Contribution
A review of probiotics acting on mood through the microbiota–gut–brain axis, organized around why preclinical findings fail to translate. Its case studies include strains whose human trials were null.
Key insight
The rodent work shows that gut microbes can drive affect-related behaviour. The human work has not shown that they do, robustly, at doses and in populations that matter: “the translation of these findings from mouse to man has proven difficult”.
Assumptions
- Industry authorship. Verbatim: “At the time of writing, all authors were employed by IFF Health & Biosciences. IFF Health & Biosciences manufactures and commercializes probiotics.” The review is narrative, with no search method.
- Healthy-volunteer trials. These are taken as relevant to mood in general. The authors note that “most findings from these clinical trials cannot be extrapolated to patients with clinically diagnosed depression.”
Key results
- Route. “The vagus nerve is the most direct route of communication between the gut and the brain, thus enabling bidirectional communication.” In mice the effects are lost after vagotomy.
- Germ-free animals. They show an exaggerated stress-hormone response that colonization normalizes. This is replicated across rodent strains.
- Faecal transplant to rodents. Abnormal behaviour “indicating at least partial transfer of the clinical psychiatric phenotype”. The donors were few, and “the gut microbiome is the only humanized component of the rodent”.
- Healthy humans. “A recent meta-analysis demonstrated that probiotic consumption could result in a reduction of subjective stress levels in healthy volunteers”. Single trials report reduced reactivity to sad mood, perceived stress and anxiety, some only under stress or in subgroups.
- Failures. L. rhamnosus JB-1, after “comprehensive preclinical evidence”, “was not found to have an effect on mood, anxiety, stress, sleep quality, memory performance, attention, or anti-inflammatory cytokine levels compared with the placebo.” Another strain did not alleviate exam stress. A combination left depression scores identical to placebo.
Claims
| id | claim | strength | support |
|---|---|---|---|
| C1 | Gut microbes signal to the brain by vagal, immune, endocrine and metabolite routes | moderate in animals; asserted in humans | vagotomy; germ-free studies |
| C2 | Faecal transplant from depressed humans transfers the phenotype to rodents | weak to moderate | ”at least partial”, few donors |
| C3 | Probiotics reduce subjective stress in healthy people | moderate, mixed | one meta-analysis; RCTs with nulls |
| C4 | Probiotics treat clinical depression | weak | small trials; nulls |
| C5 | Preclinical psychobiotic effects translate to humans | weak; repeatedly failed | JB-1, 1714, strain screen |
Connections
- The owner’s example (CLAIM-166, CASE-047). In animals, a lower-level collective changes affect-related behaviour, and a transplant moves the change with the microbes. That is the drive-from-below the owner describes. In humans the evidence that everyday affect and experience are moved by one’s own microbiome is modest: small effects on subjective stress, which is experience, alongside clean nulls.
- Felt affect (THEORY-045). The vagus is an interoceptive channel, so this is a route from the microbiome into the bodily state that, on THEORY-045, felt affect reports. That bearing is the record’s.