Irish researchers discover that the transition from milk to solids not only changes babies’ diets but also significantly reshapes their gut microbiome, aiding in the digestion of more complex foods and supporting early development.
A new study from Irish researchers suggests that weaning does more than change what babies eat: it appears to reshape the gut ecosystem in ways that help infants digest more complex foods. Published in Scientific Reports in late September, the work followed mother-and-infant pairs in Ireland and found that the move away from an all-milk diet coincided with major shifts in the infant microbiome, the community of microbes living in the intestines.
The research drew on 125 mothers and babies enrolled in the Cork Nutrition and Development Maternal-Infant Cohort Study. Scientists analysed 250 stool samples taken before and after solid foods were introduced, comparing an early milk-only stage, at an average age of about 87 days, with a later stage after complementary feeding began, at around 222 days. On average, babies started solids at about 147 days, with formula-fed infants beginning roughly 17 days earlier than breastfed babies.
According to the findings, breast-feeding played an important role in shaping microbial development, with longer exposure to breast milk linked to greater diversity in the gut community. The shift to solids also appeared to alter the function of the microbiome, helping babies move from digesting the simple sugars in milk to breaking down more complex carbohydrates found in solid food. The researchers said this transition was accompanied by a rise in bacteria better suited to processing starch and other fibres, alongside an increase in the production of vitamins and a noticeable rise in bowel movements once solids were introduced.
The results fit with earlier work showing that weaning changes the infant gut in a predictable way. A 2017 Nature study found that adding cereals such as wheat, sorghum, rice and oats increased bacteria linked with fibre breakdown while reducing Enterobacteriaceae and boosting acetate, a short-chain fatty acid associated with healthy digestion. More recent research indexed by PubMed reported similar changes, including higher levels of Ruminococcus-related groups and lower levels of Bifidobacterium, Lactobacillus and Escherichia after solids were introduced. Other studies have suggested that both human milk oligosaccharides, the special sugars found in breast milk, and dietary fibre help select for microbes that support microbiome maturation during the weaning period.
For parents and caregivers, the broader message is that the first foods offered in infancy are not just a matter of taste or timing. They help guide the gut’s development at a critical stage, alongside milk feeding and the gradual move towards family foods. Researchers say the process is a normal and important part of early life, as the digestive system adapts to a more varied diet.
Disclaimer: This content is for informational purposes only and is not intended to be a substitute for professional medical judgment, advice, diagnosis, or treatment.
