L-Theanine vs Vitamin D for cognition: which has better evidence?
A tie on evidence grade for cognition (L-Theanine Grade C vs Vitamin D Grade C). For cognition, the two are evidence-matched; the difference is in effect and safety.
Platform evidence comparison
These existing platform assessments are separate from the source-reported findings below. They do not represent a direct trial comparing these treatments.
| L-Theanine | Vitamin D | |
|---|---|---|
| Grade | grade C | grade C |
| Certainty | moderate | low |
| Effect | improves attention and cuts down distraction, especially paired with caffeine | Systematic review and meta-analysis: lower vitamin D levels are associated with poorer cognition and higher dementia risk. The association is mostly observational; supplementation trials have not established that raising vitamin D improves cognition. |
| Studied dose | 100–200 mg | not applicable — observational association |
| Population | adults | older adults |
| Safety | 0 modeled interaction(s) | 1 modeled interaction(s) |
Grade C / moderate: probably a small effect in specific populations; do not expect a general benefit.
Direct comparison evidence
There are no direct RCTs in our corpus comparing L-Theanine and Vitamin D head-to-head for cognition. What we present above is an indirect comparison: each side's own trials, contrasted. This is standard practice for supplement comparisons — dose, population, and outcome-instrument differences between the two evidence bases add uncertainty on top of each side's own grade.
Can you take both?
No modeled interaction between L-Theanine and Vitamin D in our corpus. Absence of a recorded interaction is not proof of additive benefit — combined-effect trials are usually absent for supplement pairs, so treat "stack" claims skeptically.
Decision guidance
- If certainty matters most — choose whichever side carries the higher certainty rating at a similar grade.
- If you're on medications — check each side's interaction pages before deciding; safety differences dominate small evidence-grade gaps.
- If your population is under-studied — the smaller-effect side with better population evidence often wins over the higher-grade side studied in a different demographic.
- If cost matters — form and dose page differences (L-Theanine vs Vitamin D) can invert the practical winner.
Grade history
No grade changes yet. Initial grade assigned; re-reviewed on every 6 months (Grade C+) — next review by . Methodology v1.
Re-review cadence: every 6 months (Grade C+) · next scheduled by · methodology v1.
Frequently asked
Which has stronger evidence: L-Theanine or Vitamin D for cognition?
L-Theanine carries Grade C at moderate certainty; Vitamin D carries Grade C at low. The stronger evidence points to a tie, subject to the caveat that direct head-to-head trials are usually absent for supplement pairings.
Can I take L-Theanine and Vitamin D together?
No modeled interaction between L-Theanine and Vitamin D; combining is not documented to be problematic, but combined-effect trials are also absent, so treat additive benefit claims skeptically.
Have L-Theanine and Vitamin D been compared head-to-head?
Direct RCTs comparing the two for cognition are uncommon. This page contrasts each side's own trial evidence — that is an indirect comparison, which introduces confounds from population and dose differences. See the direct-comparison block above.
What decides between L-Theanine and Vitamin D for me?
Read the decision guidance below — the split usually comes down to population fit, interaction risk with your other medications, and how much certainty you want behind the recommendation.
Related
Cite this page
Citation is encouraged. The proposed reuse license is CC-BY-4.0; status: pending-governance-ratification.
Plain-text citation
Substrate. L-Theanine vs Vitamin D for cognition. https://evidencebased.info/compare/theanine-vs-vitamin-d-for-cognition. Updated 2026-07-14.
BibTeX
@misc{substrate_vitamindforcognition,
author = {Substrate editorial},
title = {L-Theanine vs Vitamin D for cognition},
year = {2026},
url = {https://evidencebased.info/compare/theanine-vs-vitamin-d-for-cognition},
note = {evidencebased.info}
}