L-Theanine for Cognitive Function
The evidence for L-theanine improving cognitive function is weak. On its own it produces only a small, short-lived speed-up in simple reaction time; the broader "focus and memory" benefits that supplements advertise mostly come from pairing it with caffeine, not from L-theanine alone.
Why this grade7-layer evidence engine
This earns a weak (Tier C) grade because the positive findings are small, acute, and narrow. A 2025 systematic review and meta-analysis (PMID 40314930) found that L-theanine taken alone modestly sped up choice reaction time (SMD -0.35), but most other cognitive gains came from L-theanine combined with caffeine (attention switching SMD 0.33; digit vigilance SMD 0.20). Small crossover trials agree: single doses of 100-200 mg shaved roughly 16 ms off simple reaction time (PMID 36263942) and 400 mg shifted an attention-related brainwave marker (PMID 32777998), yet neither improved sustained attention or self-control.
Crucially, the one longer, real-world test failed. A 12-month trial in older adults with cognitive decline (PMID 39213264) saw no improvement on standard memory and daily-function scores, even though matcha contains both L-theanine and caffeine. Other matcha trials (PMID 39275223) and an athlete study (PMID 37815006) again show benefits tied to the caffeine combination rather than to L-theanine by itself.
Authorities reinforce the caution. The US FDA treats L-theanine as safe (GRAS, 'FDA has no questions') but has approved no cognitive claim, and the EU's EFSA explicitly ruled that 'a cause and effect relationship has not been established' between L-theanine and improved cognition. The Cleveland Clinic likewise notes 'still limited research on the benefits.' Bottom line: do not expect meaningful brain or memory enhancement from L-theanine alone.
Scoring transparency
All scores computed by a 7-layer evidence engine — fully auditable▸View the full decision path (audit trail)
- compute_raw_score — 加權公式: L2×0.30 + L3×0.25 + L5×0.25 + L11×0.10 + L1×0.10 = 0.477
- tier_from_score — 依分數區間映射至 tier letter
- apply_hec_rules — 無高階證據可裁決
- tier_strict_requirement_check — Tier 條件達標，未降階
- detect_disputes — 偵測到 0 個 hard + 0 個 soft dispute
- decide_status — 依 tier + dispute 結果決定 status