Probiotic improves respiratory and gastrointestinal health, immune homeostasis, and gut microbiota composition in infants: a randomized controlled trial

Introduction The early postnatal period is a critical window for shaping the gut microbiota, which plays a pivotal role in immune maturation, infection resistance, and metabolic programming. Disruptions to this process may predispose infants to infections and allergic or metabolic disorders. Probiotics such as Bifidobacterium infantis have shown promise in modulating gut microbial ecology and immune function, but strain-specific and mechanistic evidence in infants remains limited. This study aimed to evaluate the effects of B. infantis YLGB-1496 supplementation on clinical outcomes, immune markers, and gut microbiota composition in healthy infants below one year of age. Methods In a 12-week, randomized, double-blind, placebo-controlled trial, 119 healthy infants were enrolled ( B. infantis YLGB-1496 n =59, placebo n =60). Participants received one daily sachet of B. infantis YLGB-1496 (1 × 10¹⁰ CFU) or placebo. Clinical outcomes for respiratory health and gastrointestinal (GI) health were assessed via validated questionnaires. Oral and fecal samples were collected for analysis of sIgA, cortisol, and cytokines (TNF-α, IFN-γ, IL-1β, IL-10, calprotectin). Gut microbiota was profiled by 16S rRNA sequencing, and diversity indices and taxonomic shifts were analyzed. Results Compared with placebo, B. infantis YLGB-1496 supplementation was associated with consistent numerical reductions in respiratory symptom days, although these did not remain statistically significant after false discovery rate (FDR) adjustment. In contrast, gastrointestinal outcomes showed robust improvements after FDR correction, including reduced stomach ache ( q = 0.010), lower diarrhea incidence ( q 0.001), and fewer diarrhea-related clinical visits ( q 0.001). Fecal sIgA remained elevated in the B. infantis YLGB-1496 group ( P = 0.138 vs P = 0.000 in placebo), accompanied by increased IL-10 ( P 0.001) and reduced IL-1β ( P = 0.002). Oral sIgA was enhanced ( P = 0.001), while cortisol declined similarly in both groups. Microbiota analysis revealed enrichment of beneficial taxa in the B. infantis YLGB-1496 group with concurrent reductions in pathobionts. In contrast, the placebo group exhibited increases in Campylobacter, Staphylococcus , and Desulfovibrio desulfuricans , and decreases in Faecalibacterium prausnitzii and Anaerostipes caccae , indicative of dysbiosis. These compositional changes support improved gut barrier function and immune development. Clinical trial registration https://clinicaltrials.gov/study/NCT05794815?term=NCT05794815rank=1 , Identifier: NCT05794815.