Maternal synbiotic supplementation during early lactation and infant growth in the first year of life: a randomized, triple blind, placebo-controlled trial

Background Breast milk plays an important role in supporting infant growth and development during early life. Maternal synbiotic supplementation during lactation may also influence infant growth; however, clinical evidence supporting this potential effect remains limited and inconsistent. Objective The primary aim of this study was to investigate whether short-term synbiotic supplementation of lactating women during early lactation affects infant growth during the first year of life. Methods In this randomized, triple-blind, placebo-controlled trial, healthy lactating women with exclusively breastfed term infants ( n = 96) received either a synbiotic supplement (four Lactobacillus species plus fructooligosaccharides) or an identical placebo for 28 days. Infant weight-for-age z-score (WAZ), length-for-age z-score (LAZ), and head-circumference-for-age z-score (HCAZ) were measured at baseline, post-intervention, 6 months, and 12 months. Maternal plasma manganese, iron, and copper concentrations were measured before and after supplementation. All participants were analyzed by intention-to-treat using linear mixed models. Results At 6 months, infants in the synbiotic group had higher WAZ than those in the placebo group (0.08 ± 1.49 vs. −0.63 ± 0.98; p = 0.008; Cohen’s d = 0.56), and this difference remained at 12 months (0.24 ± 1.19 vs. −0.53 ± 0.97; p = 0.001; Cohen’s d = 0.71). In the longitudinal analysis, WAZ increased more from baseline in the synbiotic group than in the placebo group at both 6 months (β = 0.477; 95% CI, 0.045–0.909; p = 0.030) and 12 months (β = 0.528; 95% CI, 0.093–0.963; p = 0.017). No significant differences in growth trajectories were observed for LAZ or HCAZ. Maternal plasma manganese, iron, and copper concentrations were not associated with infant growth indices before or after the intervention. Conclusion Short-term maternal synbiotic supplementation during early lactation was associated with a more favorable trajectory of infant weight-for-age z-score over the first year of life. No clear differences were seen for length or head circumference. The underlying biological mechanisms remain unclear, and further well-designed studies are needed to confirm these findings and clarify their clinical significance. Trial registration IRCT20181020041387N2, registered on 28 June 2024. Retrospectively registered.