higher oxygen cost (+141 ± 42 mL·min⁻¹, p<0.05), reduced gross efficiency (−0.75%, p<0.05), and increased mean heart rate (+10 ± 5 bpm, p<0.01). Time to exhaustion was also significantly reduced following KD (−221 ± 103 s, p<0.05), indicating impaired high-intensity endurance performance. No significant changes were observed in the control group.
CONCLUSION:
A 10-day ketogenic diet enhances fat oxidation but substantially reduces muscle glycogen availability and increases the oxygen cost of exercise. These metabolic changes impair exercise efficiency and compromise high-intensity endurance performance, suggesting that short-term KD adaptation may be insufficient to support performance when high-intensity efforts are required.