For sessions of about an hour, large carbohydrate intakes aren't needed; a mouth rinse or small amounts can slightly improve hard efforts, and eating beforehand matters more than fuelling during
Interpretation
A systematic review of 61 studies (679 subjects) found carbohydrate benefits grow with duration and that around 1 h the effect appears to be oral rather than metabolic (Stellingwerff 2014); duration-based guidance says large amounts are unnecessary for 30 min to 1 h (Jeukendrup 2014). A meta-analysis of 13 rinse studies found a small improvement in mean power (SMD 0.25) but not completion time (Brietzke 2019); in 9 cyclists (7 M, 2 F) a rinse cut 1 h time-trial time from 61.37 to 59.57 min (Carter 2004); in 12 male cyclists the rinse helped more when fasted (+3.4%) than fed (+1.8%), with fed plus rinse best (Lane 2013). Feeding before exercise helped sessions over 60 min but not short ones across 46 studies (Aird 2018). Gaps: mostly men, continuous time trials rather than intervals, no triathletes.
Practical implication
Use water for easy hour-long indoor sessions, consider a carbohydrate drink or rinse for hard hour-long sessions especially when fasted, eat beforehand when the session matters, and use longer indoor rides to practise race fuelling. Avoid habitually skipping fuel to save energy. Evidence mostly from trained men in time trials.
Sources
7 sources, independently verified.
50 of 61 placebo-controlled studies (82%) found a significant performance benefit from carbohydrate; benefit increased with exercise duration and carbohydrate dose. Around 1 h, benefits appear to come from oral receptor exposure acting on brain reward centres, with amount and oxidation largely irrelevant; beyond 2 h, high delivery rates matter as glycogen falls.
Duration-based carbohydrate guidelines: for about 30 min to 1 h, large amounts are unnecessary and a mouth rinse or small amounts can benefit performance; up to about 60 g/h of a single carbohydrate for 2-3 h; about 90 g/h with multiple transportable carbohydrates for ultra-endurance. Recommendations should be adjusted downwards at lower absolute intensities; framed for well-trained athletes.
About 1 h time trial (914 kJ) with a 6.4% maltodextrin or water rinse every eighth of the trial, spat out. Completed in 59.57 vs 61.37 min, mean power 259 vs 252 W; heart rate and RPE unchanged. Authors suggest a central drive or motivation effect. Small, mostly male; feeding state not stated in the abstract.
Carbohydrate mouth rinse improved mean power output in cycling time trials vs placebo (SMD 0.25, 95% CI 0.04-0.46) but not completion time (SMD -0.13, 95% CI -0.36 to 0.10); low risk of bias, I² 0%.
Pre-exercise feeding did not significantly improve short-duration aerobic performance (P = .687) but did improve prolonged aerobic exercise over 60 minutes (P = .012). Fasted exercise raised circulating free fatty acids; authors suggest fasted exercise may carry metabolic adaptation benefits needing further study.
Four 60-minute simulated time trials, fed (2 h after a 2.5 g/kg carbohydrate meal) or after an overnight fast, with a 10% maltodextrin or placebo rinse. Rinse raised mean power 3.4% when fasted (273 to 282 W) and 1.8% when fed (281 to 286 W); fed plus rinse produced the highest power. All-male.
700 kJ self-paced time trial at 32 °C, 40% RH in still air or with 16, 30 or 44 km/h airflow. Mean power 232 W in still air vs 247, 250 and 248 W with airflow; airflow conditions did not differ. Evaporative capacity 160 vs 298-324 W/m²; peak core temperature 39.4 vs 39.0, 38.8 and 38.8 °C; RPE higher in still air. Hot lab conditions, all-male.