Probiotics: What They Actually Do - and When They Are Worth Taking
Probiotic supplements are a multi-billion-dollar industry. The evidence for specific strains in specific conditions is stronger than people realise - but so is the evidence that most people are buying the wrong product for the wrong reason.
The Strain-Specificity Problem
Probiotic evidence is not transferable between strains. Lactobacillus rhamnosus GG has strong evidence for reducing antibiotic-associated diarrhoea. This says nothing about whether any other Lactobacillus strain in any other product has the same effect. Yet most probiotic marketing implies that the entire category shares the evidence for specific strains - a logical error with significant commercial consequences.
Where Probiotics Have Strong Evidence
- Antibiotic-associated diarrhoea: Multiple strains (LGG, S. boulardii) have consistent RCT evidence for prevention. This is the most evidence-backed use case.
- Infectious diarrhoea in children: Significant reduction in duration, particularly with LGG and Bifidobacterium lactis.
- IBS symptom management: Modest but consistent evidence for several strains (Bifidobacterium infantis 35624, VSL#3) in reducing bloating and pain scores.
- Pouchitis (post-surgical bowel condition): VSL#3 has strong evidence for prevention and maintenance.
Where Evidence Is Weaker or Absent
- General "immune support" claims: not supported by RCTs in healthy adults at standard supplement doses
- Mental health and anxiety: promising animal and small human studies; no robust clinical evidence yet
- Weight management: negligible effects in the existing trials
- "Restoring microbiome balance": this concept is not well-defined enough to test
"Most probiotic products on the market have never been tested in humans in clinical trials. The gap between marketing and evidence is enormous." - Dr. Mary Ellen Sanders, International Scientific Association for Probiotics
Probiotics in Practice
If taking antibiotics, a probiotic containing LGG or S. boulardii taken simultaneously (but not within 2 hours of the antibiotic) has meaningful evidence for reducing GI side effects. For IBS, discuss a trial of Bifidobacterium infantis 35624 with your healthcare provider. For general gut health in healthy individuals, fermented foods (kefir, yoghurt with live cultures, kimchi, sauerkraut) have broader immune-modulating benefits and stronger food-matrix evidence than most supplements.
