Abstract
eine (CAF) is among the most extensively researched dietary supplements worldwide. However,
Department of Psychiatry, College of Medicine and Medical Sciences, Arabian Gulf University, Manama 323, Bahrain; hjahrami@health.gov.bh 9 Ministry of Health, Manama 410, Bahrain 10 High Institute of Sport and Physical Education Ksar-Saïd, Manouba University, Mannouba 2010, Tunisia * Correspondence: acammar@uni-mainz.de (A.A.); h_chtourou@yahoo.fr (H.C.); Tel.: +49-15236403235 (A.A.); +216-22872095 (H.C.) Abstract:Caffeine (CAF) is among the most extensively researched dietary supplements worldwide. However, little is known about the relationship between dosage and performance enhancement, particularly in female athletes. This study aimed to explore the effects of three different CAF dosages (3 mg·kg −1 , 6 mg·kg −1 , and 9 mg·kg −1 ) on high-intensity exercise and the prevalence of undesirable side effects related to these doses among female team-sports athletes. All participants (n = 16; age: 16.9±0.6 y; height: 1.64±0.1 m; BMI: 21.6±1.5 kg·m −2 ) were mild CAF consumers. This study had a randomized, crossover, double-blind design in which each athlete performed four experimental sessions after ingesting either a placebo (PLAC), 3 mg·kg −1 CAF (CAF-3), 6 mg·kg −1 CAF (CAF-6), or 9 mg·kg −1 of CAF (CAF-9), with an in-between washout period of at least 72 h. In each experimental session, 60 min after ingesting the capsules, participants underwent a countermovement jumps test (CMJ), modified agility t-test (MATT), repeated sprint ability (RSA) test, and a rating of perceived exertion (RPE) and completed the CAF side effects questionnaire. Our findings revealed that in comparison to the PLAC condition, the MATT, RSAmean, and RSA bestperformances were significantly greater only under the CAF-6 and CAF-9 conditions. Although the RPE scores remained unchanged, CMJ performance improved under all CAF conditions. All the performance outcomes were better for the CAF-6 and CAF-9 conditions than for the CAF-3 condition. Notably, no significant difference between the CAF-6 and CAF-9 conditions was observed for any of these parameters despite the highest incidence of side effects being noted for the CAF-9 condition. In summary, our findings highlight the recommendation for a moderate CAF dosage of 6 mg·kg −1 rather than 3 or 9 mg·kg −1 to enhance various aspects of short-term maximal performance in mild-CAF-consumer female team- sports athletes
and CAF-9 conditions was observed for any of these parameters despite the highest incidence of side effects being noted for the CAF-9 condition. In summary, our findings highlight the recommendation for a moderate CAF dosage of 6 mg·kg −1 rather than 3 or 9 mg·kg −1 to enhance various aspects of short-term maximal performance in mild-CAF-consumer female team- sports athletes while mitigating the occurrence of adverse CAF side effects. Keywords:caffeine intake; dosages; mild consumers; side effects; female athletes; team sports; athletic performance Nutrients2024,16, 640.
Nutrients2024,16, 640 2 of 20 1. Introduction Caffeine (CAF) is a key ingredient in coffee and is the most popular drink after water. It is estimated that people around the world consume more than 2 billion cups of this beverage every day [1]. Despite its lack of nutritional value and nonessential role in biological functions, CAF (1,3,7-trymethylxantine), a pharmacologically active substance, is exceptionally popular, as it is present in a variety of foods, beverages, and nutritional supplements and is estimated to be consumed daily by approximately 80% of the world’s population [2,3]. CAF consumption among athletes, particularly in endurance sports, has significantly increased since its removal from the World Anti-Doping Agency’s prohibited substances list in 2004, with usage trends rising with age [4,5]. CAF is widely recognized as one of the most commonly utilized ergogenic aids in sports, and numerous studies have validated its effectiveness in enhancing both anaerobic and aerobic performance [6]. Due to its lipophilic nature, CAF is quickly taken into the body after it is ingestedorally, and it is capable of crossing all biological barriers, including the blood–brain barrier [7]. This psychoactive substance is proposed to enhance performance through various potential mechanisms, which include preserving muscle glycogen by inhibiting phosphodiesterase [8,9], promoting calcium release from the sarcoplasmic reticulum [10,11], and counteracting the effects of adenosine A1 and A2 receptors in the central nervous system [12]. Therefore, it is plausible that either a single factor or an amalgamation of these factors might contribute to the enhancement in exercise performance following CAF intake. The efficacy of CAF as a performance enhancer depends on variables such as dosage, form, training status, timing of consumption, habitual CAF intake, sex, and exercise type [13]. Moreover, recent studies suggest that genetic variations in the CYP1A2 and ADORA2A genes influence the impact of CAF on exercise performance [6,14]. This calls for more extensive investigations and a greater understanding of the appropriateness of CAF in different scenarios. Although CAF supplementation may improve performance in individual sports, its effectiveness is less evident in team sports, where success is determined by a combination of physical condition, technical
the CYP1A2 and ADORA2A genes influence the impact of CAF on exercise performance [6,14]. This calls for more extensive investigations and a greater understanding of the appropriateness of CAF in different scenarios. Although CAF supplementation may improve performance in individual sports, its effectiveness is less evident in team sports, where success is determined by a combination of physical condition, technical skill, and tactical understanding [15]. Research on this topic, however, has been inconclusive; recent meta-analyses findings have shown that CAF can be effective for jumping performance [16,17], agility performance [18], and repeated sprint ability [16,17], while others have indicated that it has no effect on jumping and agility performance [19] or repeated sprint bouts [20]. These contradictory results could be due to the different dosages of CAF used in various studies, an aspect that requires additional attention within athletic groups. On the other hand, recent data show a predominant focus on male participants, with male-only samples ranging from 72% to 100% across the 21 meta-analyses from an umbrella review [13]. Current CAF supplementation guidelines, primarily based on studies involving male athletes, are identically applicable to females despite a lack of research specifically analyzing female athletes, which raises practical concerns [6]. In this context, recent findings underline the need for sex-specific guidelines in athletics, highlighting that conclusions drawn from observations in male groups may not be applicable to female groups due to potential sex differences [21]. In female team-sports athletes, lower doses, namely between 1 and 3 mg·kg −1 , have been found to have little to no effect on aspects such as jumping [22,23], agility [22–24], or repeated sprint [22,23,25]. However, interestingly, the same dosage was shown to have beneficial effects on jumping [26] and agility [26]. On the other hand, a moderate dose of 6 mg·kg −1 did not have any noticeable effect on jumping [27,28], agility [29,30], or repeated sprint ability [30,31]. However, contradictory effects were reported in other studies in which the same dose was found to improve jumping performance [30,32,33], agility [32], and repeated sprinting [33]. Recent findings indicated that (5 mg·kg −1 ) of CAF
hand, a moderate dose of 6 mg·kg −1 did not have any noticeable effect on jumping [27,28], agility [29,30], or repeated sprint ability [30,31]. However, contradictory effects were reported in other studies in which the same dose was found to improve jumping performance [30,32,33], agility [32], and repeated sprinting [33]. Recent findings indicated that (5 mg·kg −1 ) of CAF intake has been linked to improved CMJ and grip strength among female volleyball players [34]. These results highlight the varied responses to CAF intake in the context of female sports performance, suggesting a need for further research. Investigations into various CAF effects
Nutrients2024,16, 640 3 of 20 on female team-sport athletes have proven to be insightful but also expose a deficiency in existing data. Karayigit et al. [1] showed that consuming low (3 mg·kg −1 ) and moderate (6 mg·kg −1 ) doses of CAF in the form of coffee may equally enhance lower body muscular endurance. Meanwhile, another study conducted by Arazi et al. [35] indicated that ingesting a higher (5 mg·kg −1 ) dose of CAF may reduce pain perception during muscular endurance tests in female karate athletes, an effect not observed with smaller doses of 2 mg·kg −1 . Furthermore, Karayigit et al. [36] illustrated that low (3 mg·kg −1 ) and moderate (6 mg·kg −1 ) doses of CAF consumption could increase the average peak power score during repeated sprints test on a cycle ergometer on trained female team-sports players. Despite these insights, the difference in outcomes between lower, moderate, and high doses of CAF on critical aspects of the performance of high-intensity exercise in team sports, such as vertical jump, agility, and repeated sprints, are not well documented, presenting a fertile opportunity for further exploration. To explain this divergence, the data around CAF dosages of 3–6 mg·kg −1 are incon- clusive, suggesting that not everyone benefits from these levels and that future studies should consider that lower CAF doses (≤3 mg·kg −1 ) primarily affect the CNS, while higher doses (6–9 mg·kg −1 ) may have peripheral effects [8,37,38]. The impact of increased CAF consumption should be investigated in relation to individual tolerance and sensitivity to CAF. Thus, further investigations involving the administration of higher doses of CAF (i.e., 6–9 mg·kg −1 ), as opposed to the use of lower doses of CAF (≤3 mg·kg −1 ), should be carried out [37]. CAF has been shown to enhance exercise performance across various dosage ranges (2–9 mg·kg −1 ), although the physiological processes facilitating these im- provements at higher doses have not been determined [37]. A more effective methodology for studying CAF effects might involve conducting trials with both low and high CAF doses within each study, thus
should be carried out [37]. CAF has been shown to enhance exercise performance across various dosage ranges (2–9 mg·kg −1 ), although the physiological processes facilitating these im- provements at higher doses have not been determined [37]. A more effective methodology for studying CAF effects might involve conducting trials with both low and high CAF doses within each study, thus enabling a direct comparison. Despite the apparent logic of this approach, its implementation in research remains surprisingly rare [39]. However, further studies are needed to determine whether a linear relationship exists between dosage and performance enhancement. Interestingly, studies have not thoroughly investigated the impact of CAF on women at either high doses exceeding 9 mg·kg −1 or at extremely low doses [17]. This understanding could significantly contribute to the interpretation of the physiological processes associated with CAF effectiveness, particularly in female athletes. Moreover, the increase of the CAF dose should be based on the individual’s response to the substance, the habitual CAF intake classification (low, mild, moderate, or heavy), the type of physical exercise, sex, and the prevalence of side effects after pre-exercise CAF intake. Surprisingly, studies exploring the impacts of CAF withdrawal on exercise per- formance are lacking, indicating a pressing need for research in this area to develop an effective restriction protocol. Despite the positive impact of CAF on physical factors, some argue that team-sport athletes should carefully use CAF due to its potential impact on the technical and tactical aspects, with associated side effects like nervousness possibly leading to a decline in accuracy performance [16]. However, the potential negative side effects of CAF consumption, particularly at the levels taken to enhance performance, have received relatively little focus [40]. Furthermore, the trend of early specialization in sports seems to be on the rise among young athletes [41,42], with the pressure to focus on one sport coming from coaches, parents, and peers [43]. This may lead young athletes to start using supplements at a young age without specific recommendations for their age group, potentially leading to adverse effects for some athletes. In this context, limited studies have explored CAF’s
to be on the rise among young athletes [41,42], with the pressure to focus on one sport coming from coaches, parents, and peers [43]. This may lead young athletes to start using supplements at a young age without specific recommendations for their age group, potentially leading to adverse effects for some athletes. In this context, limited studies have explored CAF’s impact on young athletes, resulting in scarce evidence that draws definitive conclusions or guidelines. The performance advantages seen in adults may not extend to younger individuals, highlighting the need for additional research in the youth athlete demographic [16]. Limited data are available regarding the effects of CAF on young female athletes. Therefore, our study seeks to investigate the impact of CAF intake on this specific demographic. In light of the above considerations, the objective of this study was to examine the effects of three distinct dosages of CAF (3 mg·kg −1 , 6 mg·kg −1 , and 9 mg·kg −1 ) on high-
Nutrients2024,16, 640 4 of 20 intensity physical performance and the prevalence and severity of any possible side effects associated with these dosages among young female team-sports athletes. 2. Materials and Methods 2.1. Participants Following the recommended guidelines proposed by Beck [44], we used G*Power software (version 3.1.9.6; Kiel University, Kiel, Germany) [45] to pre-determine the required sample size. The significance level (α) was established at 0.05, with a desired statistical power (β) of 0.95. Based on Karayigit et al. [1] and discussed between authors, we approxi- mated the effect size to be 0.5. To attain the requisite statistical power, it was determined that a sample size of at least 10 athletes would be sufficient, therefore minimizing the probability of a type 2 statistical error. Among the 39 reviewed surveys, 25 females team-sports players were considered suitable and volunteered to participate in the study. However, during the experimental phase, nine participants withdrew due to logistical (one player) and menstrual cycle (eight players) reasons. The data gathered from the 16 participants who successfully completed all the sessions of the experiment are shown in Table. Before providing their written consent, the athletes and their parents were briefed about various aspects of the experiment, including the schedule, the type of exercise, and the evaluations they would be required to undergo. All protocols and methods received approval from the local research ethics committee of the University of Jendouba (054-2023), adhering to the latest version of the Declaration of Helsinki. Table 1.General characteristics of the study subjects (n = 16). Minimum Maximum Mean Age(years) 16 18 16.9 ±0.6 Body mass(kg) 50.3 74.1 60.1 ±5.7 Height(m) 1.6 1.8 1.64 ±0.1 Body mass index(kg·m −2 ) 18.3 23.3 21.6 ±1.5 CAF habitual intake(Mg·kg·day −1 ) 0.99 1.52 1.09 ±0.3 Sleep duration(h) 6.9 9.1 7.7 ±0.6 MEQ questionnaire score(au) 44 58 48.9 ±4.5 Practice experience(years) 4 7 5.1 ±0.9 Training sessions frequency/week(au) 3 5 4.3 ±0.7 Daily caloric intake(Kcal) 1650 2996 2335.8 ±404.4 Menstrual cycle length(days) 25 31 27.69 ±1.93 MEQ, Morningness–Eveningness Questionnaire of Horne and Ostberg (1976). The minimum, maximum, mean, and standard deviation values of the participants’
±0.3 Sleep duration(h) 6.9 9.1 7.7 ±0.6 MEQ questionnaire score(au) 44 58 48.9 ±4.5 Practice experience(years) 4 7 5.1 ±0.9 Training sessions frequency/week(au) 3 5 4.3 ±0.7 Daily caloric intake(Kcal) 1650 2996 2335.8 ±404.4 Menstrual cycle length(days) 25 31 27.69 ±1.93 MEQ, Morningness–Eveningness Questionnaire of Horne and Ostberg (1976). The minimum, maximum, mean, and standard deviation values of the participants’ characteristics are shown in the table. The study included young female team-sports athletes (well-trained handball (n = 10) and football (n = 6) players) who volunteered to participate in this investigation (Table). Participants were selected based on the following inclusion criteria: (1) aged between 15 and 20 years; (2) actively participated in team sports for at least 3 years, with a minimum of 3 times/week for the last 6 months; (3) had daily CAF intake less than 2.99 mg·kg.day −1 ; and (4) had regular menstrual cycles with a variance of no more than 3 days over the last 4 months [46]. The individuals who were excluded from the study met the following exclusion criteria: (1) had a history of a menstrual disorder within the past four months; (2) had taken oral contraceptives or medications in the previous four months, including pills, patches, injections, implants, and intrauterine devices; (3) had a positive alcohol and/or smoking status; (4) had a history of diseases and/or the use of any medications for any chronic
Nutrients2024,16, 640 5 of 20 medical condition; (5) had experienced an injury within the last 3 months; (6) had taken stimulants, narcotics, mind-altering drugs, nutritional enhancers, or participated in any strict dietary regimen during the previous four months that could influence hormone balances or sporting performance; (7) had extreme chronotypes; (8) were allergic to CAF; and (9) had PSQI scores more than 5. Player chronotypes for this study were identified through Horne and Ostberg (1976) self-assessment questionnaire [47]. This questionnaire measures preferences for sleep and activity using 19 items on a Likert scale and was used to avoid circadian typology influ- encing the study results. Participants with extreme morning or evening tendencies were excluded. The selected study participants were those with “neither type” chronotype. According to the Pittsburgh Sleep Quality Index (PSQI), all the selected athletes had sleep durations of approximately 7.7±0.6 h during the month leading up to the experimental procedure [48]. All the athletes had PSQI scores less than 5, indicating good sleep qual- ity [49]. Furthermore, the study recruited participants who were all classified as mild CAF consumers (1.09±0.3 mg·kg·day −1 ), based on the recent classification proposed by Filip et al. [50]. This classification was introduced to standardize the categorization of athletes, aiming to minimize the probability of disparities in the qualification of daily CAF intake across various studies. These data were assessed during the four weeks before the commencement of our experiment by a reliable semi-quantitative self-reported CAF intake questionnaire [51]. Using a mobile application, Mycalendar ® Period Tracker, which tracks key events throughout the menstrual cycle, each participant was evaluated during the fol- licular phase, the luteal phase, or both phases of their menstrual cycle [52]. However, recent investigations suggest that the bodily absorption and effects of acute caffeine consumption are constant across different menstrual phases, and female athletes consistently gain from their intake [53,54]. 2.2. Experimental Design A double-blind, placebo-controlled, randomized crossover design was used in this investigation, where each player acted as her own control. Players performed four different experimental sessions with at least a 72 h washout period between each session
Description
The study investigates caffeine dosage effects on exercise performance in young female athletes.