Background Caffeine is a popular ergogenic aid in high-intensity sports, but it can cause side effects like cardiovascular stress and tolerance buildup. Theacrine, a structurally similar purine alkaloid, may offer comparable performance benefits with fewer adverse effects. This study aimed to assess the acute effects of theacrine, caffeine, and their combination on performance and physiological responses in elite canoe sprint athletes. Methods Twenty highly trained male canoe sprint athletes participated in a randomized, double-blind, placebo-controlled, crossover trial with four conditions: placebo (P), caffeine (K), theacrine (T), and a combination of caffeine and theacrine (KT). Each participant completed a 500-meter time trial and a 30-second Wingate Anaerobic Test (WAnT) on a kayak ergometer under each condition. Primary outcomes included race time, power output, muscle oxygen saturation (SmO2), total hemoglobin (THb), and maximal accumulated oxygen deficit (MAOD), which were measured using near-infrared spectroscopy (NIRS) and VO2 analysis. Results Caffeine significantly improved the 500-meter time trial (-1.872 s vs. P, p < 0.001) and WAnT distance (+2.31 m, p = 0.005), while KT produced smaller but still significant effects. Theacrine alone (T) did not lead to statistically significant performance improvements. Caffeine increased MAOD (p < 0.005), whereas KT and T showed non-significant changes. SmO2 and THb slopes were not significantly affected in any condition, suggesting that performance gains were mediated by mechanisms other than local oxygen delivery. No significant differences were observed in heart rate, blood pressure, or perceived exertion across conditions. Strong negative correlations between race time and power output—especially under caffeine—indicated that mechanical output is the primary factor influencing performance. Conclusion Caffeine remains a powerful ergogenic aid in sprint paddling, enhancing both mechanical output and muscle oxygen utilization. Although theacrine alone showed limited ergogenic effects, its combination with caffeine produced additional improvements compared to placebo, indicating a possible complementary role. These findings support targeted supplementation strategies in elite sports.
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