Dose-dependent effect of caffeine supplementation on judo-specific performance and training activity: a randomized placebo-controlled crossover trial

Thirty trained male judoists were initially enrolled in this study. However, 22 judoists completed the study and were included in analyses (Fig. 1; Table 1). The athletes were members of the Polish Judo Clubs from Wrocław, Poznań and Jarocin. They occupied leading positions in national competitions and also represented Poland in international competitions.

The primary recruitment strategy was to contact the coaches responsible for training judo athletes. They enabled the identification, inclusion and confirmation of the inclusion criteria aspects declared by the participants. They also supported training control and supplementation compliance. The inclusion criteria were: good health, a valid and up-to-date medical certificate that confirmed the athlete’s ability to practice sports, at least 4 years of training experience and participation in a minimum of four judo workout sessions a week. The exclusion criteria were: current injury, any health-related contraindication, declared general feeling of being unwell and unwillingness to follow the study protocol. The drop-outs were predominantly independent from the study protocol (Fig. 1). The reasons for drop outs were: personal, infections and/or minor injuries during customary training. The entire study protocol for each participant lasted 5 weeks. All athletes declared that before and during the study protocol they did not introduce any changes in their lifestyles, training elements, nutrition or supplementation, and that they had not been using any medications and/or supplements with potential ergogenic effects, other than those supplied by the authors of this study. For this study, athletes were also divided depending on daily regular caffeine consumption into - “caffeine consumers” - athletes who consumed more than 160 mg of CAF each day, and “caffeine non-consumers” - those who consumed less than 160 mg of CAF on a daily basis or consumed products containing CAF less frequently. The amount of 160 mg of CAF refers to the average amount of CAF found in two cups (240 ml per cap) of coffee [28]. During the study, the participants were in the usual and non-starting training period in which they did not regulate body weight or did not make any specific changes to training procedures. In accordance with the 1975 Declaration of Helsinki, all participants consented to participate in the research procedures before the study began. This study was approved by the Bioethics Committee at Poznan University of Medical Sciences and was registered at ClinicalTrials.​gov (NCT03822663). The study was registered retrospectively, since registration was not required when study enrolment started. The authors confirm that all ongoing and related trials associated with this intervention are registered. The trial was conducted from May to June 2017 and May to June 2018. It complies with the CONSORT statement for randomised trials, as shown in Fig. 1 and Additional file 1: Table S1.

G*Power software (version 3.1.9.4, Universität Düsseldorf, Germany) was used to calculate sample size required to obtain a power of approximately 80% (α = 0.05) and large effect size partial eta squared 0.14 in analysis of variance (ANOVA) with repeated measurements (RM) within factors. Analysis indicated that a sample size of 9 would be suitable for detecting a difference between five measurements. In one previous study, a sample of 18 was appropriate to detect significant differences in SJFT results after acute CAF supplementation in young judoists [29].

The study protocol consisted of acute supplementation with three doses of CAF or PLA in a randomised double-blind placebo-controlled crossover design. The primary outcomes were changes in judo-specific exercise performance and subsequent combat activities. Between treatments, a 7-day washout period was introduced. This period was likely sufficient given the kinetics of CAF excretion from the body [3]. The participants were intimated on the testing procedures, protocols and equipment before study commencement.

The participants were first enrolled by the authors and then randomly assigned (stratified randomisation) to the supplementation groups with specific codes by an impartial biostatistician. The main study protocol involved five separate visits (T₁–T₅) and included exercise tests (SJFT and judo sparring combats (Randori)) conducted in natural conditions at the judo training centres. The participants performed exercise tests before (baseline) and after each treatment (CAF or PLA). All testing was performed at the same time of day (afternoon hours). Moreover, 3 hours before the exercise tests, participants consumed standardised small meals. They were also instructed to avoid strenuous exercise for the 24 h preceding each test session.

The experimental procedure for each athlete included an acute 3, 6 or 9 mg/kg body weight CAF supplementation and PLA treatment in a crossover regimen. CAF (pure pharmaceutical caffeine, KFD Nutrition, Poland) and PLA (maltodextrin) were administered dissolved in 350 mL water. On testing days, the supplements were taken 60 min before the exercise capacity test session (Fig. 1). The preparations were administrated to each participant in containers marked with a unique code. In accordance with the recommended blind procedure, the preparations were made in advance (after morning anthropometric measurements) by the research team member who did not directly participate in the investigations. Regarding double blinding, neither the researchers nor the participants knew whether CAF or PLA was administered. Only the head of the department had access to the randomisation information, which was only revealed after protocol cessation. The preparations were administered at a strictly specified time before the exercise tests, and consumption compliance was controlled by the investigators along with the trainers of the studied athletes.

Anthropometric measurements were taken with the participants in a fasted state during the morning hours at each visit. Body mass and height were measured using a professional medical scale with a stadiometer (WPT-60/150OW, RADWAG®, Poland). Further, the proper hydration level was verified via urine specific gravity measurement, with URYXXON® Relax (Macherey-Nagel, Germany), and results < 1.020 indicated proper hydration.

During each exercise session (T₁–T₅), all athletes performed: 1) a standardised 18-min warm-up; 2) one SJFT and 3) three sparing combats (Fig. 1). All test procedures (SJFT and Randori combats), were evaluated in real time by two independent coaches. Additionally, they were visually registered, and the final evaluation and verification was done by a member of the research team (judo black belt coach). This triple control allowed precise counting of all technically well-performed throws and attacks. At the end of each testing session (4.5 min after the final combat), the athletes were asked to rate their perceived exertion (RPE) using the Borg scale (9–20) according to published recommendations [30]. RPE was evaluated at the end of testing session to minimise effects of data collection on exercise performance.

All variables were checked for normal distribution using the Shapiro-Wilk test. For variables with normal data distribution, the effects of CAF dosage or PLA on SJFT and fight performance were tested by ANOVA with RM. Two-way ANOVA with RM was performed to evaluate the effect of habitual CAF consumption on supplementation effect (CAF dosage x customary CAF consumption). A Huynh-Feldt adjustment was made when sphericity was violated (as indicated by Mauchly’s test). Post-hoc comparisons were performed by Fisher’s least significant difference (LSD) test. Effect sizes were calculated using partial eta squared (\( {\eta}_p^2 \); 0.01 – small effect, 0.06 – medium effect and 0.14 – large effect [34‐36]). For variables not normally distributed, Friedman’s ANOVA, followed by pairwise Wilcoxon signed-rank test, were applied. Effect sizes were calculated using Kendall’s concordance coefficient W (0 – no agreement and 1 – complete agreement). The same approach was implemented to verify the effect of the number of attacks in Randori combat outcomes within particular CAF doses. Differences between SJFT_R2 and SJFT_R3 outcomes were tested with a T test for dependent variables or pairwise Wilcoxon signed-rank test, with effect size expressed as Cohen’s d or rank correlation coefficient r_c. With regard to variables not normally distributed, the effect of habitual CAF consumption was verified by dividing the entire studied group into “consumers” and “non-consumers” subgroups. Further analyses in these two subgroups were performed according to above described scheme, starting from checking the data distribution. Moreover, the statistical analysis was complemented by comparing all the variables between CAF ‘consumers’ and ‘non-consumers’, via a T test for independent variables (for normally distributed data; effect size expressed as Cohen’s d; 0.20 – small effect, 0.50 – medium effect and 0.80 – large effect [34]) or the Mann-Whitney U test (variables not normally distributed; effect size expressed as Glass’s rank-biserial correlation coefficient (r_g); interpretation according to correlation coefficient). HR (robust to violations of normality) and RPE (ordinal scale) results were analyzed with parametric and non-parametric test, respectively, regardless of data distribution. Statistical significance was set at P <  0.05, and data were analysed using the STATISTICA-13.3 software program (StatSoft Inc., USA).

A few studies examined the effects of CAF on performance in combat sports. Yet, there are no trials that compared different supplementation strategies (doses) in judo. One previous study in endurance athletes suggested that significant performance increases can only be achieved by low to moderate CAF doses (3–6 mg/kg), while high doses (9 mg/kg) may overstimulate the CNS [50]. In the current study, we showed that CAF doses of 6 and 9 mg/kg resulted in better SJFT performance compared to BASE, PLA or 3 mg/kg. Further, at 9 mg/kg CAF the number of throws between SJFT_R3 and SJFT_R2 was comparable. Astley et al. [29] noted that acute ingestion of 4 mg/kg CAF increases the number of throws in SJFT compared to PLA in young judoists, while Lopes-Silva et al. [25] and Felippe et al. [51] did not observe any effects of 6 mg/kg CAF alone on the number of throws. Inconsistences between findings in the aforementioned studies may arise from different methodological approaches, e.g., different athlete weight categories, alternative pre-supplementation preparation (e.g., 5-day weight loss weight [25]) or small sample sizes [25, 51].

Other studies that investigated the effects of 5 mg/kg of CAF supplementation on judo performance [44, 52] utilised measures we did not implement, namely the Wingate Anaerobic Test [52], countermovement jump test, handgrip strength test or judo grip strength test [44]. Athayde et al. [44] did not observe any positive significant effect of CAF supplementation in neuromuscular tests. Souissi et al. [52], however, noted that CAF ingestion improves peak and mean power after morning compared to afternoon supplementation. CAF effectiveness was also investigated in wrestling [53, 54], taekwondo [55, 56], Brazilian jiu-jitsu (BJJ) [49, 57] and boxing [58]. Four [54] and 5 mg/kg [53] doses were ineffective; only 10 mg/kg CAF [54] improves performance in wrestlers. Further, supplementation with 5 mg/kg CAF produced mixed results in taekwondo athletes [55, 56]; 3 mg/kg enhances combat intensity and muscular performance in BJJ [49, 57] and 6 mg/kg induces greater duration of high-intensity actions in boxing [58].

Judo combat is dependent on the opponent’s strategy of attacks and defense, and it is composed of acyclic tasks, factors which make it difficult to identify the effect of CAF during combats. However, high-intensity efforts lead to central and muscle fatigue (related to metabolite accumulation, energy substrate depletion, neuromuscular junction failure and muscle contractile potential disturbance) [59]. These phenomena apparently explain why during consecutive matches the number of attacks decreases [44]. However, in our study we did not observe substantial differences in the number of attacks in three consecutive judo combats at any CAF dose, PLA or BASE. There is only one previous study [44] that measured the number of attacks during a judo match. Ingesting 5 mg/kg CAF did not increase the number of attacks [44]. This finding is contrary to our results, where we observed increased TOT_attacks in 3 combats at 6 and 9 mg/kg CAF comparing to PLA.

Ingesting CAF can influence both HR at rest [8, 40] and during exercise [4, 41], namely due to CNS activation (e.g., catecholamine release), phosphodiesterase inhibition, adrenal cortex stimulation (corticosteroid release) and influence on the renal system [60]. Low CAF doses (3 mg/kg) do not alter HR at rest [29, 50]. In our study 6 and 9 mg/kg CAF increased HR_RA and HR_1minAF compared to 3 mg/kg, and 9 mg/kg additionally compared to PLA. However, 3 mg/kg CAF did not affect HR, and this result is in accordance with the current literature [29, 50]. In contrast, two former studies that measured HR in judo did not observe any significant differences in HR after 4 [29] or 6 mg/kg [25] CAF supplementation.

CAF ingestion may reduce RPE, which could be caused by the ability of CAF to cross the blood-brain barrier and compete with adenosine receptors [8, 13, 14]. This mechanism is also responsible for reduced pain perception and sleepiness [25]. On the one hand, our current study did not show any direct significant effect of CAF supplementation on RPE measured by the Borg scale and are in line with a study by Fellipe et al. [51], who utilised 6 mg/kg CAF. On the other hand, it should be emphasized, that in the current study 6 and 9 mg/kg CAF doses improved judo-specific performance. An increase in performance is inherent with, among others, greater work performed. In practice, the participants were able to perform more effectively without perceiving greater exertion. Thus, it must be stated that CAF supplementation positively affected RPE. Contrarily, report by Lopes-Silva et al. [25] showed reduced RPE in judoists who ingested 6 mg/kg CAF, but no increase in performance.

It is important to note that, to our knowledge, there is no data on the influence of customary CAF consumption on the effectiveness of CAF supplementation in combat sports. This aspect was investigated in athletes who represented other sport disciplines [20, 61, 62]. Moreover, none of the studies showed significant differences in performance between CAF users and nonusers [20, 61, 62]. However, in CAF non-consumers, the post-ingestion effect lasted 3 h longer compared to customary consumers [61]. In our study we showed that CAF consumers needed to ingest 9 mg/kg to achieve better SJFT results, while in non-consumers the dose of 6 mg/kg was equally effective as 9 mg/kg. Further, in CAF consumers, in contrast to non-consumers, the implemented supplementation did not affect HR after exercise. Habitual CAF consumption also had no effect of combat activity.

Knowledge of the impact of different CAF dosages on judo performance could have significant beneficial implications for athletes who compete in international matches. Our data can therefore be translated into a real-world setting and hold implications for improving performance and overall competition efficiency. Information on whether higher CAF doses promote more throws and/or attacks could be important to establish discipline-dependent recommendations for CAF usage. Importantly, given the physiologically similar nature of the effort, our results could also be used also in ergogenic supplementation support of athletes in wrestling, BJJ, and other “grappling” combat sports disciplines.

It should be noted that our research has some limitations. First, we did not analyse blood biochemical marker (e.g., CAF) concentration in athletes during our study, which in our opinion would be worth performing in future investigations. However, our primary outcome was a practical assessment of the final CAF influences on discipline-specific exercise capacity, which we achieved. Second, the result of combat activity may be also related to the opponent’s experience level and the load that the opponent makes during the combat. Still, in our opinion this is in practice an unavoidable problem in combat sports. However, to minimise this effect, judoists fighting with each other were selected in terms of weight category and experience level, as well as in the same order of opponents (in T₁-T₅). Further, the result of combat activity may be affected by the fatigue that occurred after SJFT. According to RPE expressed by the Borg scale [30, 63], the participants perceived the exertion after SJFT as somewhat hard. Finally, we did not account for carry-over effect. However, it needs to be emphasised that at each laboratory visit we administered a single CAF dose to each participant, and each participant visited the laboratory five times with at least 7 days of wash-out. Since the elimination half-life of CAF ranges between 2.5 and 10 h [3], we believe that this wash-out period was sufficient to remove CAF from the judoist’s systems.

The unquestionable strength of our study is application of a four-fold crossover design protocol and implementation in all study participants each of the three CAF doses as well as PLA; such an approach has never been deployed in judo before. For the scientific value of our work, double-blind masking and full control of compliance of the delivery of the administered supplement, which was taken at the set time under the control of a member of the research team, are also important. Moreover, there was close cooperation among the research team and the coaches to ensure proper study protocol conduct (e.g., motivated athlete effort and commitment and analysed and assessed the correctness of techniques and actual attacks during combats). It is also worth noting that we used the discipline-specific judo performance test. The SJFT was developed to evaluate both anaerobic and aerobic potential, and it can be considered more appropriate to evaluate judo performance capacity than the Wingate Anaerobic Test [31‐33]. It is also worth taking into account the high training level of studied judoists and high familiarisation of the studied group to perform all efforts. We also accounted for customary consumption of CAF-containing products. We revealed that recommended doses of CAF in judo should vary according to usual caffeine consumption.