In plain terms: GABA (Gamma-Aminobutyric Acid) shows early signals in human research but the evidence is limited -- small trials, surrogate endpoints, or not yet independently replicated. It is an emerging area, not yet a proven one.
GABA (Gamma-Aminobutyric Acid)
The evidence as the record states it
Guimarães et al. (J Diet Suppl, 2024; PMID 38321713): randomized double-blind placebo-controlled trial, 30 sedentary women with overweight or obesity who were also starting an exercise program, 200 mg/day for 90 days; Pittsburgh Sleep Quality Index, negative affect and DASS-21 depression scores improved and heart-rate variability rose; the abstract reports no stress result. Hepsomali et al. (Front Neurosci, 2020; PMID 33041752): systematic review of 14 placebo-controlled trials of natural or biosynthetic oral GABA; limited evidence for stress and very limited evidence for sleep; first author at a consumer-goods company's research arm. Byun et al. (J Clin Neurol, 2018; PMID 29856155): the one polysomnography trial, 40 adults with insomnia symptoms randomized 3:1 to 300 mg/day rice-germ GABA or placebo for 4 weeks; sleep latency fell and sleep efficiency rose within the GABA group, with the abstract reporting within-group change rather than a between-group difference. Grade C: small trials on self-report and surrogate endpoints, with the only objective-outcome trial reporting within-group change; oral GABA's passage into the brain is debated.
Why Grade C and not Grade B
Grade C reflects early or limited human evidence -- small trials, surrogate endpoints, or preliminary findings not yet independently replicated. The evidence points in a promising direction but does not yet meet the standard for Grade B. A single well-powered, independently replicated trial with a meaningful endpoint would move this to Grade B or higher.
What would move this grade
A well-powered randomized trial with an objective, clinically meaningful primary endpoint and at least one independent replication would support Grade B or higher.