Complementary Feeding and Infant Gut Microbiota: A Narrative Review

Background: The complementary feeding period, spanning from 6 to 24 months of age, marks the transition from an exclusive liquid diet in infants to a dietary pattern requiring the introduction of solid foods to meet nutritional demands. Complementary feeding coincides with other critical development windows, including the maturation of the gut microbiome. However, the effects of specific solid foods on gut microbiota and the subsequent influence on health outcomes require further investigation. Methods: This narrative review analyzes published research from January 2004 to October 2024 and aims to summarize the current evidence of the effects of complementary feeding on the infant gut microbiota. Results: A total of 43 studies were included in this review. Overall, multiple studies reported an increase in alpha-diversity after solid food introduction. Bifidobacteriaceae is the predominant bacterial family during the first 6 months of life, shifting to Lachnospiraceae, Ruminococcaceae, and Clostridium spp. after the introduction of solid foods. The timing of solid food introduction may also influence gut microbiota, though results were inconclusive. The effect of individual dietary components on the gut microbiota was conflicting, with limited evidence to make inferences. Conclusions: Because of variations in study design, dietary intake quantification, and minimal follow-up, a lack of conclusive evidence exists describing the relationship between complementary feeding and gut microbiota outcomes in infants. Future research to describe these relationships should focus on the impact of individual foods on microbial diversity and maturation, as well as the relationship between microbiota and infant health outcomes.