Effects of Caffeine and L-Theanine on Physical and Cognitive Performance in Trained Athletes

Summary

The aim of this clinical trial is to examine how caffeine and L-theanine, taken alone or together, affect both physical and cognitive performance in healthy, trained athletes. The study aims to answer whether these commonly consumed dietary ingredients can enhance strength, endurance, and hand-eye coordination under controlled laboratory conditions.

Researchers will compare four supplementation conditions:

Caffeine (CAF) - 3 mg per kilogram of body weight L-Theanine (THE) - 200 mg Combined Caffeine and L-Theanine (COM) - same doses together Placebo (CON) - mannitol (inactive substance) Each participant will complete all four test sessions in a randomized, double-blind, crossover design, with at least 72 hours between sessions. Capsules will be taken 60 minutes before testing to allow time for absorption. During each session, participants will perform a series of physical and cognitive tasks, including maximal isometric strength tests, an intermittent endurance test, and a computerized eye-hand coordination task.

The main hypothesis is that caffeine and L-theanine, alone or in combination, will improve strength and cognitive coordination compared with placebo. All participants are healthy competitive athletes between 18 and 30 years old. The study is conducted under standardized laboratory and indoor sports-hall conditions, with ethics approval obtained from the Sinop University Human Research Ethics Committee (Approval No. 2025/337).

Conditions

• Cognitive Performance
• Physical Performance Enhancement
• Caffeine and L-theanine Interaction

Interventions

DIETARY_SUPPLEMENT

Caffeine

Participants receive 3 mg of anhydrous caffeine per kilogram of body weight, encapsulated in identical gelatin capsules (five in total). Capsules are taken orally with water 60 minutes before testing. The caffeine powder (98.5% purity, Oxford Vitality Health Co., UK) is administered once per experimental session, with at least 72 hours between conditions.

DIETARY_SUPPLEMENT

L-theanine

Participants receive a single 200 mg dose of L-theanine (Orzax Pharmaceuticals, Istanbul, Turkey; active ingredient manufactured in Germany) encapsulated in identical gelatin capsules. The supplement is ingested orally with water 60 minutes before testing. All testing sessions are separated by at least 72 hours.

DIETARY_SUPPLEMENT

Combined Caffeine and L-Theanine

Participants receive a combination of 3 mg/kg anhydrous caffeine and 200 mg L-theanine, encapsulated together in visually identical gelatin capsules. The mixture is taken orally with water 60 minutes before testing to assess potential synergistic effects on performance. Each participant completes all four supplementation conditions in randomized order, separated by ≥72 hours.

DIETARY_SUPPLEMENT

Placebo

Participants receive placebo capsules filled with mannitol, a non-nutritive sugar alcohol with no known ergogenic or ergolytic effects. Capsules are identical in appearance, weight, and texture to the active supplement capsules and are taken orally with water 60 minutes before testing.

Sponsors & Collaborators

• Ulaş Can YILDIRIM

  lead OTHER

Study Design

Allocation

RANDOMIZED

Purpose

BASIC_SCIENCE

Masking

QUADRUPLE

Model

CROSSOVER

Eligibility

Min Age

18 Years

Max Age

30 Years

Sex

ALL

Healthy Volunteers

Yes

Timeline & Regulatory

Start

2025-08-01

Primary Completion

2025-08-14

Completion

2025-08-14

Countries

• Turkey (Türkiye)

Study Locations