Effects of Acute Ecklonia cava Supplementation on Submaximal Cycling and Repeated-Sprint Performance in Trained Cyclists: A Randomised Double-Blind Placebo-Controlled Crossover Trial

Background/Objectives: This study examined whether acute Ecklonia cava (EC) ingestion altered physiological responses during submaximal cycling or power output during subsequent repeated maximal sprints in experienced male cyclists. Methods: Twelve cyclists completed a randomised, double-blind, placebo-controlled crossover trial. Participants ingested 263 mg of EC extract or placebo 30 min before 60 min of cycling at 55% of the maximum work rate, followed by three 20 s maximal sprints. Blood glucose, blood lactate, heart rate, oxygen uptake and respiratory exchange ratio (RER) were measured during submaximal cycling; peak power, mean power, blood glucose and blood lactate were assessed during the repeated-sprint test. Outcomes were analysed using linear mixed-effects models incorporating treatment, time, treatment × time, experimental period and allocated sequence. Results: Blood glucose was higher overall following EC than placebo (4.23 ± 0.23 vs. 4.02 ± 0.33 mmol·L−1; mean difference, 0.213 mmol·L−1; 95% CI, 0.046 to 0.379; p = 0.023), but the treatment × time interaction was not significant (p = 0.939). EC did not affect submaximal blood lactate, heart rate, oxygen uptake or RER (all p ≥ 0.096). EC did not alter peak power (mean difference, −2.58 W; 95% CI, −29.51 to 24.35; p = 0.832), mean power (5.93 W; 95% CI, −2.47 to 14.33; p = 0.167), post-sprint glucose (p = 0.481) or post-sprint lactate (p = 0.060). No treatment × time interaction was identified for any repeated-sprint outcome (all p ≥ 0.267). Conclusions: Acute ingestion of 263 mg of EC extract produced a modest overall elevation in blood glucose during submaximal cycling, but did not improve repeated-sprint performance or materially alter the other measured physiological responses. These findings do not support an acute ergogenic effect of EC in experienced male cyclists under the conditions tested.