Probiotics for Frequent Infections

Probiotics produce small reductions in the duration and, less consistently, the frequency of respiratory infections — roughly a day or two shorter per episode in meta-analysis. Real but modest, and nowhere near what immune-boosting marketing implies.

Around 70% of immune tissue sits in the gut, in the gut-associated lymphoid tissue, which makes intestinal microbial signalling a plausible route to systemic immune modulation. Probiotic strains interact with dendritic cells and M cells in Peyer patches, influencing regulatory T cell development and cytokine balance. Documented effects include increased secretory IgA at mucosal surfaces — including, via the common mucosal immune system, at respiratory surfaces distant from the gut — enhanced natural killer cell activity, and improved phagocytic function in some strains. The distance between these mechanisms and a measurable reduction in colds is, however, considerable, and the clinical effects are correspondingly smaller than the mechanistic story suggests.

A Cochrane review of probiotics for preventing acute upper respiratory tract infections concluded that probiotics were better than placebo at reducing the number of participants experiencing episodes, reducing mean episode duration by roughly one to two days, and reducing antibiotic prescription rates — while rating the evidence quality as low, with substantial heterogeneity. Trials in children in daycare, in athletes during heavy training, and in elderly populations have shown reductions in respiratory infection incidence or duration with various Lactobacillus and Bifidobacterium strains, though results are inconsistent between products. There is also a well-documented interaction with vaccination: several trials report improved antibody responses to influenza vaccination with probiotic supplementation, which is a more specific and arguably more meaningful finding than symptom counting. The overall picture is a small, plausibly real effect with poorly defined strain specificity — the opposite of the antibiotic-associated diarrhoea evidence, where strain and indication are tightly matched.

People whose frequent infections have a structural or immunological cause — immunodeficiency, uncontrolled diabetes, immunosuppressive therapy — will not benefit meaningfully. Those whose problem is high exposure rather than susceptibility, such as parents of preschool children, are unlikely to notice much. Anyone chronically sleep-deprived is addressing a small variable while ignoring the largest one.