Effects of Timing and Types of Complementary Foods on the Gastrointestinal Microbiota in Infants up to 12 Months of Age: A Systematic Review

Context The transition from a liquid-only diet to a diet that includes complementary foods (CFs) is a sensitive period in infant development and flavor learning. Often recommended are iron- and zinc-rich foods to meet nutrient needs for growth while establishing healthy eating patterns. Recent research has focused on the influence of CFs on the infant gastrointestinal (gut) microbiome. Objective In this review we sought to assess the scientific evidence on the timing of CF introduction and the effects of singular CFs and groups of CFs on the gut microbiota of healthy infants. Data Sources Five databases were searched. Articles published since 2000 were identified for inclusion and subject to data extraction. Data Extraction Data were synthesized with a focus on microbiota outcomes and complementary feeding, including only results that pertained to timing, singular CFs, or a group or pattern of CFs and their effects on the gut microbiota. Data Analysis The search resulted in 18 reports (17 studies) for inclusion. The prior liquid diet composition, types of foods, length of intervention/follow-up, and methods to evaluate the gut microbiota varied across studies. Limited conclusive evidence exists for the effects of timing, type, or groups of CFs of alpha or beta diversity. Some studies found negative associations between dairy and dairy-based CFs and members of the Bifidobacteriaceae family and Bifidobacterium genus. Conclusions Evidence strongly suggests that the background liquid diet has a substantial influence in how timing, singular CFs, and groups of CFs modulate gut microbial composition. Dairy and dairy-based CFs were most consistently associated with reduced abundance of Bifidobacterium and/or Bifidobacteriaecae; however, the heterogeneity of infant characteristics across studies limited the ability to draw more definitive conclusions. Future intervention studies should include intervention arms with distinct liquid milk backgrounds to better elucidate relationships among the timing and types of CFs and the infant gut microbiota. Systematic Review Registration PROSPERO registration No. [CRD42024505565].