Effects of Highly Branched Cyclic Dextrin-Based Carbohydrate and Protein Co-Ingestion on Endurance Capacity and Metabolic Response in Soccer Players: A Randomized Crossover Trial

Background/Objectives: Highly branched cyclic dextrin (HBCD) may improve carbohydrate (CHO) availability during soccer-specific intermittent exercise via favorable gastrointestinal properties. Whether co-ingestion with whey protein confers additional ergogenic benefits beyond HBCD alone remains unclear. Methods: Nine male soccer players completed a single-blind, randomized crossover trial (≥7-day washout) with three conditions, Placebo (water), CHO, and isocaloric HBCD plus whey protein isolate (CHOP), during a standardized, ~120-min laboratory-based soccer-specific intermittent exercise protocol (not an actual match). The 500-mL beverage, split into two 250-mL servings, was consumed before exercise and at half-time during the exercise protocol. Respiratory quotient (RQ), substrate oxidation, heart rate, RPE, digestive symptoms, running time, and distance were assessed, with condition effects tested via repeated-measures ANOVA at each cycle. Results: Compared with Placebo, CHO increased RQ (p = 0.042) and carbohydrate oxidation (p = 0.048) while reducing fat oxidation (p = 0.034) at the final cycle, reflecting greater late-exercise CHO utilization without improving endurance performance. Compared with Placebo, CHOP significantly improved both running time (p = 0.019) and running distance (p = 0.025), with no significant difference from CHO on either outcome. RPE was significantly lower in CHOP than in Placebo during the latter stage of exercise (p = 0.016). Conclusions: HBCD ingestion before exercise and at half-time promoted greater late-exercise CHO utilization without increasing cardiovascular or gastrointestinal strain. Co-ingestion of whey protein with HBCD provided ergogenic benefits over Placebo by improving fatigue-limited endurance performance, supporting isocaloric CHOP co-ingestion as a practical nutritional strategy for prolonged soccer-specific intermittent exercise.