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Available evidence indicates that ketone bodies inhibit glycolysis in contracting muscles. Therefore, we investigated whether acute exogenous ketosis by oral ketone ester (KE) intake early in a simulated cycling race can induce transient glycogen sparing by glycolytic inhibition, thereby increasing glycogen availability in the final phase of the event. In a randomized crossover design, 12 highly trained male cyclists completed a simulated cycling race (RACE), which consisted of 3-h intermittent cycling (IMT 180′ ), a 15-min time trial (TT 15′ ), and a maximal sprint (SPRINT). During RACE, subjects received 60 g carbohydrates/h combined with three boluses (25, 20, and 20 g) ( R )-3-hydroxybutyl ( R )-3-hydroxybutyrate (KE) or a control drink (CON) at 60 and 20 min before and at 30 min during RACE. KE intake transiently increased blood  d -β-hydroxybutyrate to ~3 mM (range: 2.6–5.2 mM) during the first half of RACE ( P  < 0.001 vs. CON). Blood pH concomitantly decreased from approximately 7.42 to 7.36 (range: 7.29–7.40), whereas bicarbonate dropped from 26.0 to 21.6 mM (range: 20.1–23.7; both  P  < 0.001 vs. CON) .  Net muscle glycogen breakdown during IMT 180′  [KE: −78 ± 30 (SD); CON: −60 ± 22 mmol/kg wet wt;  P  = 0.08] and TT 15′  (KE: −9 ± 18; CON: −18 ± 18 mmol/kg wet wt;  P  = 0.35) was similar between KE and CON. Accordingly, mean power output during TT 15′  (KE: 273 ± 38; CON: 272 ± 37 W;  P  = 0.83) and time-to-exhaustion in the SPRINT (KE: 59 ± 16; CON: 58 ± 17 s;  P  = 0.66) were similar between conditions. In conclusion, KE intake during a simulated cycling race does not cause glycogen sparing, nor does it affect all-out performance in the final stage of a simulated race.   NEW & NOTEWORTHY  Exogenous ketosis produced by oral ketone ester ingestion during the early phase of prolonged endurance exercise and against the background of adequate carbohydrate intake neither causes muscle glycogen sparing nor improves performance in the final stage of the event. However, such exogenous ketosis may decrease buffering capacity in the approach of the final episode of the event. Furthermore, ketone ester intake during exercise may reduce appetite immediately after exercise.

Exogenous ketosis impacts neither performance nor muscle glycogen breakdown in prolonged endurance exercise