Acute Effects of Caffeine, L-Theanine, and Their Combination on Sprint, Jump, Change-of-Direction, and Repeated-Sprint Performance in Trained Adolescent Soccer Players: A Randomized, Double-Blind, Crossover Trial

Effects of caffeine, L-theanine, and their combination on the physical performance of adolescent soccer players compared to a placebo.

A randomized, double-blind, placebo-controlled, four-condition crossover clinical trial will be conducted in 16 trained male adolescent soccer players. Each participant will receive all four conditions: caffeine, L-theanine, caffeine plus L-theanine, and placebo. Participants will be randomly allocated to one of four treatment sequences using a simple lottery procedure with identical cards, with four participants assigned to each sequence.

This study will be conducted at Zob Ahan Football Club in Isfahan, Iran. Eligible participants will complete four experimental sessions separated by a 7-day washout period and will receive one of the following conditions at each session. Following administration of the intervention, performance tests including sprint, jump, change-of-direction, and repeated-sprint tests will be performed. Participants, performance assessors, and the statistical analyst will be blinded to treatment allocation, with experimental conditions identified using treatment codes.

Male participants aged 16 to 18 with a body mass index (BMI) of 18.5 to 24.5, at least two years of regular soccer training experience, physician-confirmed general health, and no known sensitivity to caffeine or L-theanine.

Participants will undergo four experimental conditions (in a crossover design), caffeine (3 mg/kg body mass), L-theanine (3 mg/kg body mass), caffeine plus L-theanine (3 mg/kg body mass each), and placebo. All interventions will be administered as capsules 60 minutes before testing.

20-meter sprint time; standing long jump distance; change-of-direction test time; repeated sprint time.

Randomization will be performed at the individual level using a simple randomization procedure. Four balanced treatment sequences will be predetermined for allocation of participants to the four treatment sequences. These four sequences will be: caffeine–caffeine and L-theanine combination–placebo–L-theanine; L-theanine–caffeine–caffeine and L-theanine combination–placebo; placebo–L-theanine–caffeine–caffeine and L-theanine combination; and caffeine and L-theanine combination–placebo–L-theanine–caffeine. Each sequence will be written on four identical cards; therefore, a total of 16 sequence cards will be prepared. In addition, the identification number of each of the 16 participants will be written on a separate identical card. The sequence cards and participant identification cards will be placed in two separate opaque containers. Random allocation will be performed by an independent investigator who will not be involved in intervention administration, outcome assessment, or experimental procedures. At each allocation step, one sequence card will be randomly drawn from the sequence container and one participant identification card will then be randomly drawn from the second container. The selected participant will be assigned to the treatment sequence indicated on the selected sequence card. This procedure will be repeated until all 16 participants have been allocated, resulting in four participants assigned to each treatment sequence. The unit of randomization will be the individual participant, and neither cluster randomization nor stratified randomization will be used. The random sequence will be generated and allocated using a manual lottery procedure with identical and indistinguishable cards, and no statistical software or random-number table will be used. For allocation concealment, the treatment-sequence cards will be kept in a separate container from the participant-identification cards. The investigator responsible for randomization will be independent of the researchers involved in intervention administration and outcome assessment. Therefore, the treatment sequence assigned to each participant will not be known until the lottery procedure has been performed and the cards have been drawn from the two separate containers.

The study will be conducted using a double-blind design. Participants will be blinded to the identity of the capsules and will not be informed of their assigned treatment condition. Active and placebo capsules will be visually identical and will be administered in the same number and manner across all experimental conditions. The investigators responsible for performance testing and outcome assessment will also remain blinded to treatment allocation throughout all stages of data collection. The statistical analyst will likewise be blinded to treatment identity during the statistical analyses, with the experimental conditions represented by treatment codes rather than their actual condition names. The treatment codes will be maintained by an investigator who will not be involved in outcome assessment. These codes will not be disclosed to the research team or the statistical analyst until completion of all experimental procedures and the primary statistical analyses. Thus, the identity of the treatment conditions will remain concealed throughout the experimental procedures and primary statistical analyses.

De-identified individual participant data will be made available in a public data repository or as a supplementary file to the article following the publication of the study results. Prior to sharing, all data will be processed to remove participant-identifying information. The study protocol will be made available as a supplementary file to the article or in a public scientific repository after the study is completed and the main results are published. The statistical analysis plan will be made available as a supplementary file to the article or in a public scientific repository after the study is completed and the main results are published. A data dictionary—including variable names, definitions, units of measurement, and coding schemes—will be made available alongside the de-identified individual participant data.

The period for accessing data and documentation will begin one week after the publication of the results.

Researchers working in academic and scientific institutions

The study data and documentation may be used solely for research and scientific purposes, including conducting secondary research, re-analyzing data, and verifying or replicating study results. Access to individual-level data is contingent upon the de-identification of participant information and adherence to principles of confidentiality, privacy, and ethical requirements regarding the use of research data. Data usage must be strictly confined to scientific and research objectives, with no attempt to re-identify participants

Applicants may request access to the study's data and documentation by contacting the principal investigator or corresponding author via the email address listed on the study's official webpage or in the published article. The designated contact person is Dr. Alireza Niknam (email: Alireza73.Niknam@gmail.com). Requests will be evaluated based on the research objective and verification that participant confidentiality and anonymity are maintained; upon approval, the relevant data or documentation will be provided to the applicant.

Requests for data or documentation will be reviewed by the principal investigator or the research team. The applicant must specify the purpose and intended use of the requested data or documentation. Following the initial review, the request will be evaluated for alignment with scientific objectives, adherence to confidentiality requirements, and the assurance of participant anonymity. Upon approval, the requested data or documentation will be provided to the applicant in an appropriate format. The use of data to re-identify participants or for purposes inconsistent with research ethics principles is prohibited.