G. Guppy, J. Brouner and O. Spendiff (2025). Carbohydrate mouth-rinsing does not rescue simulated time trial performance in trained endurance cyclists following a 5-day ketogenic diet. Journal of the International Society of Sports Nutrition, 22(1), 2598232. https://doi.org/10.1080/15502783.2025.2598232

PubMed 41384325

[Abstract]
BACKGROUND: Carbohydrate mouth rinsing (CHO-MR) during periods of fasting or low muscle glycogen availability could provide a more pronounced ergogenic effect compared to fed and high muscle glycogen conditions. However, there is little evidence investigating the efficacy of CHO-MR during periods of low muscle glycogen induced by ketogenic diets. Therefore, this study aimed to investigate the impact of CHO-MR vs. a placebo (PLA-MR) on cycling time trial performance in trained endurance cyclists following their habitual diet (HD) or a 5-day ketogenic diet (KD).

METHODS: Eight participants completed baseline testing and four trial conditions. For each trial, participants adhered to either their HD or a KD for 5 consecutive days. During the first 4 days of each dietary phase, they tracked daily nutrition; additionally, they recorded morning fasting blood glucose and β-hydroxybutyrate (βHB) levels for the 4 days preceding and the morning of each trial. Each trial comprised a 33.6 km simulated time trial in which rinsing was performed for ten seconds at 7 km intervals.

RESULTS: The 5-day KD significantly increased the time to completion (TTC) compared to HD (p < .001). Although no significant differences in TTC were detected between HD + CHO-MR and KD + CHO-MR (p = .670), CHO-MR did not restore KD performance to within 2% of HD conditions (±158 s; 4.8%). While a significant main effect for diet on morning fasted blood [βHB] (p = .001) was observed on day 5, it was not significantly associated with exercise time (r (14) = -.442, p = .086). Post-exercise blood [glucose] was significantly higher in the HD + CHO-MR and HD + PLA-MR conditions compared to the KD + CHO-MR (p = .038 & p = .021), and KD + PLA-MR (p = .011 and p = .003) conditions, respectively.

CONCLUSION: The data indicate that repeated 6.4% CHO-MR during endurance cycling is insufficient to overcome performance impairments induced via a 5-day ketogenic diet. This suggests that peripheral substrate availability may constrain the hepatic glucose output in response to central nervous system cues. Further research is required to elucidate how peripheral glycogen stores, central neural drive, and ergogenic interventions interact under low-carbohydrate conditions.