Probiotics

Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host — the definition formalised by WHO/FAO in 2001 and reaffirmed by ISAPP in 2014 (Hill et al.). The most studied genera are Lactobacillus, Bifidobacterium, and the yeast Saccharomyces boulardii. Effects are strictly strain-specific and cannot be extrapolated across species or formulations. Colonisation is predominantly transient; strains clear within days to weeks after supplementation ends. Mechanisms include competitive exclusion of pathogens, epithelial barrier reinforcement, SCFA production, and mucosal immune modulation — suppressing IL-6 and TNF-α while promoting IL-10 and regulatory T cells. Evidence is strongest for antibiotic-associated diarrhoea: a meta-analysis of 36 RCTs (n = 9,312; Liao et al., 2021) found a 38% incidence reduction (RR 0.62, 95% CI 0.51–0.74), with Lactobacillus rhamnosus GG and Saccharomyces boulardii showing the most consistent benefit. For irritable bowel syndrome, pooled data show modest symptom reduction (RR 0.77, NNT 7) with high heterogeneity, reflecting strong strain- and dose-dependence. A 2024 RCT (Lazou-Ahrén et al.) testing Lactiplantibacillus plantarum HEAL9 in adults over 70 found significantly reduced faecal calprotectin versus placebo; serum CRP approached but did not reach significance. Whether attenuating gut-derived inflammaging slows immune senescence remains biologically plausible but unproven by long-term trials as of 2026.