The most important concept in probiotic science — and the one most consistently ignored in supplement marketing — is strain specificity. A probiotic's effects are not transferable across strains, even within the same species. "Lactobacillus acidophilus" on a label tells you almost nothing about what the product does. The relevant question is which specific strain (identified by a three-part name: genus, species, strain code — e.g., Lactobacillus rhamnosus GG) has been tested in human RCTs for the specific outcome you care about.
The clinical trial evidence for probiotics is larger than most people realize, but it is concentrated in a small number of well-studied strains. This guide covers the strains with the strongest human RCT evidence and matches them to the clinical situations where they have actually been tested — not where general "gut health" marketing suggests they might help.
| Strain | Strongest Evidence | Dose / Duration | Notes |
|---|---|---|---|
| L. rhamnosus GG (LGG) | Antibiotic-associated diarrhea (NNT=8); acute gastroenteritis in children; C. diff prevention (adjunct) | 10^10–10^11 CFU/day; start with antibiotics, continue 1–2 weeks after | Most studied strain globally; survives gastric acid; generally heat-stable; widely available |
| L. plantarum 299v | IBS (abdominal pain, bloating — Ducrotté 2012); also studied in post-surgical infections | 10^10 CFU/day; 4–8 week trial minimum | Branded as Probi Mage; specifically IBS-D/M; less evidence for IBS-C; do not substitute other L. plantarum strains |
| B. infantis 35624 | IBS (all subtypes — Whorwell 2006, N=362): significant reduction in pain, bloating, bowel habit; global symptom relief vs placebo | 10^8 CFU/day (optimal dose from Whorwell dose-finding); higher doses NOT more effective | Branded as Align; one of the best-studied IBS probiotics; the dose-finding data (lower dose = better) is important and counterintuitive |
| S. boulardii CNCM I-745 | C. difficile recurrence prevention (Surawicz 2000); antibiotic-associated diarrhea; traveler's diarrhea | 250–500mg 2×/day (concurrent with antibiotics) | Yeast — unaffected by antibacterial antibiotics; do NOT use in immunocompromised patients (risk of fungemia); Florastor is the primary commercial brand |
| L. helveticus R0052 + B. longum R0175 | Anxiety and stress (Messaoudi 2011); reduced cortisol; improved mood reactivity | 3×10^9 CFU/day combined; 30-day minimum trial | Psychobiotic combination; must use this specific pairing — not interchangeable with other L. helveticus or B. longum strains |
| VSL#3 (8-strain combination) | Ulcerative colitis remission maintenance (Tursi 2010); pouchitis prevention after ileal pouch-anal anastomosis | 450–900 billion CFU/day (high dose); prescription-strength formula | One of the highest-CFU products with clinical data; UC and pouchitis specifically; limited evidence for IBS despite marketing |
| B. lactis BB-12 | Immune modulation (reduces respiratory infection duration); constipation; infant colic | 10^9–10^10 CFU/day; 4+ weeks | Chr. Hansen strain; widely used in yogurts and supplements; constipation data most consistent in elderly populations |
CFU count — more is not always better: The Whorwell 2006 B. infantis 35624 trial is the definitive example: 10^8 CFU outperformed 10^10 CFU for IBS outcomes; the effective dose varies dramatically by strain; as a general guide: 10^9–10^11 CFU/day covers most evidence-based applications; mega-dose products (50+ billion CFU) are not better for most indications and may destabilize the microbiome transiently; the strain designation on the label is more important than the CFU number.
Survivability to the colon: A probiotic's CFU count means nothing if the organisms don't survive gastric acid (pH 1.5–3.5) and bile; LGG, VSL#3, and B. infantis 35624 have documented acid survival; spore-forming probiotics (Bacillus coagulans, B. subtilis) survive gastric passage in spore form and germinate in the intestine — highly stable at room temperature; encapsulated or delayed-release formulations improve survival of non-spore-forming strains.
Refrigeration — the real requirement: Many Lactobacillus and Bifidobacterium strains require refrigeration after opening (live cultures lose viability at room temperature within weeks); check manufacturer storage requirements; shelf-stable products use freeze-dried bacteria or spore-forming organisms; refrigeration during shipping also matters — products left in a hot warehouse or delivery truck may be significantly degraded; buy from reputable retailers with cold-chain handling for refrigerated products.
Timing with antibiotics: If using probiotics with antibiotics: take the probiotic 2+ hours away from the antibiotic dose to reduce antibiotic-mediated killing; S. boulardii (a yeast) can be taken simultaneously with antibacterial antibiotics; continue probiotics for 2–4 weeks after completing the antibiotic course (the most important window for microbiome recovery).
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