Probiotic supplements can modestly support specific aspects of immunity, such as reducing upper respiratory tract infection (URTI) incidence and duration, but evidence does not support broad improvements in "overall immunity" for healthy people.
Key Evidence from Trials
Meta-analyses of randomized controlled trials show probiotics (e.g., Lactobacillus, Bifidobacterium strains) reduce URTI risk by 10-47%, shorten illness duration by about one day, and lower severity in adults and children. In athletes, they increase salivary IgA (a mucosal immune marker) and modulate cytokines like IFN-γ (up) and TNF-α/IL-10 (down), aiding post-exercise recovery. Benefits appear strongest with multi-strain formulas taken for 4-12 weeks.
Mechanisms Involved
Probiotics enhance gut barrier function, promote anti-inflammatory responses, and boost innate immunity via Toll-like receptors and antibody production. They compete with pathogens and regulate immune cells like macrophages and T-cells.
Limitations and Context
Effects are strain-specific and modest—not a "boost" to overall immunity like vaccines—and minimal in already healthy individuals without infections. No strong evidence for preventing all illnesses or enhancing vaccine responses broadly. Risks include minor GI upset; benefits are clearest for gut-respiratory axis support.
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