Introduction We previously showed that post-exercise ketone supplementation (PEKS) suppresses the development of overtraining symptoms, stimulates muscular adaptations, and improves endurance performance during endurance overload training. However, it is unclear whether PEKS also improves endurance performance and promotes training adaptations during well-balanced training. Methods Therefore, 28 well-trained active males were enrolled in an 8-week fully supervised cycling training intervention. Throughout the training program, participants received post-exercise and before sleep either 25g of the ketone ester (R)-hydroxybutyl (R)-hydroxybutyrate (KE, n = 14), or an isocaloric placebo drink (CON, n = 14). Outcome parameters included exercise performance, along with muscular and cardiac adaptations and were evaluated before (PRE), after week 3 (MID) and 7 (POST), and following a taper week (POST+1week). Results The training intervention improved 30-minute time trial performance (TT30min), absolute and relative whole-body oxygen uptake (VO2max), peak power output during the VO2max test (PPOVO2max), citrate synthase activity, and peak cardiac output (all p < 0.05 for PRE vs. POST). However, TT30min (CON: 291 ± 27 W vs. KE: 302 ± 28 W, p < 0.001) and PPOVO2max (CON: 457 ± 43 W vs. KE: 473 ± 41 W, p = 0.091) were 3.5% higher in KE compared to CON at POST. Furthermore, the relative VO2max showed a greater increase in KE (+12%) compared to CON (+ 6%, p < 0.001). Citrate synthase activity was at POST 14% higher in KE (9.37 ± 1.36 mol.h-1.kg protein-1) compared to CON (8.21 ± 0.97 mol.h-1.kg protein-1, p = 0.035), and OXPHOS complex II muscle protein content increased only significant in KE (+25%, p = 0.032 vs. PRE). Conclusion In conclusion, these data indicate that PEKS is a potent nutritional strategy to improve endurance performance and mitochondrial adaptations during regular training.
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