Effects of isolated and combined supplementation of caffeine and capsaicin on a self-paced 10-km running performance

Abstract Purpose To investigate the acute effects of isolated and combined caffeine and capsaicin supplementation on performance and physiological, perceptual, and subjective responses during a self-paced 10-km run. Methods This randomized, double-masked, placebo-controlled crossover trial included 27 runners, of whom 20 (32.3 ± 8.2 years; 8 women, 12 men) completed four conditions: caffeine (CAF; 6 mg·kg⁻ 1 ), capsaicin (CAP; 0.35 mg·kg⁻ 1 ), combined caffeine and capsaicin (CAF+CAP), and placebo. Supplements were ingested 45 min before running. Completion time, rating of perceived exertion (RPE), heart rate (HR), blood lactate, and subjective responses were assessed. Continuous and subjective outcomes were analyzed using linear and generalized linear mixed-effects models, respectively. Results The factorial model showed a significant main effect of caffeine on completion time ( p = 0.0049), with no capsaicin effect or caffeine × capsaicin interaction. Pairwise comparisons showed a lower completion time only for CAF+CAP versus placebo (Tukey-adjusted p = 0.048; Dunnett-adjusted p = 0.028). Post-exercise blood lactate was higher with CAF+CAP than with all other conditions (all Tukey-adjusted p ≤ 0.022). Perceived vigor increased with CAF and CAF+CAP, while caffeine-containing conditions produced more frequent insomnia, anxiety, headache, and tachycardia. HR did not differ among conditions. Conclusion Combined CAF and CAP reduced 10-km completion time and increased post-exercise blood lactate and perceived vigor versus placebo. Session-RPE and heart rate did not differ among conditions; RPE differed only between CAF and CAP. CAF-containing conditions also produced more frequent transient adverse effects. Trial registration Brazilian Registry of Clinical Trials, U1111-1333-3665, registered on February 2026.