Probiotic supplementation is associated with higher faecal secretory IgA (sIgA) in healthy full-term infants: a systematic review and meta-analysis

BACKGROUND: Early probiotic supplementation influences infant immune development, but effects on specific immune biomarkers remain unclear. METHODS: We searched six databases (2000-2025) for randomised controlled trials of oral probiotic supplementation initiated before 2 years of age that reported faecal secretory IgA (sIgA) and/or cytokines. For sIgA, quantitative synthesis was restricted to healthy full-term infants. Random-effects meta-analyses were conducted for outcomes reported as means, and a quantile-estimation approach was applied to studies reporting medians. Risk of bias was assessed using the RoB 2 tool. RESULTS: Thirty-nine randomised controlled trials met the inclusion criteria. Twenty-four trials reported sIgA, 18 reported cytokines, and three reported both outcomes. Seven sIgA trials in healthy full-term infants (440 probiotic, 414 control) were meta-analysed. Probiotic supplementation was associated with higher faecal sIgA, with a pooled mean difference of 435 µg/g stool (95% CI 196-674; I² = 0%). Larger effects were observed when supplementation began at or before 4 weeks of age. Cytokine outcomes were derived from a broader, clinically heterogeneous evidence base, and pooled analyses showed no consistent effects. CONCLUSION: Probiotic supplementation in healthy full-term infants is associated with higher faecal sIgA, whereas effects on cytokine outcomes remain uncertain. IMPACT: = 0%) The pooled sIgA estimate favoured probiotics and remained similar in sensitivity analyses. Earlier initiation ( ≤ 4 weeks) was associated with larger sIgA effects than later initiation. Cytokine outcomes were derived from more clinically heterogeneous populations, and pooled analyses did not show consistent effects. Faecal sIgA may be a practical immunological endpoint for mechanistic infant probiotic trials.