Effects of Theacrine Supplementation on Heart Rate Variability Following a Resistance Exercise Session: A Randomized, Double-Blind, Placebo-Controlled Crossover Pilot Study

Background/Objectives: Resistance exercise (RE) reduces heart rate variability (HRV) in healthy young individuals, reflecting increased cardiovascular demand and autonomic stress during recovery. Given that theacrine (TEA) acts on the central nervous system and may influence autonomic cardiovascular regulation, its effects on HRV metrics remain unclear and have not yet been investigated. Methods: Seventeen young, healthy resistance-trained participants (8 females) completed two RE sessions under TEA and placebo (PLA) conditions in a randomized, double-blind, crossover design. HRV indices related to vagal modulation (RMSSD and HFnu), systolic blood pressure (SBP), diastolic blood pressure (DBP), and heart rate (HR) were assessed at three time points: pre-supplementation, pre-exercise (90 min after supplementation), and 30 min post-exercise. Results: In the TEA condition, RMSSD increased at pre-exercise compared with pre-supplementation (p = 0.01), with no significant difference between pre-supplementation and post-exercise (p = 0.140). The PLA condition showed significant post-exercise reductions in RMSSD compared to both pre-supplementation and pre-exercise (p < 0.01). Similar patterns were observed for HFnu. SBP, DBP, and HR increased significantly after exercise in both conditions, with no between-condition differences. No significant time × condition interactions or sex effects were observed for any HRV or cardiovascular variable. Conclusions: Acute TEA supplementation did not significantly alter autonomic or cardiovascular responses compared with PLA following a single RE session. Although distinct post-exercise HRV patterns were observed between conditions, the time × condition interaction did not support a significant treatment effect. Larger studies are needed to clarify the potential effects of TEA on post-exercise autonomic recovery.