A 3-month randomized, double-blind, placebo-controlled pilot in 18 sedentary adults aged 65+ with mild depressive symptoms found that 48g/day of blueberry powder (roughly 2 cups of fresh berries) produced statistically significant shifts in gut microbial gene abundance at several Enzyme Commission levels — most notably EC 3.6.3.31 Polyamine Transporting ATPase, a pathway upstream of GABA synthesis. Species-level abundance, MetaCyc pathways, and short-chain fatty acid metabolites remained unchanged. Depressive symptom scores trended toward greater reduction in the blueberry group, though between-group differences did not reach significance.
The GABA connection is the headline mechanism here. Gut-derived GABA modulation via the microbiome-gut-brain axis is an active frontier in psychiatric research, and the specificity of the EC 3.6.3.31 signal gives this pilot a mechanistic foothold that generic fiber-mood studies typically lack. Blueberry anthocyanins and prebiotic fiber are known to selectively enrich beneficial microbiota, but demonstrating functional gene-level shifts — rather than species composition changes — is a more granular and arguably more meaningful readout of microbiome activity.
The limitations, however, are substantial: n=18 is barely powered for pilot-level claims, the cohort is predominantly white and female, and without significant mood improvement between groups, the clinical relevance remains speculative. This is incremental, hypothesis-generating science — valuable for designing adequately powered trials, but far from practice-changing.
