Evidence & Sources

This page brings together neutral and independent references on the gut-liver axis and the use of probiotics for common digestive symptoms (such as bloating, functional constipation, and IBS symptoms). The goal is to inform, not replace clinical guidance.

Last update: September 23, 2025

How to read this page

  • Quality of evidence varies: studies differ in design, size, and probiotic strains evaluated.
  • Results are not universal: effects may vary by person, diagnosis, and specific strain/dose.
  • Clinical guidelines are conservative: when evidence is uncertain, guidelines tend to recommend caution.

What is the gut-liver axis?

The “gut-liver axis” describes the bidirectional communication between the gastrointestinal tract and the liver, mainly via the portal vein, mediated by microorganisms, metabolites (e.g., bile acids), and immune and epithelial barriers. Changes in this pathway can influence digestive and metabolic functions. Changes in this pathway can influence digestive and metabolic functions.

Basic reading: reviews in high-impact journals (Nature Reviews Gastroenterology & Hepatology), which explain the mechanisms, barriers, and circuits of the axis.

Probiotics: what reviews and guidelines show

Irritable Bowel Syndrome (IBS)

  • Recent meta-analyses find signs of benefit for some outcomes in specific subgroups/strains, but with heterogeneity and variable quality.
  • Clinical guidelines from American societies adopt a cautious stance and do not routinely recommend probiotics for overall control of IBS symptoms when the evidence is very uncertain.

Functional constipation in adults

  • Systematic reviews and meta-analyses (including network analyses) suggest that some interventions with probiotics, prebiotics, and synbiotics may increase weekly stool frequency in some studies.
  • The response varies by strain/dose/duration; the overall quality of evidence is moderate to low in many studies.

Safety, choice, and quality

  • Probiotics are generally safe for most adults, but are not risk-free — especially in immunocompromised individuals, hospitalized patients, those with venous catheters, or those with serious illnesses.
  • Products differ in strain, dose (CFU/day), and manufacturing quality; choose based on clear labeling and human studies of the specific strain.
  • Talk to a healthcare professional if you have medical conditions, are taking medications, or are pregnant/breastfeeding.

Transparency & Limitations

  • The sources below are independent (medical societies, government agencies, and peer-reviewed journals).
  • We do not infer effectiveness for specific brands/combinations; results from one strain do not automatically extend to others.
  • New publications may update conclusions; check the dates of articles and guidelines.

References (neutral selection)

  1. Nature Reviews Gastroenterology & Hepatology. Review on the gut-liver axis and microbiome. Access
  2. American Gastroenterological Association (AGA). Guideline on the role of probiotics in GI disorders. Access
  3. American College of Gastroenterology (ACG). IBS guideline (2021) – position on probiotics. Access
  4. NCCIH/NIH. Probiotics – safety and what the science says (lay audience). Access
  5. Gastroenterology (2023). Updated meta-analysis of probiotics in IBS (quality and heterogeneity discussed). Access
  6. BMJ Open (2024). Meta-analysis: probiotics in functional constipation (adults). Access
  7. European Journal of Nutrition (2024). Network meta-analysis: probiotics, prebiotics, and synbiotics in constipation. Access
  8. American Journal of Clinical Nutrition (2023). Review of chronic constipation and nutritional interventions (methodological context). Access
  9. NCCIH/NIH. IBS & complementary approaches – evidence and safety summary. Access

Note: these links direct you to third-party content and may be in English. Where possible, we prioritize review articles and clinical guidelines.

Important warning

This page is for informational purposes only and is not a substitute for medical advice. Individual results vary. If you have medical conditions, are taking medications, are pregnant, or are breastfeeding, seek professional advice.