Theacrine as a novel ergogenic aid: impact on canoe sprint performance

ABSTRACT

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.

1. Introduction

Canoe sprint has been an Olympic sport since 1936, featuring races in single, double, and four-person canoes and kayaks over distances of 200, 500, and 1000 meters [1]. Success in this sport depends on a complex combination of physical strength, endurance, and precise technique, as athletes strive to optimize their paddling efficiency while minimizing drag [2,3]. In addition to rigorous training methods, many athletes use ergogenic aids to boost their performance. Sports supplements, in particular, have shown to be helpful in meeting the physical demands of canoe sprint by increasing energy production, delaying fatigue, and supporting faster recovery. Recent studies have consistently confirmed caffeine’s ergogenic effects across diverse sports and populations, enhancing anaerobic performance, reaction time, and training capacity [4–9]. Evidence also indicates that caffeine can exert complementary effects when combined with other stimulants or nutrients, such as carbohydrate gels or gum-based formulations [8,9]. However, research on caffeine combined with theacrine or other purine alkaloids remains limited, with no published studies addressing such combinations in canoe sprint.

Among pre-workout supplements, caffeine is the preferred choice in canoe sprint because of its effectiveness in enhancing endurance and performance [10,11]. Caffeine functions as a competitive antagonist at adenosine receptors, affecting the sleep–wake cycle by blocking adenosine’s inhibitory effects. This leads to elevated concentrations of neurotransmitters, including dopamine and serotonin, which enhance concentration, improve mood, and reduce fatigue. Additionally, caffeine-induced adenosine blockade alters autonomic nervous system activity, resulting in increased systolic blood pressure and heart rate, particularly during physical activity [12].

Although caffeine effectively improves performance across various parameters, its use can lead to potential side effects, including increased cardiovascular stress, tolerance development with prolonged use, and the need for precise timing to maximize benefits. Concerns about side effects like tachycardia, agitation, insomnia, and dependency risks caused the World Anti-Doping Agency (WADA) to ban caffeine until 2004, and it remains on the agency’s monitoring substances list. To address these concerns, alternative compounds such as theacrine are being studied for similar benefits with fewer adverse effects.

Theacrine (1,3,7,9-tetramethyluric acid) is a purine alkaloid found in Camellia species, especially in Kucha tea from Yunnan, China, which has traditionally been used to treat colds [13–15]. Structurally similar to caffeine, theacrine shows ergogenic potential through a slower onset of action (~2 hours) and positive effects on mood and cognitive performance [16,17]. It boosts locomotor activity by modulating adenosine and dopamine receptors in the nucleus accumbens, without causing tolerance [18].

Theacrine (TeaCrine®) is safe in humans at doses of up to 300 mg/day for eight weeks, supporting consistent energy and focus without the development of tolerance. Like caffeine, theacrine acts as an adenosine receptor antagonist, but it offers more sustained benefits without the rapid decline in caffeine levels usually seen in the body [19]. Because they have similar mechanisms but different half-lives, most research has centered on combining caffeine and theacrine to achieve both quick and long-lasting ergogenic effects. Studies indicate that theacrine has no adverse effects on heart rate or blood pressure when used alone or in combination with caffeine [16,20,21]. Its strong bitterness and increased astringency—linked to stronger protein interactions—may limit its use on its own, making combining it with caffeine a more enjoyable and effective strategy [22–24].

Theacrine may enhance both physical and mental performance, particularly in sports that require a combination of aerobic, anaerobic, and mental efforts [12,25]. When combined with caffeine and Dynamine®, (methylliberine, a structurally related purine alkaloid), it improves cognitive control and speed–accuracy in gaming tasks compared to caffeine alone or a placebo [26]. While research shows only modest or no gains in muscular strength among resistance-trained athletes [25,27,28], theacrine has potential for synergistic use with caffeine to fight fatigue,reduce habituation,and offer health benefits,making it a promising nootropic for athletes [29].

Therefore, this study aimed to assess the acute effects of theacrine, caffeine, and their combination on performance and physiological responses in elite canoe sprint athletes. To author's knowledge, this is the first study to simultaneously assess muscle oxygenation (SmO₂), anaerobic capacity (MAOD), and power output to evaluate theacrine–caffeine interactions in canoe sprint.

2. Methods

2.1. Trial design

This repeated-measures, randomized, placebo-controlled, double-blind study involved 20 elite canoe sprint athletes, each receiving four treatments in a crossover design: placebo (P), caffeine (K), theacrine (T), and a combination of caffeine and theacrine (KT). No protocol changes occurred after the study began. Both participants and investigators were blinded to the treatments until the study's completion. At the end of the study, participants were asked to identify the supplements ingested during each trial; responses did not differ from chance, indicating that the blinding procedure was effective. The study followed the CONSORT 2025 guidelines, with the design shown in Supplementary Figure 1 [30].

2.2. Supplementation protocol

Supplements were provided in hydroxypropyl methylcellulose (HPMC) capsules, identical in appearance and filled with methylsulfonylmethane (MSM). Dosages were as follows: 200 mg of caffeine (K), (~2.25 mg·kg⁻¹ body mass), (Sigma-Aldrich, USA), 200 mg of theacrine (T) (TeaCrine®, Compound Solutions, USA), or a combination of 100 mg caffeine and 100 mg theacrine (KT). The caffeine dose (~2.25 mg·kg⁻¹ ) was selected based on safety recommendations for acute intake. Participants ingested these 60 minutes before testing, and this timing was kept identical across all study visits. Tests were consistently conducted in the morning after an overnight fast of at least 12 hours. Trials were spaced at least 72 hours apart to ensure complete elimination of the substances. Participants were instructed to avoid vigorous physical activity and caffeine intake from any source while maintaining their usual diet during the 24 hours prior to each test.

2.3. Sample size

A minimum sample of 20 participants was calculated using power analysis (via G*Power 3.1.9.3, Heinrich-Heine-Universität Düsseldorf), assuming an effect size of 0.3, an alpha error probability of 0.05, and a power of 0.95. The analysis was based on a single group with four repeated measures (placebo, caffeine, theacrine, and their combination). The treatment order for each participant was randomly generated by a computer program, with an independent person (not involved in the study) assigning the sequence to ensure blinding.

2.4. Participants

Twenty highly trained male canoe sprint athletes (mean age: 21.9 ± 4.5 years; body mass: 88.7 ± 10.4 kg; height: 186.1 ± 4.8 cm; VO_2max: 61.9 ± 6.1 mL/kg/min) voluntarily took part in the study. All participants were current members of national canoe sprint teams with 7.9 ± 1.8 years of training and 5.1 ± 1.4 years of competitive experience, engaging regularly in structured training programs, including on-water paddling and gym workouts. Although participants typically engaged in >10 training sessions per week, testing sessions were scheduled in collaboration with coaches to coincide with lighter training days, and all athletes were instructed to refrain from vigorous activity for at least 24 h prior to each trial, which they confirmed through training logs and self-report. Among them, four were finalists at the Paris 2024 Olympic Games, and one was a former European champion. Inclusion criteria required participants to (1) be between 18 and 35 years old; (2) have at least 5 years of continuous training in sprint canoeing or kayaking; (3) be free of injury or illness for the past three months; and (4) have no known cardiovascular, metabolic, or neurological conditions. Participants were excluded if they reported habitual caffeine intake exceeding 6 mg/kg body mass per day or had a history of supplement sensitivity. Habitual caffeine intake was assessed through direct questioning: No participant exceeded >6 mg·kg⁻¹ ·day⁻¹ . Twelve athletes reported no regular use, while eight consumed ~2 cups of coffee per day (~160–200 mg; ~1.8–2.2 mg·kg⁻¹·day⁻¹). None habitually used caffeine-containing supplements, which were taken only during competitions to avoid tolerance. Additionally, athletes were instructed to abstain from all caffeine-containing products (coffee, tea, chocolate, energy drinks, supplements) for 24 h before each trial, and compliance was confirmed by self-report. Before taking part, all athletes provided written informed consent after being briefed on the procedures, benefits, and potential risks of the study. The study followed the Declaration of Helsinki and was approved by the Institutional Ethics Committee of the Faculty of Medicine, Novi Sad, with approval number 01−39/62/1/2025.

2.5. Muscle oxygenation measurements

Muscle oxygen saturation (SmO₂) was measured at the right vastus lateralis (VL) using a wearable near-infrared spectroscopy (NIRS) sensor (Moxy Monitor, Fortiori Design LLC., Hutchinson, MN, USA). The Moxy Monitor is a standalone, relatively affordable, continuous-wave NIRS device that quantifies an arbitrarily scaled heme volume, reflecting both hemoglobin (Hb) and myoglobin (Mb) concentrations within the illuminated tissue as SmO₂ on a 0–100% scale. In addition to SmO₂, total hemoglobin (THb) was recorded as a relative measure of changes in local blood volume during exercise and recovery. The sensor was placed over the muscle belly of the right VL, located at one-third of the distance between the patella and the greater trochanter, with participants seated and the knee flexed at 90°. The device was secured with adhesive tape and the manufacturer-provided light shield to reduce ambient light interference and motion artifacts. Although a low skinfold is recommended for NIRS accuracy, all participants were included to represent typical elite-athlete profiles, with no adjustments made to their skinfold measurements. Data were collected at 0.5 Hz (one measurement every 2 seconds) and smoothed with a 5-second moving average, using the manufacturer’s default settings [31].

2.6. Oxygen uptake (VO₂) measurement

Oxygen uptake was measured using the portable VO₂ Master Analyzer (VO₂ Master Health Sensors Inc., Vernon, BC, Canada), a validated device that provides breath-by-breath VO₂ measurements using flow-volume and oxygen fraction sensors. Before each testing day, the device was calibrated according to the manufacturer’s protocol, which included flow calibration with a 3 L calibration syringe and oxygen gas calibration using ambient air. During testing, the analyzer was fitted to a silicone face mask covering the nose and mouth to ensure airtight sampling during paddle ergometer efforts. VO₂ data were collected continuously at a sampling rate of 1 Hz throughout both the 500 m and Wingate trials, capturing key metrics such as peak VO₂, time to peak, and VO₂ kinetics. Raw data were visually inspected using the accompanying software (VO₂ Master Manager), and artifacts were removed; no additional smoothing was applied. All VO₂ outcomes were normalized to body mass (mL·kg⁻¹ ·min⁻¹) to allow for comparison between athletes [32].

To allow for individualized estimation of anaerobic capacity during the main trials, maximal oxygen uptake (VO_{2 max}) was assessed in a separate pre-experimental session using an incremental kayak ergometer test performed to voluntary exhaustion. The test was conducted on a Dansprint kayak ergometer (Dansprint ApS, Hvidovre, Denmark), with the workload increasing progressively at regular intervals. VO₂ was measured breath-by-breath, and VO₂max was defined as the highest 30-second average of oxygen uptake during the test. Achievement of VO₂max was confirmed through commonly accepted physiological criteria, including a VO₂ plateau, RER ≥ 1.10, near-maximal heart rate, and/or RPE ≥ 18. The VO₂max values obtained from this preliminary test were used to estimate the theoretical maximum aerobic contribution during supramaximal efforts and served as the basis for calculating individual maximal accumulated oxygen deficit (MAOD) in subsequent trials. The same Dansprint kayak ergometer was used for both VO_2max assessments and main trials to ensure consistency of equipment. Prior to each test, athletes’ body mass was entered into the software so that the propeller-based braking system could adjust resistance according to manufacturer’s instructions, thereby simulating on-water conditions.

2.7. Maximal accumulated oxygen deficit (MAOD)

Maximal accumulated oxygen deficit (MAOD) was used to estimate anaerobic energy contribution during the 500-meter time trial. VO₂ was recorded continuously using a breath-by-breath metabolic system, and VO_2max was determined during a prior incremental test. For each sprint, accumulated VO₂ was calculated as the area under the VO₂-time curve. Theoretical oxygen demand was estimated by multiplying VO_2max by sprint duration (in minutes), assuming a constant maximal aerobic rate. MAOD (mL·kg⁻¹) was calculated as the difference between the estimated oxygen demand and the measured oxygen uptake:

$$\mathrm{MAOD}\left({\mathrm{mL·kg}}^{-1}\right)=({\mathrm{VO}}_{2\max }\times \mathrm{t})-{\int }_0^t{\mathrm{VO}}_2\left(\mathrm{t}\right)\mathrm{dt;}$$

This method reflects anaerobic energy contribution, as used in previous sprint models [33].

2.8. Statistical analysis

To assess the effects of different supplements on performance and physiological outcomes, a robust statistical framework incorporating both within-subject comparisons and mixed-effects modeling was used. Before analysis, raw data were systematically checked for outliers using interquartile range (IQR) and standard deviation thresholds. Outlier values were conservatively replaced with group medians or adjusted values based on context, except for primary outcomes such as race time, RPE, and blood pressure, which were left unchanged to preserve individual variability. The number of outliers identified per variable is reported in Supplementary Table S1. Assumptions of normality were verified using the Shapiro–Wilk test. For each test, a linear mixed-effects model was fitted with supplement as a fixed effect and athlete identity as a random effect to account for repeated measures and individual differences. The placebo was consistently used as the reference group.

Additionally, a Pearson correlation analysis was conducted between primary outcome parameters (e.g. race time, MAOD, power output, fatigue index, total distance) and secondary physiological variables (SmO₂, THb, RPE, HR, stroke count). Slopes of SmO₂ and THb during and after trials were calculated to reflect dynamic oxygen use and blood flow responses. Significance was set at p < 0.05. All analyses were performed using R statistical software (version 4.5.0).

2.9. Experimental protocol

Each participant underwent all four conditions in a randomized order during four different trial sessions and one familiarization session.

2.9.1. 500-meter kayak ergometer time trial

For the 500-meter kayak ergometer time trial, systolic and diastolic blood pressure, as well as resting heart rate, were measured in a seated position to exercise testing using an automated upper-arm monitor. Participants then completed a standardized warm-up involving 3–5 minutes of low-intensity paddling with several short accelerations. After the warm-up, each participant was equipped with a VO₂ Master Analyzer (VO₂ Master Inc., Canada) to measure oxygen uptake (VO₂), a Moxy Monitor placed on the vastus lateralis (VL) muscle of the dominant leg to assess muscle oxygen saturation (SmO₂), and a chest strap heart rate monitor. Participants then performed a maximal 500-meter effort, recording their race time on the kayak ergometer (Dansprint PRO), during which VO₂, heart rate (HR), and SmO₂ were continuously recorded. Immediately after finishing the race, blood pressure and heart rate were reassessed to capture acute cardiovascular responses, and all parameters (VO₂, HR, SmO₂) continued to be monitored for an additional 5 minutes during passive recovery. The RPE was collected immediately after the race using the Borg 6–20 scale [34].

2.9.2. Wingate anaerobic test (WAnT)

Maximal power (PP), average power (MP), and fatigue index (FI) were recorded during the 30-second Wingate Anaerobic Test (WAnT), conducted 60 minutes after participants ingested the supplements to ensure adequate absorption. The Moxy Monitor was placed on the vastus lateralis (VL) muscle, and heart rate (HR) was continuously monitored during the WAnT and for 3 minutes post-exercise. The Ratings of Perceived Exertion (RPE) were collected immediately after the exercise using the Borg 6–20 scale.

3. Results

3.1. 500-meter kayak ergometer time trial

In the 500-meter kayak ergometer time trial, caffeine supplementation (K) significantly enhanced performance compared to a placebo (P). The average race time decreased by 1.872 seconds (p < 0.001, d = 0.65), while the combination of caffeine and theacrine (KT) resulted in a reduction of 1.383 seconds (p = 0.018, d = 0.49). Theacrine alone (T) caused a minor decrease of 0.462 seconds, which was not statistically significant (p = 0.430, d = 0.12). These results suggest that caffeine, either alone or combined with theacrine, benefits short-duration, high-intensity performance. However, theacrine alone has limited ergogenic effects under these conditions. These findings are illustrated in Figure 1, where both K and KT demonstrate a clear shift toward faster race times compared to the placebo. In addition, Supplementary Table 2 is provided with absolute values for each condition.

Figure 1.

Violin plot displaying individual and group-level changes (Δ) in 500-meter kayak ergometer time trials under three supplementation conditions compared to placebo: caffeine + theacrine (KT), caffeine alone (K), and theacrine alone (T). Negative values signify performance improvements (faster times). Each dot represents an athlete. Solid vertical bars indicate the interquartile range (IQR), and horizontal lines show the average race time for each condition.

Caffeine supplementation also resulted in a significant increase in anaerobic capacity, with MAOD rising by 327.4 J compared to a placebo (p < 0.005, d = 0.54). Figure 2a shows the VO₂ kinetics of a representative athlete, illustrating how MAOD was calculated as the area between VO_2max and the VO₂ curve during the 500-meter time trial. The caffeine/theacrine combination and theacrine alone caused moderate, non-significant increases in MAOD of 90.8 J (p = 0.438, d = 0.22) and 31.8 J (p = 0.786, d = 0.08), respectively. Similarly, average power output was significantly higher in the K condition (p < 0.001, d = 0.52) and moderately higher with KT (p < 0.005, d = 0.41), while theacrine alone had a marginal, non-significant effect (p = 0.319, d = 0.10).

Figure 2.

(a) Representative oxygen uptake (VO₂) curves during the 500-meter kayak ergometer time trial for a single athlete under placebo (500_P), theacrine (500_T), caffeine (500_K), and caffeine + theacrine (500_KT) conditions. The shaded area beneath each curve shows the total oxygen consumption during the trial. The horizontal dashed line marks this athlete’s VO_2max, measured in a pre-test. The area between the VO_2max line and each VO₂ curve indicates the Maximal Accumulated Oxygen Deficit (MAOD), which estimates the contribution of anaerobic energy. (b) Muscle oxygen saturation (SmO₂) responses from the same athlete during and after the 500-meter kayak ergometer time trial under four conditions: placebo (P), theacrine (T), caffeine (K), and caffeine + theacrine (KT). Data were collected through continuous near-infrared spectroscopy (NIRS) monitoring of the vastus lateralis.

Physiological measurements taken during the 500-meter time trial did not show statistically significant effects of supplementation on the rate of muscle oxygen extraction, as indicated by the SmO₂ slope. Although the theacrine (T) condition showed a trend toward a less negative slope (p = 0.074), suggesting a possible reduction in muscle deoxygenation, this effect was not statistically significant. Figure 2b presents a representative example of real-time SmO₂ dynamics from a single athlete, highlighting differences in deoxygenation and reoxygenation kinetics across supplement conditions. Neither the caffeine (K) nor the caffeine + theacrine (KT) conditions produced significant differences from placebo (p > 0.690 and p > 0.740, respectively). These results suggest that muscle oxygenation dynamics during sprinting were not significantly influenced by supplementation. Other mechanisms, such as increased mechanical power output, likely mediate the performance-enhancing effects of caffeine.

The total hemoglobin (THb) slope, which indicates changes in blood volume and muscle perfusion during exercise, was not significantly affected by any of the supplementation conditions. No notable differences were seen for caffeine (p = 0.968), theacrine (p = 0.764), or the caffeine + theacrine combination (KT) (p = 0.378) compared to placebo. Absolute values for THb slopes are provided in Supplementary Table 3. These findings imply that the performance gains observed with supplementation are probably not driven by changes in THb dynamics or muscle perfusion, as measured by NIRS during the 500-meter sprint.

No significant differences were observed between supplement conditions for systolic or diastolic blood pressure, heart rate (before or after exercise), or ratings of perceived exertion (RPE), with all p-values exceeding 0.1, as shown in Figure 3. Absolute values for these outcomes are provided in Supplementary Table 4. These findings indicate that the observed performance improvements were not driven by cardiovascular stress or changes in perceived effort, but rather by enhancements in muscular efficiency and oxygen utilization.

Figure 3.

Boxplots displaying changes (Δ) in cardiovascular parameters from before to after a 500-meter kayak ergometer time trial across supplement conditions: placebo (P), caffeine + theacrine (KT), caffeine (K), and theacrine (T). (a) Δ Systolic blood pressure (mmHg), (b) Δ Diastolic blood pressure (mmHg), and (c) Δ Heart rate (bpm). Each dot represents an individual athlete. No statistically significant differences were observed across conditions (all p > 0.1).

Correlation analysis showed several strong links between physiological variables and race time in the 500-meter time trial (Figure 4a). Among all factors, average and maximum power had the strongest negative correlations with race time, confirming the importance of mechanical output for performance. Specifically, athletes who consistently produced higher average (r = –0.95, p < 0.001) and peak power (r = –0.73, p < 0.001) finished the race faster, underscoring the significance of sustained force application during the event. Additionally, minimum power also demonstrated a notable negative correlation with race time (r = –0.60, p = 0.004), suggesting that maintaining a higher baseline effort is beneficial to performance.

Figure 4.

Heatmaps of Pearson correlation coefficients showing relationships among performance, physiological, and subjective variables. (a) During the 500-meter kayak ergometer time trial, race time had strong negative correlations with average power (r = –0.95), maximum power (r = –0.73), and minimum power (r = –0.60). In contrast, SmO₂ and cardiovascular variables showed weaker or inconsistent associations. (b) In the Wingate test, maximal paddled distance was strongly correlated with average power (r = 0.81), peak power (r = 0.69), and minimum power (r = 0.67). Fatigue Index (FI) was negatively related to both distance (r = –0.39) and minimum power (r = –0.89).

Muscle oxygenation metrics, including SmO₂ and THb slopes, showed weak to moderate positive correlations with race time when all participants were analyzed together. This suggests that faster oxygen extraction or increased perfusion was not necessarily linked to better performance across all athletes. However, when examined by supplement condition, the KT group displayed moderate positive correlations between race time and both SmO₂ slope (r = 0.39, p = 0.09) and THb slope (r = 0.51, p = 0.03), indicating that muscle oxygenation responses during combined supplementation may involve different physiological mechanisms or compensatory strategies (Table 1). These results support the physiological importance of near-infrared spectroscopy (NIRS)-based metrics in capturing real-time oxygen and hemodynamic changes during high-intensity sprint efforts.

Table 1.

Pearson correlation coefficients between selected physiological and performance variables and (a) 500-meter race time and (b) total distance paddled during the Wingate test across all supplement conditions (P—placebo, T—theacrine, K—caffeine, KT—caffeine + theacrine).

	Variables	P	T	KT	K
500 m kayak ergometer time trial Correlation with RaceTime	Avg_Power	−0.953	−0.945	−0.959	−0.952
	Diastolic_After	0.257	−0.107	0.153	0.214
	Diastolic_Before	0.315	−0.129	−0.278	0.004
	HR_After	−0.339	0.171	0.02	0.158
	HR_Before	0.017	0.163	0.076	−0.089
	MAOD	−0.417	0.419	−0.18	0.027
	Max_Power	−0.463	−0.825	−0.724	−0.835
	Min_Power	−0.557	−0.487	−0.537	−0.849
	RPE	0.269	0.168	0.635	0.382
	SmO2_Slope	0.294	0.112	0.39	0.078
	SmO2-second_Slope	0.616	0.206	0.375	0.443
	Systolic_After	0.3	−0.131	0.193	0.33
	Systolic_Before	0.098	0.108	−0.005	0.062
	THb_Slope	0.17	-0.113	0.508	0.131
	THb-second_Slope	0.131	0.118	−0.054	0.128
Wingate test Correlation with Distance	Avg_Power	0.912	0.86	0.663	0.784
	FI	−0.544	−0.189	−0.366	−0.471
	Max_Power	0.772	0.76	0.607	0.67
	Min_Power	0.729	0.634	0.58	0.729
	RPE	0.131	−0.138	−0.061	−0.106
	SmO2_Slope	−0.047	−0.064	−0.116	0.117
	THb_Slope	0.028	−0.358	0.056	0.056

Subjective measures, such as the rating of perceived exertion (RPE), showed a weak overall correlation with race performance, suggesting that perceived effort remained relatively stable across different performance outcomes. However, under specific supplement conditions—especially KT (r = 0.64, p = 0.02) and K (r = 0.38, p = 0.11)—RPE displayed moderate positive correlations with race time. This could indicate heightened sensitivity to perceived exertion due to stimulant effects or fatigue-related feedback in these groups. Post-exercise heart rate showed a weak overall link with performance. Nonetheless, under caffeine and theacrine, higher heart rates were modestly associated with slower times, hinting at individual differences in recovery under stimulant conditions. Blood pressure analysis revealed weak correlations between post-race systolic and diastolic pressures and race time in most conditions. However, under caffeine (K), systolic_after (r = 0.33, p = 0.15) and diastolic_after (r = 0.21, p = 0.38) exhibited slightly stronger positive correlations, possibly reflecting a more pronounced sympathetic nervous system response among faster athletes.

The role of MAOD was inconsistent across supplement conditions. Under placebo, a moderate negative correlation with race time (r = –0.42, p = 0.07) was observed, indicating that greater anaerobic capacity was associated with improved performance. However, this relationship reversed under theacrine (r = + 0.42, p = 0.08) and was nearly neutral under caffeine (r = + 0.03, p = 0.91) and KT. These different patterns may reflect individual differences in how the anaerobic energy system is engaged or how each supplement influences the contributions of energy pathways during maximal efforts.

When performance was analyzed by supplement condition, caffeine (K) produced the most consistent pattern of beneficial correlations. Under caffeine, race time was strongly and negatively associated with average power (r = −0.95, p < 0.001), maximum power (r = −0.84, p < 0.001), and minimum power (r = −0.85, p < 0.001), reinforcing caffeine’s effectiveness in enhancing mechanical output. Additionally, a weak negative correlation was observed between MAOD and SmO₂ slope under the caffeine condition, suggesting that subtle contributions may come from anaerobic metabolism and oxygenation efficiency.

The KT combination also showed strong negative correlations between race time and power outputs, as well as moderate links to oxygenation measures. These patterns suggest that combined supplementation may provide complementary effects, although with slightly more variability compared to caffeine alone. In contrast, theacrine alone (T) was associated with moderate negative correlations with average (r = –0.61, p = 0.01) and maximum power (r = –0.67, p = 0.004) but showed weaker and less consistent relationships with other performance measures. Under placebo, most correlations with race time were weak or inconsistent, further emphasizing the role of active supplementation in influencing performance outcomes and physiological responses.

3.2. Wingate anaerobic test (WAnT)

In the 30-second Wingate Anaerobic Test, caffeine (K) supplementation significantly enhanced performance compared to placebo (P). Specifically, caffeine increased total distance paddled by 2.31 meters (p = 0.005, d = 0.47) and improved maximal power output by 48.9 W (p < 0.001, d = 0.39), indicating a robust ergogenic effect on peak performance. There was also a trend toward improved average power output with caffeine, increasing by 14.7 W (p = 0.088, d = 0.34), although it did not reach statistical significance. The Fatigue Index (FI) showed no significant change with caffeine (p = 0.365, d = 0.10), suggesting no measurable improvement in power sustainability over the test duration.

The combination of caffeine and theacrine (KT) also produced statistically significant improvements. KT increased the average total distance paddled by 1.88 meters (p = 0.024, d = 0.33) and maximal power by 25.9 W (p = 0.034, d = 0.36). However, KT had no significant effects on average power output (p = 0.390, d = 0.10) or Fatigue Index (p = 0.470, d = 0.07), suggesting that while KT supports higher peak output, it may not enhance endurance-related aspects of anaerobic performance.

In contrast, theacrine (T) alone did not significantly impact the measured performance parameters. Changes in total distance paddled (β = + 0.01, p = 0.989), average power (p = 0.798), maximal power (p = 0.665), and Fatigue Index (p = 0.395) were all statistically non-significant, indicating limited ergogenic potential when theacrine is administered by itself. These outcome variables across supplement conditions are shown in Figure 5a–c (the absolute values mean, SD and 95% CI are presented in Supplementary Table 5).

Figure 5.

Violin plots showing changes in (a) maximal power output, (b) average power output, (c) total distance paddled, and (d) Rating of Perceived Exertion (ΔRPE) covered during the Wingate kayak ergometer test after ingestion of theacrine (T), caffeine (K), and caffeine + theacrine (KT), compared to placebo. Each point represents an athlete; central lines indicate group means with standard deviation.

During the Wingate test, physiological responses did not differ significantly among supplementation conditions. The muscle oxygen saturation (SmO₂) slope was higher in all supplemented groups compared to the placebo, but the differences were not statistically significant. Caffeine (p = 0.684), KT (p = 0.136), and theacrine (p = 0.200) all showed modest increases in SmO₂ slope, indicating slightly slower desaturation; however, this may reflect individual variability rather than a consistent effect. Similarly, total hemoglobin (THb) slope showed no significant differences across all supplementation groups. Group distributions of SmO₂ and THb slopes are shown in Figure 6. Neither caffeine (p = 0.837), KT (p = 0.937), nor theacrine (p = 0.750) had a significant impact on perfusion-related responses during the effort phase.

Figure 6.

Violin plots illustrating changes in (a) muscle oxygen saturation slope (ΔSmO₂) and (b) total hemoglobin slope (ΔTHb) during the 30-second Wingate test after ingestion of theacrine (T), caffeine (K), and caffeine + theacrine (KT), compared to placebo. Each point represents an athlete; central lines indicate group means with standard deviations.

Consistent with findings from the 500-meter time trial, heart rate measured both before and after the Wingate test, as well as ratings of perceived exertion (RPE), showed no significant differences between supplement conditions. Likewise, systolic and diastolic blood pressure values remained unchanged across trials (p > 0.1), supporting the interpretation that performance improvements were driven by local muscular adaptations rather than cardiovascular or perceptual shifts. Figure 5d displays individual and group-level RPE changes across the supplement conditions.

Correlation analysis clarified performance determinants in the Wingate test (Figure 4b). The total distance covered was positively correlated with maximal power output (r = 0.69) and average power (r = 0.81), underscoring the critical roles of both peak and sustained anaerobic output in success in short-duration efforts. Additionally, distance correlated negatively with Fatigue Index (r = –0.39), indicating that athletes who were better able to sustain their power output over time covered a greater distance.

In contrast, the muscle oxygen saturation slope (SmO₂ slope) and total hemoglobin slope (THb slope) showed no significant correlation with distance, indicating that muscle oxygenation and perfusion may not be limiting factors in this short, high-intensity anaerobic effort. Similarly, RPE did not correlate with any performance parameter (e.g. r = 0.03 with distance), highlighting the limited role of perceptual effort in differentiating performance during high-intensity, short-duration efforts.

Caffeine (K) showed the strongest positive correlations with distance covered during the Wingate test, especially with average power (r = 0.912) and maximal power (r = 0.772). It also exhibited a moderate negative correlation with the Fatigue Index (r = –0.544), indicating both enhanced peak performance and better fatigue resistance (Table 1). The combination of caffeine and theacrine (KT) showed similarly high correlations with average (r = 0.860) and maximal power (r = 0.760). However, it had a weaker association with the Fatigue Index (r = –0.189), suggesting a less pronounced impact on power maintenance. Theacrine alone demonstrated moderate correlations with average power (r = 0.784) and the Fatigue Index (r = –0.471), indicating its effect lies between that of caffeine and the placebo. Notably, it was the only condition showing a positive correlation with the SmO₂ slope (r = 0.117), which may reflect improved muscle oxygenation dynamics.

4. Discussion

This study is the first to explore the immediate effects of theacrine, caffeine, and their combination on both performance and physiological responses in elite canoe sprint athletes. The findings show that caffeine significantly improved 500-meter race times and power output in the Wingate test, especially increasing maximal and average power. These performance improvements were associated with enhanced oxygen extraction efficiency (as indicated by the SmO₂ slope), increased anaerobic capacity (measured by MAOD), and improved muscle perfusion (reflected by the THb slope). While theacrine alone showed modest and mostly statistically non-significant effects, its combination with caffeine led to additional improvements in several parameters, indicating a possible synergistic effect between the two compounds.

4.1. Performance outcomes: caffeine vs. theacrine

Caffeine’s ergogenic effects on both aerobic and anaerobic performance are well-documented and were reaffirmed in this study, showing clear improvements in race time, power output, and fatigue resistance. Although theacrine is chemically similar to caffeine, it did not match caffeine's effectiveness in enhancing performance. This discrepancy may be due to theacrine’s more prolonged onset of action (approximately 2 hours) and its primarily central nervous system effects, compared to caffeine’s quicker and peripheral actions. Since our testing was done 60 minutes after ingestion, peak theacrine bioavailability might not have been reached yet, which could partly explain its weaker results. These findings align with those of Casareo et al. [27], who reported no improvements in muscular strength, endurance, or power after acute supplementation with either 200 mg or 300 mg of theacrine in resistance-trained men, supporting the limited ergogenic potential of theacrine when used alone. Similar results were found by Cerqueira et al. [35], who saw no performance benefits from a 200 mg dose of theacrine in amateur team-sport athletes across various field tests. While both studies suggest theacrine has limited efficacy when used alone, differences in athlete skill level, type of performance, and testing environment might partly explain variations in response.

Interestingly, when combined with caffeine, theacrine contributed to improved performance and physiological measures beyond those of the placebo, although these gains did not consistently surpass those seen with caffeine alone. However, some studies have reported limited or inconsistent effects. Dillon et al. [29] found that a multi-ingredient supplement containing both caffeine and theacrine did not enhance performance, cognitive function, or perceived readiness in Division I athletes. Notably, readiness scores decreased after ingestion, suggesting that individual responses and sport-specific demands may influence the supplement's effectiveness. The KT condition improved muscle perfusion (THb slope) and fatigue resistance, indicating that theacrine may support recovery or extend caffeine’s stimulatory effects. Pharmacokinetic studies have also shown that co-administration of caffeine and theacrine may shift theacrine’s time to peak concentration closer to the time of exercise, potentially increasing its physiological relevance during acute testing [16]. Additionally, this aligns with previous findings that theacrine can reduce habituation to caffeine and maintain alertness without increasing cardiovascular strain [20].

While caffeine alone produced the strongest effects, combining caffeine and theacrine (KT) also resulted in notable improvements in maximum power output and distance covered despite containing only half the caffeine dose. Although this study was not designed to examine dose-response relationships, these findings suggest that theacrine may help sustain ergogenic effects when caffeine intake is lowered. This could be particularly important for athletes seeking to manage caffeine tolerance or minimize potential side effects while still reaping ergogenic benefits.

4.2. Muscle oxygenation and perfusion

This study underscored the importance of SmO₂ and THb as sensitive markers of muscular metabolic status during sprint paddling. Both the SmO₂ slope and SmO₂ decline were strongly linked to performance, confirming that rapid and profound muscle deoxygenation indicates higher work rates and effective oxygen use. Caffeine and KT supplementation showed trends toward improved oxygenation dynamics, suggesting that adenosine receptor antagonism may enhance oxygen availability and use during high-intensity efforts [25]. These results are consistent with those of Paquette et al. [36], who found that sprint interval training on a kayak ergometer caused the most significant muscle deoxygenation, especially in the vastus lateralis, compared to other HIIT formats, highlighting the key role of peripheral oxygen extraction in sprint kayak performance. Theacrine’s lack of impact on the SmO₂ slope might be due to differences in pharmacodynamics, specifically weaker stimulation of peripheral circulation. Still, the moderate correlations observed with theacrine alone suggest individual differences in response that could be worth further investigation.

4.3. Central vs. peripheral effects

A notable finding of this study is the consistent lack of change in heart rate, blood pressure, and RPE across supplement conditions, despite significant improvements in performance. This suggests that the performance enhancements were not due to increased cardiovascular strain or altered perception of effort, but instead to local muscular and metabolic adaptations. These findings support the proposed nootropic and motivational effects of theacrine, which may boost neural drive without overstimulating the cardiovascular system [16]. Recent research on tactical personnel has also shown that a caffeine–theacrine combination improves cognitive performance under fatigue without increasing cardiovascular strain [37]. Similar results in e-sports research suggest that theacrine’s central effects could enhance precision and decision-making, further indicating its role in cognitive and neural modulation rather than cardiovascular stimulation. Evans et al. [26] demonstrated that combining caffeine with theacrine and Dynamine improved cognitive control and reaction time, likely through increased neural activation. Supporting this, Tartar et al. [17] found that a mix of caffeine, theacrine, and methylliberine improved reaction time and inhibitory control in e-gamers without adverse mood effects. Although we did not directly measure cognitive function, these central nervous system mechanisms may partly explain the performance benefits observed without an increase in cardiovascular strain. This idea is further supported by Bello et al. [12], who found that adding theacrine to caffeine improved post-match cognitive performance in elite soccer players without raising heart rate or perceived exertion, highlighting the potential for centrally mediated effects in real-world fatigue conditions.

4.4. Practical applications

For coaches and athletes involved in canoe sprint and similar high-intensity sports, these findings confirm that caffeine remains a dependable ergogenic aid for enhancing anaerobic capacity and muscle efficiency. Theacrine alone did not produce measurable performance gains but showed potential when combined with caffeine, suggesting modest additive effects. Since both substances are currently permitted in sports, caffeine-based strategies should remain the primary approach, while theacrine may be considered only as part of a combined protocol.

4.5. Study limitations

This study has several limitations that should be acknowledged. The ergogenic effects of threatine may depend on timing, and the 60-minute pre-ingestion period might not have aligned with its peak pharmacological activity. The inclusion of only male athletes limits the applicability of the results to female populations. Although the sample size was sufficient to detect main effects, it may have been too small for more detailed subgroup analyses. Complete avoidance of training is not fully feasible in elite athletes; however, the standardized scheduling minimized potential variability. Finally, recent evidence suggests that theacrine supplementation offers minimal long-term benefits in amateur athletes [35], highlighting the need for further research into its chronic use, particularly among elite athletes.

4.6. Future directions

Further research should investigate the long-term effects of theacrine and caffeine supplementation over extended training cycles to understand better tolerance development, physiological adaptation, and recovery processes. Including female athletes and examining other sports with mixed energy system demands (e.g. rowing, CrossFit) would improve the applicability of the findings. Additionally, studying cognitive or neurophysiological markers (e.g. reaction time, executive function) would help clarify the nootropic effects of theacrine and its potential influence on sports that require quick decision-making and sharp mental focus.

5. Conclusion

Acute caffeine supplementation significantly improved canoe sprint performance by increasing maximal power output, total distance traveled, and anaerobic energy contribution, all without raising cardiovascular strain or perceived effort. Theacrine alone did not yield notable performance benefits; however, when combined with caffeine (KT), it led to statistically significant gains in key performance measures. Interestingly, these improvements occurred despite a lower caffeine dose in the KT condition, implying a possible synergistic effect between caffeine and theacrine. Although this study was not designed to examine dose-response relationships, the results may have practical relevance for athletes seeking to reduce caffeine intake while maintaining ergogenic effects. Overall, the findings support the strategic use of caffeine—and possibly caffeine-theacrine combinations—in short-duration, high-intensity sports, such as canoe sprint. Future research should investigate the long-term effectiveness, sex-based differences, and optimal dosing protocols for theacrine.

Supplementary Material

Supplementary Material

Supplementary tables and figures.

Supplemental Material

Supplemental data for this article can be accessed at https://doi.org/10.1080/15502783.2025.2590097.

Author contributions

Conceptualization: Pavle Jovanov, Borislav Obradović, Milan Vraneš; Methodology: Pavle Jovanov, Borislav Obradović, Milan Vraneš; Formal analysis and investigation: Pavle Jovaanov, Aleksandar Marić, Maksim Rapaić, Nikola Maravić; Writing — original draft preparation: Pavle Jovanov; Writing — review and editing: Pavle Jovanov, Otto Barak, Milan Vraneš, Borislav Obradović.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Funding

This research was supported by the Ministry of Science, Technological Development and Innovation, Republic of Serbia (Contract No. 451−03−136/2025−03/200222).

Ethics approval statement

The study adhered to the Declaration of Helsinki and was approved by the Institutional Ethics Committee of the Faculty of Medicine, Novi Sad, with approval number 01−39/62/1/2025. All athletes provided written informed consent after being briefed on the procedures, benefits, and potential risks involved.