Hypothesis

Administration of the human milk oligosaccharide 2'-fucosyllactose (2'-FL) at 5 g/day together with moderate‑intensity aerobic exercise (30 min, 5 days/week) will produce a greater reduction in depressive symptom scores than either intervention alone, mediated by increased hippocampal brain‑derived neurotrophic factor (BDNF) through a gut‑brain axis involving short‑chain fatty acid (SCFA) production, vagal afferent activation, and epigenetic modulation of BDNF transcription.

Mechanistic Rationale

• 2'-FL selectively stimulates Bifidobacterium adolescentis and Akkermansia muciniphila, raising luminal acetate and propionate levels【2】.
• Acute exercise transiently increases intestinal permeability, permitting greater portal uptake of SCFAs【3】.
• SCFAs activate enteroendocrine FFAR2 receptors, boosting serotonin release that stimulates vagal afferents.
• Vagal signaling elevates hippocampal BDNF expression via the cAMP‑CREB pathway.
• Propionate also crosses the blood‑brain barrier through monocarboxylate transporters and inhibits histone deacetylases (HDACs), directly promoting BDNF transcription.
• This dual route—neural (vagus) and epigenetic (HDAC inhibition)—provides a mechanistic explanation for why the combination may outperform monotherapies.

Testable Predictions

1. Primary outcome: After 8 weeks, the combined 2'-FL + exercise group will show a significantly larger reduction in Montgomery‑Åsberg Depression Rating Scale (MADRS) scores compared with 2'-FL alone, exercise alone, and placebo (effect size > 0.8).
2. Secondary outcomes:
   • Fecal acetate and propionate concentrations will be highest in the combined group (↑ ≥ 30 % vs baseline).
   • Serum BDNF levels will rise proportionally to SCFA increases (r > 0.5).
   • Heart‑rate variability (an index of vagal tone) will improve most in the combined arm.
   • Hippocampal BDNF mRNA expression (measured via peripheral blood exosomes) will show decreased HDAC activity (↑ acetyl‑histone H3).
3. Dose‑response: Varying exercise intensity (40 % vs 60 % VO₂max) will modulate the magnitude of SCFA uptake and BDNF response, allowing mechanistic dissection.

Potential Outcomes and Falsifiability

• If the combined group demonstrates superior depressive symptom reduction accompanied by the predicted biomarker changes, the hypothesis is supported.
• If no significant difference is observed between the combined group and the best monotherapy, or if biomarker changes do not correlate with clinical improvement, the hypothesis is falsified, indicating that the proposed gut‑brain pathways are insufficient or that other mechanisms dominate.

This design directly addresses the current gap noted in prebiotic‑depression literature—lack of head‑to‑head RCTs against established treatments—and provides a clear, falsifiable roadmap for translating mechanistic insights into a clinically testable adjunctive strategy.