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    Acute effect of different doses of caffeinated chewing gum on exercise performance in caffeine-habituated male soccer players
    (Frontiers Media Sa, 2023) Yildirim, Ulas Can; Akcay, Neslihan; Alexe, Dan Iulian; Esen, Ozcan; Gulu, Mehmet; Cirtita-Buzoianu, Cristina; Cinarli, Fahri Safa
    The ergogenic benefits of caffeine have been well established, but there is scarce research on its chewing gum form. The present research aimed to examine the effects of different doses (100 and 200 mg) of caffeinated chewing gum on muscle strength, vertical jump performance, and ball-kicking speed in trained male soccer players. In a double-blind, randomized counterbalanced, and crossover research design, 14 male soccer players (age = 22 +/- 2 y; body mass = 74.2 +/- 7.1 kg; height = 180.0 +/- 6.8 cm; habitual caffeine intake = 358.9 +/- 292.4 mg/day) participated in three experimental trials. In each trial, participants performed isometric handgrip strength, quadriceps and hamstring strength, ball-kicking speed, and 15 s countermovement jump test 10 min after chewing 100 mg (LCAF) or 200 mg (MCAF) of caffeinated gum or placebo (PLA). MCAF improved quadriceps strength (53.77 +/- 5.77 kg) compared to LCAF (49.62 +/- 8.81 kg, p = 0.048) and PLA (49.20 +/- 7.20 kg, p = 0.032). However, neither LCAF nor MCAF had a significant effect on the isometric handgrip and hamstring strength, ball-kicking speed, and 15 s countermovement jump test (all p > 0.05). These findings support chewing gum as an alternative mode of caffeine administration which can be used as a nutritional ergogenic aid for trained soccer players, at least for quadriceps strength.
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    Acute effects of combined and isolated caffeine and theanine supplementation on physical and cognitive performance in competitive athletes: a randomized, double-blind, placebo-controlled crossover study
    (Frontiers Media Sa, 2026) Tuncer, Selin Yildirim; Ozdenk, Serhat; Yildirim, Ulas Can; Erkan, Dilara; Sari, Cengizhan; Gundem, Mehmet Can; Sar, Halit
    Introduction Modern athletic performance is driven not only by physical capacity but also by rapid decision-making, attentional control, and visuomotor coordination. Evidence regarding the acute effects of caffeine (CAF), L-theanine (TEA), and their combination remains inconsistent, particularly with respect to their combined influence on physical and cognitive performance in athletic populations. This study examined the acute effects of isolated and combined CAF and TEA supplementation on maximal strength, intermittent aerobic endurance, and eye-hand coordination in competitive athletes. It was hypothesized that the combined ingestion of CAF and TEA would differentially affect physical performance and eye-hand coordination outcomes compared with isolated CAF or TEA intake.Methods Twenty trained athletes completed four randomized, double-blind, placebo-controlled crossover conditions: CAF (3 mgkg-1), TEA (200 mg), CAF+TEA (COM), and placebo (CON), with >= 72 h washout. Outcomes included isometric leg, back, and handgrip strength; Yo-Yo Intermittent Endurance Test Level 1 performance; and visuomotor coordination (CogniFit (R)). Data were analyzed using repeated-measures ANOVA with Bonferroni corrections (alpha = 0.05).Results Condition effects were found for leg strength (p = 0.004, eta p2 = 0.24) and back strength (p = 0.008, eta p2 = 0.19). In the COM condition, no additional or synergistic effect on maximal strength was observed. Additionally, no significant difference was found between the conditions in aerobic endurance, maximum isometric handgrip strength, and hand-eye coordination results. Caffeine ingestion did not differ from placebo for any strength outcome under the present conditions. Finally, acute ingestion of TEA was associated with reduced maximal isometric leg and back strength compared with CAF and CON.Discussion Findings challenge the prevailing assumption of CAF + TEA synergy and underscore the need for task-specific interpretation of co-supplementation strategies. Future studies should evaluate dose-response interactions, habitual caffeine intake, sex-specific responses, and broader cognitive domains beyond visuomotor control. The randomized controlled trial was registered at ClinicalTrials.gov, under the registration number NCT07268573.Clinical Trial Registration ClinicalTrials.gov, NCT07268573.
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    Combined but Not Isolated Ingestion of Caffeine and Taurine Improves Wingate Sprint Performance in Female Team-Sport Athletes Habituated to Caffeine
    (Mdpi, 2021) Karayigit, Raci; Naderi, Alireza; Saunders, Bryan; Forbes, Scott C.; Coso, Juan Del; Berjisian, Erfan; Yildirim, Ulas Can
    Previous studies have investigated caffeine (CAF) and taurine (TAU) in isolation and combined during exercise in males. However, the potential synergistic effect during high-intensity exercise remains unknown in female athletes. Seventeen female team-sport athletes participated (age: 23.4 +/- 2.1 years; height: 1.68 +/- 0.05 m; body mass: 59.5 +/- 2.2 kg). All participants were habitual caffeine consumers (340.1 +/- 28.6 mg/day). A double-blind randomized crossover design was used. Participants completed four experimental trials: (i) CAF and TAU (6 mg/kg body mass of CAF + 1 g of TAU), (ii) CAF alone; (iii) TAU alone; and (iv) placebo (PLA). Supplements were ingested 60 min before a 30-s Wingate Anaerobic Test (WAnT). Heart rate and blood lactate (BL) were measured before and immediately after the WAnT; and ratings of perceived exertion (RPE) were recorded immediately after the WAnT. Peak power (PP) was significantly higher following co-ingestion of CAF+TAU compared to PLA (p = 0.03) and TAU (p = 0.03). Mean power (MP) was significantly higher following co-ingestion of CAF+TAU compared to PLA (p = 0.01). No other differences were found between conditions for PP and MP (p > 0.05). There were also no observed differences in fatigue index (FI), BL; heart rate; and RPE between conditions (p > 0.05). In conclusion, compared to PLA the combined ingestion of 6 mg/kg of CAF and 1 g of TAU improved both PP and MP in female athletes habituated to caffeine; however; CAF and TAU independently failed to augment WAnT performance.
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    Comparative effects of caffeine and paraxanthine on rowing performance and sleep quality: a randomized crossover study
    (Taylor & Francis Inc, 2026) Bingol Diedhiou, Azize; Yildirim, Ulas Can; Ozdenk, Serhat; Erkan, Dilara; Karakulak, Izzet; Yildirim Tuncer, Selin; Turgut, Murat
    BackgroundAlthough caffeine is widely used in athletes due to its ergogenic effects, the effects of its main metabolite, paraxanthine, on performance and sleep have not been adequately investigated.PurposeThis study aimed to comparatively investigate the effects of caffeine and its main metabolite paraxanthine on rowing performance and sleep quality.MethodsThe study was designed as a randomized, double-blind, crossover study and included 14 male university-level rowers (21.6 +/- 1.9 age; 2.2 +/- 1 years of rowing experience). The participants participated in 2000-m rowing ergometer time trials under four different supplementation conditions (caffeine + paraxanthine, caffeine + placebo, paraxanthine + placebo, and placebo with 200 mg each). Performance data (completion time, mean power, and heart rate), sleep quality, and daytime sleepiness were assessed by subjective scales. The data were analyzed by repeated-measures analysis of variance (ANOVA) and Bonferroni post-hoc tests.ResultsCompared with the placebo, the combined caffeine + paraxanthine condition was associated with faster 2000-m performance and higher mean power output (p = 0.044; Cohen's d = 0.30). Caffeine alone and paraxanthine alone did not show clear evidence of performance improvement in this sample, although estimates favored both conditions versus placebo. Conditions containing caffeine were associated with poorer subjective sleep quality, whereas paraxanthine alone showed more favorable sleep-related outcomes.ConclusionThese results indicate that the combined ingestion of caffeine and paraxanthine elicited the most pronounced performance benefits, while paraxanthine alone did not demonstrate clear standalone ergogenic efficacy at the administered dose. However, paraxanthine was associated with better subjective sleep outcomes compared to caffeine, suggesting that its potential value may be related more to tolerability rather than superior performance enhancement, particularly for athletes training in the evening. Study limitations, including the small sample size and lack of objective sleep measures, should be considered when interpreting the results.Practical applicationsParaxanthine may represent a practical option for athletes who prioritize sleep quality or experience sensitivity to caffeine-related sleep disturbances, although further dose-response studies are required to clarify its ergogenic potential.
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    Kafein ve Sportif Performans
    (Sakarya Uygulamali Bilimler Üniversitesi, 2023) Yildirim, Ulas Can
    Kahve, çay ve kakao gibi dogada bulunan bazi bitkilerin tohumlarinda, yapraklarinda ve meyvelerinde dogal olarak bulunan bir uyarici olan kafein, günümüzde bazi yiyecek ve içeceklerde ve bazi ilaçlarin içerigine de eklenerek tüketiciye sunulmaktadir. Dogrudan veya dolayli bir biçimde dünya genelinde her gün milyarlarca insan tarafindan tüketilen kafein, merkezi sinir sistemi ve diger sistemler üzerinde uyarici etkilere neden olmaktadir. Kafeinin birincil etki mekanizmasi adenozin reseptörlerini bloke etme yetenegidir. Yapi olarak adenozine çok benzemesi sebebi ile organizmayi kandirarak adenozin reseptörlerinde antagonist bir etki yaratmaktadir. Agri ve yorgunluk hissi ile alakali olan bu reseptörler g proteinleri ile gerekli baglantiyi kuramadigindan dolayi metabolizma agriyi ve yorgunlugu inhibe eder, böylece agri ve yorgunlugun ortaya çikisi gecikebilmektdir. Diger bir etkisi ise hormonal sisteme olmaktadir. Kafeinin alimi epinefrin ve norepinefrin gibi nörotransmitterlerin saliniminda artisa yol açar. Bu nörotransmitterler, bilissel islevi ve uyanikligi artirmada çok önemli bir rol oynamaktadir. Ayrica organizmanin agri kesicisi olarak da bilinen Beta Endorfin hormonunun saliniminda da kafein alimi ile artis ortaya çiktigi düsünülmektedir. Dahasi, glikoz ve yag asitleri gibi enerji substratlarinin mevcudiyetini artirdigi da bilinmektektedir. Bunlarin yanisira dolasim ve solunum sistemleri gibi diger fizyolojik sistemleri etkileyebilir ve dogru dozda tüketildiginde bazi potansiyel saglik faydalari olabilir. Ayrica isiri tüketim ve tolerasyon seviyesine bagli olarak elektroansefalografi (EEG) düzenini, duygulanimi ve uyku düzenini de degistirdigi, ayrica yine alim miktarina bagli olarak anksiyeteyi artirabildigi saptanmistir. Bahsi geçen mekanizmalar sayesinde kafeinin sportif performans üzerinde de çesitli etkileri olmaktadir. Arastirmalar, kafeinin glikojenin mobilizasyonunu ve yaglarin utilizasyonunu artirarak vücudun sinirli glikojen depolarini korudugu, böylece yorgunlugun baslangicini geciktirerek, egzersizin süresini artirdigini, güç ve kuvvet gibi performans parametrelerini etkiledigi söylemektedir. Sonuç olarak, kafein, birçok biyomotor özelligi gelistirerek, sportif performans üzerinde olumlu etkiler ortaya koyabilir. Bu literatür incelemesi kafeine genel bir bakisin yaninda, farkli parametreler üzerinde ne gibi etkileri meydana getirdigini tespit ederek sonuçlari okuyucular ile aktarmayi amaçlamistir.
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    Varying doses of evening caffeine ingestion have different effects on rowing ergometer performance, sleep quality, and wakefulness scores
    (Frontiers Media Sa, 2025) Karakulak, Izzet; Yildirim, Ulas Can; Erkan, Dilara; Karayigit, Raci; Eyuboglu, Ender; Diedhiou, Azize Bingol; Gundem, Mehmet Can
    Introduction: This study investigated the dose-dependent effects of evening caffeine ingestion on rowing performance, sleep quality, and daytime sleepiness in trained male rowers. Methods: Using a double-blind, randomized, crossover design, 13 university-level rowers (mean age = 22.07 +/- 2.21 years; mean body mass = 77.66 +/- 6.45 kg) completed four 2,000 m time-trial sessions between 19:00 and 20:00 h under placebo (PLA), low-dose capsule caffeine (3 mg/kg, LDC), moderate-dose capsule caffeine (6 mg/kg, MDC), and high-dose capsule caffeine (9 mg/kg, HDC) conditions. Performance metrics, heart rate, and subjective sleep assessments were collected. Rowing performance was assessed by a standard 2,000 m rowing ergometer (Concept II, United States) time trial. Sleep quality was measured with a numerical rating scale in the morning after each trial, and daytime sleepiness was measured with the Karolinska Sleepiness Scale. Results: Results indicated significantly improved rowing times and power output with HDC and MDC compared to PLA (p < 0.05), with HDC yielding the most notable enhancements (d = 0.40-0.41). However, these ergogenic benefits were accompanied by significantly impaired sleep quality and elevated daytime sleepiness in both HDC and MDC groups (p < 0.01; d = 1.3-1.5). Notably, adverse effects such as headache, insomnia, and gastrointestinal discomfort were predominantly reported in the HDC condition (p < 0.05). Although LDC offered mild performance improvements with minimal sleep disruption, only the high dose condition exhibited large physiological and perceptual trade-offs. Discussion: These findings indicate a clear dose-response relationship, wherein higher evening caffeine intake improves performance but has detrimental effects on sleep and recovery markers. Coaches and athletes should carefully balance caffeine dosing against potential recovery costs, especially in evening training or competition contexts.