CONTEXT: Prior research has explored the effect of synbiotics, the combination of probiotics and prebiotics, on the gut microbiota in clinical populations. However, evidence related to the effect of synbiotics on the gut microbiota in healthy adults has not been reviewed to date. OBJECTIVE: A systematic review and meta-analysis was conducted to comprehensively investigate the effect of synbiotics on the gut microbiota and inflammatory markers in populations of healthy adults. DATA SOURCES: Scopus, PubMed, Web of Science, ScienceDirect, MEDLINE, CINAHL, and The Cochrane Library were systematically searched to retrieve randomized controlled trials examining the primary outcome of gut microbiota or intestinal permeability changes after synbiotic consumption in healthy adults. Secondary outcomes of interest were short-chain fatty acids, inflammatory biomarkers, and gut microbiota diversity. DATA EXTRACTION: Weighted (WMD) or standardized mean difference (SMD) outcome data were pooled in restricted maximum likelihood models using random effects. Twenty-seven articles reporting on 26 studies met the eligibility criteria (n = 1319). DATA ANALYSIS: Meta-analyses of 16 studies showed synbiotics resulted in a significant increase in Lactobacillus cell count (SMD, 0.74
95% confidence interval [CI], 0.15, 1.33
P = 0.01) and propionate concentration (SMD, 0.22
95% CI, 0.02, 0.43
P = 0.03) compared with controls. A trend for an increase in Bifidobacterium relative abundance (WMD, 0.97
95% CI, 0.42, 2.52
P = 0.10) and cell count (SMD, 0.82
95% CI, 0.13, 1.88
P = 0.06) was seen. No significant differences in α-diversity, acetate, butyrate, zonulin, IL-6, CRP, or endotoxins were observed. CONCLUSION: This review demonstrates that synbiotics modulate the gut microbiota by increasing Lactobacillus and propionate across various healthy adult populations, and may result in increased Bifidobacterium. Significant variations in synbiotic type, dose, and duration should be considered as limitations when applying findings to clinical practice. SYSTEMATIC REVIEW REGISTRATION: PROSPERO no. CRD42021284033.