Acute 1,3-Butanediol Co-Ingestion with Carbohydrate Does Not Improve Endurance Performance in Chronically Ketogenic Male Athletes

Background/Objectives: Exogenous ketone supplementation has been proposed to enhance endurance performance by altering substrate metabolism and sparing muscle glycogen. Prolonged carbohydrate restriction may upregulate ketolytic capacity and could therefore modulate the response to exogenous ketone ingestion. This study investigated the impact of increasing circulating ketone bodies through exogenous ketone supplementation on endurance performance in athletes chronically adapted to a ketogenic diet. Methods: Participants (n = 9 recreationally active males, with ≥1 year of ketogenic adherence) visited the lab four times. The first visit determined their V.O2max. The second visit accustomed them to test procedures. In a counterbalanced design, visits three and four tested the consumption of a 60 g carbohydrate beverage (CHO), or a beverage comprising 60 g carbohydrate and 0.5 g/kg 1,3-butanediol (CHO + BD). This was followed by a 60 min set-intensity exercise bout, followed by a 16.1 km time trial. Results: βHB significantly increased in the CHO + BD group (rest: 0.72 ± 0.33 mmol/L; post-exercise: 1.69 ± 0.30 mmol/L) compared to CHO (rest: 0.80 ± 0.53; post-exercise: 0.55 ± 0.30) (p < 0.001). There was no significant difference in time trial performance between groups (1606 ± 138 s; 1620 ± 134 s; p > 0.05). During set intensity exercise, there was no significant main effect of supplementation on lactate concentration, substrate oxidation, oxygen consumption at a fixed workload, RPE or heart rate (p > 0.05). However, glucose was significantly lower at minutes 40 and 60 of set intensity exercise in CHO + BD compared to CHO. Conclusions: No significant effect of 1,3-butanediol ingestion on endurance performance was detected in this sample of nine chronically ketogenic male athletes. βHB significantly increased from resting levels; however, time trial performance did not differ between conditions. Given the small sample and limited statistical power, these preliminary findings indicate no detectable ergogenic effect under the present conditions rather than evidence of no effect, and larger studies are warranted.