The Gut Microbiota: A Promising Target in the Relation between Complementary Feeding and Child Undernutrition.

Child undernutrition is a major public health challenge that is persistent and disproportionately prevalent in low- and middle-income countries. Undernourished children face adverse health, economic, and social consequences that can be intergenerational. The first 1000 days of life, from conception until the child's second birthday, constitute the period of greatest vulnerability to undernutrition. The transition process from milk-based diets to solid, semi-solid, and soft food and liquids other than milk, referred to as complementary feeding (CF), occurs between the age of 6 mo and 2 y. CF practices that do not meet the WHO's guiding principles and are lacking in both quality and quantity increase susceptibility to undernutrition, restrict growth, and jeopardize child development and survival. The gut microbiota develops toward an adult-like configuration within the first 2-3 y of life. Recent studies suggest that significant changes in the gut microbial composition and functional capacity occur during the CF period, but these studies were conducted in high-income countries. Research in low- and middle-income countries, on the other hand, has implicated a disrupted gut microbiota in child undernutrition, and animal experiments reveal the potential for a causal relation. Given the growing body of evidence for a plausible role of the gut microbiota in the link between CF and undernutrition, microbiota-targeted complementary food may be a promising treatment modality for undernutrition management. The aims of this paper are to review the evidence for the relation between CF and undernutrition and to highlight the potential of the gut microbiota to be a promising target in this relation. Our summary of the current state of the knowledge in this area provides a foundation for future research and helps inform the design of interventions targeting the gut microbiota to combat child undernutrition and promote healthy growth.