Probiotics

Probiotics are live microbes that, given in large enough amounts, bring a health benefit to the host. WHO/FAO set that definition in 2001. ISAPP reaffirmed it in 2014 (Hill et al.). The most-studied groups are Lactobacillus, Bifidobacterium, and the yeast Saccharomyces boulardii. Effects are strictly strain-specific. You cannot assume one strain's results carry over to another species or product. Colonization is mostly temporary, too; strains clear within days to weeks after you stop. How do they help? They crowd out pathogens and strengthen your gut barrier. They make short-chain fatty acids. And they tune your gut immune response, lowering IL-6 and TNF-α while raising IL-10 and regulatory T cells. The evidence is strongest for antibiotic-associated diarrhea. A meta-analysis of 36 trials (n = 9,312; Liao et al., 2021) found a 38% drop in cases (RR 0.62, 95% CI 0.51 to 0.74). Two strains were the most consistent: Lactobacillus rhamnosus GG and Saccharomyces boulardii. For irritable bowel syndrome, pooled data show a modest symptom drop (RR 0.77, NNT 7) with high variability, reflecting strong strain- and dose-dependence. A 2024 trial (Lazou-Ahrén et al.) tested Lactiplantibacillus plantarum HEAL9 in adults over 70. It found clearly lower fecal calprotectin versus placebo; serum CRP came close but did not reach significance. Whether easing gut-driven 'inflammaging' slows immune aging stays plausible but unproven by long-term trials as of 2026.

Last reviewed: June 22, 2026

This definition is educational and is not medical advice, a diagnosis, or treatment. Talk to a doctor about any health decisions. Read our full medical disclaimer