Rethinking the Caffeine-Theanine Synergy in Sports Nutrition Formulations

A randomized, double-blind, placebo-controlled crossover study published in Frontiers in Nutrition challenges some assumptions of the synergy of caffeine and l-theanine, demonstrating that the combination does not provide additional performance advantages and that isolated L-theanine may actually hinder specific physical outcomes in trained athletes.¹

Interest in supplements that enhance focus and decision-making speed has been increasing, the researchers note. The pairing of caffeine and l-theanine has gained attention because of the stimulatory benefits of caffeine while leveraging the calming properties of L-theanine for an alert yet controlled state of attention.

How Was the Clinical Trial Structured?

The trial investigated the acute effects of isolated and combined caffeine and L-theanine supplementation on physical performance metrics and cognitive-visuomotor functions. Twenty competitive, trained athletes completed the study, navigating four distinct experimental conditions separated by a washout period of at least 72 hours.

The four randomized treatments consisted of:

• Isolated Caffeine (CAF): Administered at a dose of 3 mg·kg⁻¹ of body weight.
• Isolated L-Theanine (TEA): Administered at a fixed dose of 200 mg.
• Combined Formulation (COM): Ingestion of both 3 mg·kg⁻¹ CAF and 200 mg TEA.
• Placebo Control (CON): An inactive control substance.

The primary physical performance outcomes measured included maximum isometric leg strength, back strength, and maximum isometric handgrip strength. Intermittent aerobic endurance was evaluated using the Yo-Yo Intermittent Endurance Test Level 1. Eye-hand coordination and visuomotor control were assessed using the standardized CogniFit software platform. Data were analyzed using repeated-measures ANOVA.

What Data and Performance Outcomes Were Discovered?

The statistical analysis revealed condition effects specifically for isometric leg strength and isometric back strength. However, the data failed to support the prevailing hypothesis of an additive or synergistic effect within the combined (COM) group.

Contrary to expectations, the combined ingestion of caffeine and L-theanine did not enhance maximal physical strength or endurance compared to the other groups. Furthermore, no significant differences were observed between any of the experimental conditions regarding maximum isometric handgrip strength, aerobic endurance capacities, or eye-hand coordination performance.

Isolated caffeine administration did not yield statistically significant improvements over the placebo control for any strength parameter under the specific testing conditions of this protocol. Additionally, acute intake of isolated L-theanine was associated with a decrease in maximal isometric leg and back strength compared to both the isolated caffeine and placebo control conditions.

“Findings challenge the prevailing assumption of CAF + TEA synergy and underscore the need for task-specific interpretation of co-supplementation strategies,” the researchers concluded. “Future studies should evaluate dose–response interactions, habitual caffeine intake, sex-specific responses, and broader cognitive domains beyond visuomotor control.”

The researchers identified limitations to the study, including a lack of control for genetic factors that could affect a response to caffeine as well as an allowance of participants to continue habitual caffeine consumption, and a limited measurement of cognitive performance.

What Are the Strategic Implications for Supplement Formulators?

For industry stakeholders, these findings indicate that the performance interactions between caffeine and L-theanine are highly task-specific. The absence of cognitive or physical enhancement from the combined formula suggests that the common "stimulant-plus-calming" formulation framework may not translate into tangible performance gains for high-intensity physical tasks or basic visuomotor coordination in elite athletic demographics.

In the feature Finding Nuance in Energy Products from the June issue of Nutritional Outlook, industry experts highlight a growing market shift away from simple caffeine spikes toward new approaches that improve focus and stress management.² The Frontiers in Nutrition study suggests a potential boundary for formulators: while the tempering strategy from the inclusion of l-theanine may support mental focus, it might not enhance maximal athletic strength.

References

1. Tuncer SY, Ozdenk S, Yildirim UC, Erkan D, et al. 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. Front Nutr. 2026;12:1751673. doi:10.3389/fnut.2025.1751673
2. Krawiec S. Finding Nuance in Energy Products. June 11, 2026. Accessed June 19, 2026. https://www.nutritionaloutlook.com/view/finding-nuance-energy-products