Effect of single or combined caffeine and L-Theanine supplementation on shooting and cognitive performance in elite curling athletes: a double-blind, placebo-controlled study

ABSTRACT

Purpose

Previous studies have investigated the effect of single or combined caffeine (CAF) and L-theanine (THE) intake on attention performance. However, its effect on shooting performance and cognitive performance in a sport is unknown. The aim of this study was to investigate the hypothesis “Does single or combined CAF and THE supplementation have an effect on shooting and cognitive performance in elite curling athletes?.” It is predicted that over the next decade, studies based on nutritional ergogenic supplements in the developing sport of curling will continue to increase, leading to a significant increase in studies examining the effects of CAF and THE supplementation, alone or in combination, on throwing and cognitive performance in elite curling athletes.

Methods

In this double-blind, randomized controlled crossover study, twenty-two elite national curling athletes (age 20.20 ± 1.61 and sports age 6.20 ± 0.51 years, height 174.10 ± 7.21 cm, BMI 21.80 ± 3.47 kg/m²) were randomly assigned to CAF (6 mg/kg single dose CAF), THE (6 mg/kg single dose THE), CAF*THE (6 mg/kg CAF and 6 mg/kg THE combined) and PLA (400 mg maltodextrin) groups at each of four sessions. 60 minutes after taking the supplement, the athletes were first given the Stroop test and then asked to shoot.

Results

Our main findings have shown that the performance of athletes in guard (F=_3.452, P < .001, η_p² = .842), draw (F=_1.647, P < .001, η_p² = .485), and take-out (F=_3.121, P < .001, η_p² = .743) shot styles significantly improved when comparing the combined intake of CAF and THE to the PLA. Regarding cognitive performance evaluation through the Stroop test, during the NR task (F=_4.743, P = .001, η_p² = .653), the combined intake of CAF and THE significantly improved reaction times compared to the intake of single CAF, THE, or PLA. The best reaction times during the CR and ICR (respectively; F=_2.742, P = .004, η_p² = .328; F = 1.632, P < .001, η_p² = .625) tasks were achieved with the combined CAF and THE intake, showing a significant improvement compared to PLA. During the NER (F=_2.961, P < .001, η_p² = .741), task, the combined intake of CAF and THE significantly improved error rates compared to the intake of CAF, THE, or PLA single. The best accuracy rates during the CER and ICER (respectively; F=_4.127, P < .001, η_p² = .396; F=_3.899, P < .001, η_p² = .710) tasks were achieved with the combined CAF and THE intake, leading to a significant reduction in error rates compared to PLA. Based on these findings, it has been demonstrated in this study that the best shooting scores and cognitive performance were achieved, particularly with the combined intake of CAF and THE.

Conclusions

Based on these findings, it has been demonstrated in this study that the best shooting scores and cognitive performance were achieved, particularly with the combined intake of CAF and THE. The combined use of these supplements has been found to be more effective on shooting and cognitive performance than their single use.

KEYWORDS:

• Caffeine
• L-Theanine
• curling
• cognitive performance

1. Introduction

Curling is a team sport played on ice, characterized by competition and strategy. Despite being perceived as slow-paced, this sport places great importance on physical skills and precise shots [1]. This sport requires physical endurance, balance, coordination, and delicate touch. As curling is played on an ice rink, having a stable stance and movement skills are crucial [2]. However, the most distinct and significant feature of curling is the great importance placed on precise shots. Shots must be made with the right speed, angle, and trajectory. This relies on curling athletes’ abilities to make fine adjustments, hand-eye coordination, and strategic thinking skills. Since each shot can influence the team’s chances of winning, making the right shot at the right time provides a significant advantage. Additionally, with long matches and consecutive shots, athletes must maintain their energy levels and concentration [3]. To succeed in curling, athletes must not only have physical and shooting prowess but also maintain critical cognitive performance levels. They must move harmoniously with their teammates, jointly plan strategies, and swiftly execute decisions. This aspect is of paramount importance as it plays a crucial role in their ability to succeed in curling.

In curling, cognitive performance plays a significant role in various aspects. Cognitive performance is related to athletes’ mental processes, decision-making abilities, focus, and strategic thinking skills. Athletes need to process information quickly, predict future moves, and act in line with the team’s strategy. As curling is a team sport, the cognitive performance of team members can significantly impact the team’s success [4]. With the increasing popularity of this discipline worldwide, curling athletes have started using various ergogenic supplements in competitions to enhance their cognitive and shooting performance and gain an advantage over their competitors. However, it is not clear which supplements athletes should use.

Caffeine (CAF) (1,3,7-trimethylxanthine) is among the most commonly used supplements by athletes in all sports modalities [5–7]. Since caffeine was removed from the list of banned substances for sports in 2004, caffeine supplementation has reached a prevalence rate of 76% among international competitive athletes [8]. The first studies investigating the effects of caffeine on sports performance began to emerge in the 1970s. These studies identified an improvement in time to exhaustion during endurance tests. Such ergogenic effects have been attributed to increased lipolysis and preservation of muscle glycogen [9]. Currently, the ergogenic capacity of caffeine is explained by its blocking effect on adenosine receptors [10] A1, A_2A, and A_2B due to caffeine’s similar chemical structure to adenosine [11]. By blocking adenosine receptors at the neuromuscular level [7], caffeine enhances neuromuscular recruitment [12]. Additionally, caffeine enhances the Na⁺-K⁺ pump [13] and increases Ca²⁺ bioavailability in the myoplasm by inducing the release of Ca²⁺ from the sarcoplasmic reticulum and inhibiting its reuptake [14], resulting in the translocation of glycogen phosphorylase-b to its active form, glycogen phosphorylase-a [15]. Furthermore, caffeine maximizes glycolytic activity through increased activity of phosphofructokinase [16]. This suggests that caffeine has ergogenic effects in various sports activities, with peak blood levels occurring after 60 minutes of caffeine supplementation [12,17]. Studies examining this topic so far have focused primarily on the physical performance benefits of caffeine, while its effects on cognitive performance have received less attention. Considering the importance of cognitive performance as a crucial factor in improving shooting performance for curling athletes, the need to investigate the effects of this supplement on cognitive performance has been highlighted.

L-theanine (THE) is an amino acid naturally found in green tea leaves [18]. In recent years, the effects of THE on exercise performance have started to be investigated [19–21]. THE is known for its stress-reducing effects. It is believed to promote relaxation by increasing serotonin, dopamine, and gamma-aminobutyric acid levels in the brain. Due to these effects, THE can enhance concentration and improve mental suitability [22,23]. For exercise performance, this means achieving a more focused mental state and higher levels of concentration. THE also have the potential to reduce stress hormones such as cortisol and adrenaline. Balancing cortisol levels with THE supplementation may lead to improved endurance during exercise [24,25]. Additionally, it has been suggested that THE can enhance the activation of alpha brain waves, promoting better focus and reducing distraction during exercise [26–28]. However, more research is needed to fully understand the effects of THE on exercise performance. There are currently a limited number of studies on THE and exercise-related issues and results are inconsistent. Currently, there are limited studies on this topic, and the results are inconsistent.

In conclusion, in curling, where cognitive performance and precise shots come together, athletes’ mental processes, decision-making abilities, focus, and strategic thinking skills have a significant impact on winning the game. Considering the mechanisms to enhance these abilities, the intake of single or combined CAF and THE is seen as a promising supplement strategy. This study aims to investigate the effects of single or combined CAF and THE supplementation on shooting and cognitive performance in elite curling athletes. In this context, in order to evaluate whether CAF and THE supplementation single or combination have an effect on shooting and cognitive performance in elite curling athletes, one hypothesis was formed to evaluate our main hypothesis. This hypothesis was designed “CAF and THE supplementation single or combination has no effect on shooting and cognitive performance in elite curling athletes.”

2. Method

2.1. Participants

Twenty-two elite Curling national team athletes (age: 20.20 ± 1.61 years, sports age: 6.20 ± 0.51 years, height: 174.1 ± 7.21 cm, body mass: 66.6 ± 10.05 kg) participated in this study. The inclusion criteria were as follows: (a) being over 18 years old, and (b) having at least five years of experience as a competitive athlete in the national Curling team and participating in national and international competitions. The exclusion criteria were as follows: (a) consuming any substances within the last three months that could affect hormone levels or sports performance, such as nutritional supplements or steroids, (b) using stimulants, narcotics, and/or psychoactive substances during the testing or supplementation phase, and (c) identifying any orthopedic, neurological, cardiovascular, pulmonary, or metabolic conditions that could impair performance in various tests. Before filling out the informed consent form, participants were provided with information about the research protocol, timeline, and the types of exercises and assessments they needed to complete. All protocols and procedures were conducted in accordance with the Helsinki Declaration.

2.2. Study design

This study was designed as a double-blind, randomized, crossover trial. A total of five experimental trial sessions were conducted, including exercise sessions and anthropometric measurements. The experimental trials consisted of separate sessions for CAF, THE, the combination of CAF and THE (CAF*THE), and placebo (PLA).To prevent bias resulting from circadian rhythm interference associated with supplement intake, participants attended four sessions within a 72-hour window at the same time frame (±0.5 hours) [7,29]. During each of the four sessions, participants were randomly given one of the following supplements: PLA, CAF, THE, or CAF*THE. All test measurements were performed between 9:00 and 11:00 a.m., with randomly assigned participants at the same time three days apart. Before starting the tests, the ice surface was prepared for shots by a trained ice technician. The evaluation of shot performance was done without the sweeping effect. Before each session, a neuro-psychological test called the Stroop test was conducted to assess cognitive performance. Subsequently, the athletes performed Guard, Draw, and Take-out shots sequentially, with one-minute intervals between each type of shot. In each shot style, athletes made eight shots with 30-second intervals between each shot.

2.3. Supplementation protocol

The dosages were determined using a sensitive analog scale (EJ-303 Precision Scale, External Calibration 310 g × 0.001 g) and all supplements were given as powder. The compounds were dissolved in 500 ml of tap water after being placed in unidentifiable containers. Maltodextrin in the PLA was 400 mg, but the amounts in the CAF, THE, and CAF*THE groups were 6 mg/kg. The doses were calculated using a sensitive analog scale and all supplements were provided in powder form. After being placed in indistinguishable containers, the compounds were swallowed with 500 ml of tap water. The PLA contained 400 mg of maltodextrin, while the CAF, THE, and CAF*THE groups contained 6 mg/kg of CAF and 6 mg/kg of THE (Figure 1).

Figure 1. Experimental representation of the test protocol.

All supplements were provided by the same company (My Protein, Manchester, United Kingdom). The supplements were taken 60 minutes before the start of the experimental session. The dosage of CAF was selected based on the recommendations of the International Society of Sports Nutrition [30]. The dosage of THE was chosen due to its characteristic dosage found in green tea, which is already available in the market [31]. The 60-minute window was selected to allow direct comparisons with previous research and to generally reflect the highest plasma presence of both THE [32] and CAF [30].

Furthermore, the consumption of coffee, tea, and stimulant medications was limited up to 48 hours before the experimental session to prevent potential interactions with supplement elimination. To ensure this, a list was prepared for participants, advising them to avoid caffeine-rich foods and beverages such as coffee, tea, mate, energy drinks, soft drinks, chocolate beverages, and chocolate.

2.4. Stroop Test

The Stroop test is a neuropsychological test that reflects frontal lobe activity. In this test, it has been discovered that saying the names of objects or colors takes longer than reading words associated with them, and this phenomenon is referred to as the “Stroop interference effect” [33]. The Stroop task consists of three categories: neutral, congruent, and incongruent. Participants were asked to press the “←” or “→” arrow key on a keyboard with their right index and ring fingers. Reaction times and error rates were measured. The Stroop task consisted of 3 blocks with 30 neutral, 30 congruent, and 30 incongruent trials in total. Baseline measurements were taken for 30 seconds at the beginning and end of the task. The stimuli remained on the screen until a response was made or until 2000 ms elapsed. The stimuli were presented at intervals of 1000 ms. Responses were given within 200 to 2000 ms after the presentation of the stimulus was considered acceptable. Responses that did not fall within this time range (i.e. 200–2000 ms) and responses where the participant pressed the wrong color button were considered incorrect. All words were written in Turkish characters. The Stroop task was designed using Psychtoolbox in MATLAB 2018.

2.5. Guard shot

The Guard shot is a strategic technique in curling where a stone is positioned in the Free Guard Zone (FGZ) without touching the scoring area to set up the game. The FGZ consists of four regions. Athletes were given 5 minutes for warm-up and familiarization with the ice before the shot performance. To evaluate the shot performance, a total of 8 shots were made into the designated 4 regions. The scoring for the shots is as follows: 1st region − 4 points, 2nd region − 3 points, 3rd region − 2 points, 4th region − 1 point, and stones that remained outside the designated areas were not scored (Figure 2).

Figure 2. Positional display and scoring of shots.

2.6. Draw shot

The Draw shot is a technique in curling where the stone is aimed directly toward the center of the house. The main objective of this shot is to make the closest shot to the center of the house and accumulate the highest possible score. The scoring for the shots is as follows: shots to the center of the house − 4 points, shots to the four-foot circle − 3 points, shots to the eight-foot circle − 2 points, shots to the twelve-foot circle − 1 point, and stones that remained outside the designated areas were not scored (Figure 2).

2.7. Take out shot

The Take-out shot is performed to remove the opponent’s stones from play by pushing them out of the playing area. In the study, a stone was positioned at the center of the house, and the athletes’ goal was to hit this stone and either keep it at the center of the house or place it with minimal deviation. The scoring for the shots is as follows: shots to the center of the house − 4 points, shots to the four-foot circle − 3 points, shots to the eight-foot circle − 2 points, shots to the twelve-foot circle − 1 point, and stones that remained outside the designated areas were not scored (Figure 2).

3. Familiarization

Each participant in this study was carefully selected. Due to their elite athlete status, participants were already familiar with the protocols of guard, draw, and take-out shots. However, to ensure no issues arose, all participants were informed about the practice/trial process and the cognitive performance tests and shots that would be performed. Subsequently, three days before the start of the experimental sessions, all participants took part in a familiarization session where the test protocols were explained to them. This way, they became acquainted with the procedures and trial process beforehand. A pre-determined, visually illustrated experimental flowchart was created for this study.

3.1. Statistical analyses

The mean and standard deviation (SD) of the data are presented. To ascertain whether all the acquired variables displayed a normal distribution, the Shapiro-Wilk test was applied. Repeated measurements were conducted using an analysis of variance over time (ANOVA-RM). Using the Mauchly test, Greenhouse-Geiser corrections for asymmetric distributions were assessed, and Bonferroni corrections were used for post hoc comparisons. 0.25, 0.26–0.63, and > 0.63 were determined as the small, medium, and large effect sizes, respectively, for the ANOVA-RM, according to the partial eta squared calculation [34]. IBM Corp., Armonk, New York, USA, provided the IBM SPSS Statistics for 25.0 for use in the tests. P 0.05 was used as the significant level. These images were produced by the applications GraphPad (10.0.2, Windows and Mac), and Biorender (GraphPad, San Diego, CA, USA; accessible at: https://www.graphpad.com/quickcalcs/randomize1.cfm and https://www.biorender.com/accessed on 1 July 2023). These images were produced by the applications GraphPad and Biorender (GraphPad, San Diego, USA; accessible at: https://www.graphpad.com/quickcalcs/randomize1.cfm).

4. Results

This study examined the effects of PLA, CAF, THE, and CAF*THE supplements on cognitive performance and shot performance in elite curling athletes. The obtained data were presented using tables and graphs.

The minimum, maximum, mean, and standard deviation values of the participants’ characteristics are shown in Table 1.

Table 1. Characteristics of the participants.

Variables	Mean ± SD	Min-Max
Age (years)	20.20 ± 1.61	18–22
Sports age (years)	6.20 ± 0.51	5–8
Height (cm)	174.10 ± 7.21	160–178
Body mass (kg)	66.60 ± 10.05	58–79
BMI (kg/m^2)	21.80 ± 3.47	18–25
Fat mass (%)	8.42 ± 5.12	5.9–25.5
Muscle mass (%)	91.58 ± 6.23	74.5–94.1

Abbreviations: BMI: Body mass index.

Supplement of different conditions between repeated measures one-way ANOVA, as a result of guard shot (score) (F=_3.452, P < .001, η_p² = .842), draw shot (score) (F=_1.647, P < .001, η_p² = .485), and take out shot (score) (F=_3.121, P < .001, η_p² = .743) revealed a statistically significant difference in the variables (Table 2).

Table 2. Shooting points of curling athletes.

	PLA Mean ± SD	CAF Mean ± SD	THE Mean ± SD	CAF*THE Mean ± SD	F	P	ηp^2
Guard Shot (score)	1.40 ± 2.22	14.10 ± 4.25	15.10 ± 3.90	18.10 ± 3.44*	3.452	<.001	.842
Draw Shot (score)	14.30 ± 5.16	18.50 ± 3.27	18.50 ± 5.33	24.70 ± 6.09*	1.647	<.001	.485
Take Out Shot (score)	14.20 ± 5.43	21.70 ± 4.27*	24.00 ± 4.52*	26.70 ± 5.51*	3.121	<.001	.743

Abbreviations: PLA: placebo, CAF: caffeine, THE: L-Theanine, mean±SD: mean±standard deviation, η_p²: partial eta square coefficient, *: Significantly different according to PLA values (P < .05).

A one-way ANOVA test was applied to evaluate repeated measurements of different supplementation protocols. As a result, NR (ms) (F=_4.743, P = .001, η_p² = .653), CR (ms) (F=_2.742, P = .004, η_p² = 0.328), ICR (ms) (F=_1.632, P < .001, η_p² = .625), NER (error rate) (F=_2.961, P < .001, η_p² = .741), CER (error rate) (F=_4.127, P < .001, η_p² = .396) and ICER (error rate) (F=_3.899, P < .001, η_p² = .710) revealed a statistically significant difference in the variables (Table 3).

Table 3. Reaction times and error rates were obtained from measurements under different conditions.

	PLA Mean ± SD	CAF Mean ± SD	THE Mean ± SD	CAF*THE Mean ± SD	F	P	ηp^2
NR (ms)	83.60 ± 11.52	741.00 ± 150.66	761.50 ± 113.87	568.50 ± 113.18*^#^Ϯ	4.743	<.001	.653
CR (ms)	72.00 ± 129.18	624.00 ± 114.81	616.00 ± 108.95	527.50 ± 122.27*	2.742	.004	.328
ICR (ms)	1025.00 ± 122.24	889.00 ± 160.51	902.50 ± 144.05	778.00 ± 144.89*	1.632	<.001	.625
NER (error rate)	9.20 ± 3.49	6.90 ± 1.85	7.40 ± 2.11	4.20 ± 2.34*^#^Ϯ	2.961	<.001	.741
CER (error rate)	6.00 ± 1.56	4.00 ± 2.21	4.20 ± 1.93	2.40 ± 1.07*	4.127	<.001	.396
ICER (error rate)	11.70 ± 3.52	7.30 ± 2.54	7.90 ± 2.18	5.30 ± 2.79*	3.899	<.001	.710

Abbreviations: PLA: placebo, CAF: caffeine, THE: L-Theanine, NR: Neutral reaction, CR: Congruent reaction, ICR: Incongruent reaction, NER: Neutral error rate, CER: Congruent error rate, ICER: Incongruent error rate, mean±SD: mean±standard deviation, η_p²: partial eta square coefficient, *: Significantly different according to PLA values (P < .05), ^#: Significantly different according to CAF values (P < .05), ^Ϯ: Significantly different according to THE values (P <0.05).

Figure 3. PLA: placebo, CAF: caffeine, THE: L-Theanine, NR: neutral reaction, CR: congruent reaction, ICR: incongruent reaction. Stroop task accuracy (ms) in different supplement conditions. *: significantly different according to PLA values, #: significantly different according to CAF values, Ϯ: significantly different according to the values (P < .05).

Figure 4. PLA: placebo, CAF: caffeine, THE: L-Theanine, NER: neutral error rate, CER: congruent error rate, ICER: incongruent error rate. Stroop task accuracy (error rate) in different supplement conditions. *: significantly different according to PLA values, #: significantly different according to CAF values, Ϯ: significantly different according to the values (P < .05).

5. Discussion

This study is the first to investigate the effects of individual and combined caffeine (CAF) and theanine (THE) intake on cognitive and shooting performance in elite curling athletes and to evaluate the impact of CAF and THE supplementation in curling. Our main findings have shown that the performance of athletes in guard, draw, and take-out shot styles significantly improved when comparing the combined intake of CAF and THE to the PLA. Additionally, while an increase in performance was observed in guard and draw shots with individual CAF and THE intake compared to PLA, this increase was not significant. However, a significant difference in shooting performance was observed when comparing individual CAF and THE intake to PLA in the take-out shot style. Regarding cognitive performance evaluation through the Stroop test, during the NR task, the combined intake of CAF and THE significantly improved reaction times compared to the intake of CAF, THE, or PLA single. The best reaction times during the CR and ICR tasks were achieved with the combined CAF and THE intake, showing a significant improvement compared to PLA (Figure 3). During the NER task, the combined intake of CAF and THE significantly improved error rates compared to the intake of CAF, THE, or PLA single. The best accuracy rates during the CER and ICER tasks were achieved with the combined CAF and THE intake, leading to a significant reduction in error rates compared to PLA (Figure 4). Based on these findings, it has been demonstrated in this study that the best shooting scores and cognitive performance were achieved, particularly with the combined intake of CAF and THE.

For curling athletes to be successful, whether as part of a team or individually, it is crucial to maintain their shooting performance throughout the competition. In long-duration curling matches, it is essential to preserve the decline in physical parameters that may occur due to fatigue and, most importantly, ensure the sustainability of cognitive performance, including decision-making abilities, focus, concentration, and strategic thinking. Cognitive performance is a fundamental element for success in curling. As a result, the use of nutritional supplements that directly impact these processes forms the unique nature of this study. These supplements aim to support and sustain cognitive abilities, which play a critical role in curling achievement.

In the current study, the most significant improvement in shooting scores and cognitive performance was observed after the combined intake of CAF and THE. When reviewing the literature, no previous studies were found that specifically evaluated the effects of CAF and THE supplementation on shooting and cognitive performance in any sport, including curling. Therefore, conducting a sport-specific evaluation becomes challenging. However, previous research has reported improvements in cognitive levels with the intake of CAF and THE, indicating their potential positive effects on cognitive performance [19,24,35–38]. Caffeine intake is based on the idea that it inhibits the release of inhibitory neurotransmitters like adenosine and dopamine (an excitatory neurotransmitter) in the brain. The antagonistic effects of caffeine can lead to the release of excitatory neurotransmitters, such as dopamine and norepinephrine, which can result in central ergogenic effects. The rationale for caffeine use is that it prevents the brain from releasing inhibitory neurotransmitters like adenosine and dopamine, which is an excitable neurotransmitter. Caffeine’s antagonistic actions may cause the release of excitatory neurotransmitters including dopamine and norepinephrine, which may have ergogenic effects in the brain [7]. Accordingly, caffeine supplementation is thought to promote a more positive mood [39,40], increase alertness, and reduce feelings of fatigue [41,42]. These effects can be considered beneficial for athletes engaging in sports with high physical and cognitive demands. In the current study, caffeine intake resulted in higher shooting scores compared to the placebo, along with faster reaction times and higher accuracy rates in cognitive performance. In various studies, caffeine has been reported to have positive effects on response times in tests such as the Stroop test and rapid visual information processing, as well as improving word learning speed and delayed recall when administered before or during exercise. Athletes who compete in high-demand physical and mental sports might benefit from these impacts. Caffeine consumption in the current study led to higher shooting scores as compared to the placebo, as well as quicker response times and greater rates of cognitive performance accuracy. Caffeine has been shown in several studies to improve word learning speed and delayed recall when taken before or during exercise, as well as response speeds in tests like the Stroop test and quick visual information processing [43,44]. Additionally, caffeine has been shown to improve reaction time, accuracy, and willingness to put physical effort into exercise [45]. It is seen that the findings we obtained with caffeine supplementation are similar to the literature.

The intake of THE supplementation has been suggested as a neuroprotective and cognitive-enhancing supplement [46]. Both peripherally and centrally administered THE has been shown to have effects on the brain [47,48], modulating monoamine levels in the central nervous system [49,50] and neurotransmissions of glutamate and glycine [51,52].

Furthermore, it has been reported that THE affects hippocampal synaptic plasticity and enhances cognitive performance. Additionally, THE is said to influence hippocampus synaptic plasticity and improve cognitive function [53,54]. In behavioral experiments, THE has also been suggested to exhibit anti-stress or antidepressant-like effects [55,56]. These effects have been associated with an increase in brain-derived neurotrophic factor expression in the hippocampus [57]. Additionally, THE’s anxiolytic effects have been observed to enhance hippocampal activity [58]. Moreover, studies have indicated that THE supplementation modulates alpha activity and plays a significant role in tasks requiring attention in electroencephalogram studies [59,60]. Furthermore, THE has been reported to have beneficial effects on mental state [61], including sleep quality [62]. However, based on our knowledge, only acute effects of THE have been investigated in healthy individuals. THE’s anti-stress effects (200 mg/day) have been observed following both once-daily [63,64] and twice-daily [65] administrations, while attention-enhancing effects were observed with four separate daily administrations of 100 mg/day [66]. Furthermore, a single administration of 200 mg/day was supported by decreased responses in functional magnetic resonance imaging. Furthermore, reduced responses in functional magnetic resonance imaging were maintained by a single dosage of 200 mg/day [67]. In terms of cognitive function, there are studies in the literature that suggest THE may cause performance decrements when administered single [62,68] or have no effect [69,70]. This circumstance indicates that regardless of the population or the application, it will not have a good effect. In this study, THE consumption led to greater shooting scores as compared to the placebo, as well as quicker response times and better cognitive performance accuracy rates. This situation predicts that it does not have a positive effect depending on the population or the application. In this study, THE intake resulted in higher shooting scores compared to the placebo, along with faster reaction times and higher accuracy rates in cognitive performance.

Several randomized controlled trials have reported improvements in cognitive performance by examining the acute effects of the CAF and THE combination of different aspects of cognitive functions [68,70–73]. The conducted studies and our findings have shown similar results, but it is evident that there is limited information in the literature regarding the sole supplementation of THE and its combined use with CAF. In contrast to our findings, some studies have also demonstrated that THE may antagonize the physiological effects of CAF [74,75]. In a meta-analysis study, Camfield, Stough [32], analyzed the effects of the Theanine-Caffeine combination on various subjective and objective measures of attention during the first two hours after administration. The authors reported a significant effect of the CAF*THE combination in improving subjective measures of alertness and attentional switching, as well as the accuracy of visual and auditory attention. Compared to the PLA, increases in shooting and cognitive performance were observed with both CAF and THE intake. However, the most notable and significant improvements were observed during the combined intake of CAF and THE. The underlying mechanisms of the reported effects in the context of this study are not well understood. At the neurochemical level, both CAF and THE are believed to directly or indirectly affect various neurotransmitter systems, including dopamine, serotonin, glutamate, and GABA. However, as far as we know, the effects of the CAF*THE combination have not been investigated at the receptor level. Further research is needed to uncover the exact mechanisms underlying the effects of CAF*THE combination intake.

Compared to PLA, the CAF*THE combination has proven to be the most effective supplement strategy for improving shooting and cognitive performance in curling athletes. THE uptake appears to exert its effects primarily through glutamatergic mechanisms [76]. It acts as a glutamate reuptake inhibitor in the hippocampus and functions as a competitive low-affinity glutamate receptor antagonist [46,76]. On the other hand, CAF intake inhibits adenosine receptors (types A1 and A_2A) in the brain, increasing dopaminergic and cholinergic transmission [7,32]. Consequently, the combined intake of CAF*THE improved cognitive performance by improving athletes’ mental processes, decision-making abilities, focus, and strategic thinking, ultimately contributing significantly to shooting performance. It is believed that exploiting its synergistic effects may be an appropriate ergogenic strategy, as neither CAF nor THE single shows more improvement compared to their combined intake.

6. Conclusions

The analysis of the available data supports the first prediction. Breakthroughs in targeted nutritional interventions and individualized intervention plans in our study on ergogenic supplementation in curling will pave the way for more precise and effective nutritional supplementation interventions. In conclusion, our current study demonstrated that CAF and THE supplementation single or combination significantly improved shooting and cognitive performance in elite curling athletes. The data largely support the hypothesis, confirming that the intake of CAF and THE supplements single or combination indeed leads to a significant improvement in curling and is in line with the expectations stated in the initial prediction. While using these supplements alone was effective, it was not more effective than using them together. Sports scientists and nutritionists may recommend the combined use of CAF*THE in sports disciplines such as archery and curling where shooting and cognitive performance are important. The experimental techniques used in this study may also be recommended for use in other sports disciplines, such as curling, where successful shooting depends on accuracy and cognitive function.

Combining 6 mg/kg CAF and 6 mg/kg THE consumption increased curling competitors’ shooting and mental performance. Although using these supplements single was effective, using them together was not more effective. Sports scientists and nutritionists may advise utilizing CAF*THE combined in sports disciplines such as archery and curling, where shooting and cognitive performance are crucial. The experimental techniques used in this study may be advised for use in other sporting disciplines, such as curling, where successful throws depend on accuracy and cognitive function. In this context, in order to evaluate whether CAF and THE supplementation single or combination have an effect on throwing and cognitive performance in elite curling athletes, one hypothesis was formed to evaluate our main hypothesis. This hypothesis was constructed as “CAF and THE supplementation single or combination has no effect on throwing and cognitive performance in elite curling athletes.” As a result of the study findings, this H₀ hypothesis was rejected and the H₁ hypothesis, “CAF and THE supplementation single or combination has an effect on throwing and cognitive performance in elite curling athletes.” was accepted. Combined intake of 6 mg/kg CAF and 6 mg/kg THE improved shooting and cognitive performance in curling athletes. While it was effective to use these supplements separately, it was not more effective than taking them in combination. In sports branches where shooting and cognitive performance are at the forefront (archery, curling), sports scientists and dietitians may recommend using CAF*THE together. The experimental procedures employed in this study can also be recommended for other sports disciplines, such as curling, where accurate throws are crucial, and cognitive performance plays a significant role in achieving success.

7. Limitations

The limited number of studies on cognitive function and shooting performance in curling athletes and especially the fact that THE supplementation has been little studied in previous studies were difficulties in the evaluation of the findings. In addition, the fact that there is a limitation in finding a large number of participants due to the developing curling sport is one of the other limitations of this study. In future studies, the limited literature can be supported by focusing on these supplements in different branches or with large participants.

Author contributions

“Conceptualization, U.Y., and Y.Buz.; methodology, U.Y., and Y.Buz.; software, Y.B; validation, Y.Buz., and M.L.P.; formal analysis, Y.Buz.; investigation, U.Y., and M.L.P.; resources, B.O and S.A.; data curation, B.O. and S.A.; writing – original draft preparation, U.Y., and Y.B.; writing – review and editing, U.Y., and H.U.; visualization, Y.Buz.; supervision, Y.Buz.; project administration, U.Y.; investigation and writing, U.Y., and H.U. All authors have read and agreed to the published version of the manuscript.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Acknowledgments

As the authors, we would like to thank the athletes of the Turkish National Team and Kuzey Yamacı FC and their coaches who participated in the measurements.

Disclosure statement

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

Additional information

Funding

This research received no external funding. Institutional Review Board Statement The study was conducted by the Declaration of Helsinki, and approved by the Erzurum Atatürk University Ethics Committee [E-70400699-060.10.06-3200456396].