The move from milk to solid food is a big nutritional shift for babies. It may also be an important turning point for the gut microbiome.
A new study from researchers at the Technical University of Denmark and Rigshospitalet suggests that certain sugars in breast milk may help select which gut bacteria thrive during weaning, especially when babies are also beginning to eat plant-based solid foods. The study, published in Nature Communications, offers a closer look at how breast milk and first foods may work together as the infant gut microbiome matures.
The research focused on human milk oligosaccharides, or HMOs, a group of complex sugars found in breast milk. Babies do not digest HMOs the same way they digest nutrients used directly for energy. Instead, HMOs help feed certain gut bacteria.
That matters because the weaning period is not just about introducing new foods. It is also a time when the gut microbiome begins shifting toward a more mature community of bacteria.
“We have long known that breastfeeding is important for infants’ health. What is new is that we can now explain how the sugars in breast milk, HMOs, also help to select the bacterial communities associated with a healthy gut microbiota later in life. This underlines the importance of combining breastfeeding with solid baby food at this stage of child’s development,” said Maher Abou Hachem, professor at DTU Bioengineering and senior author of the study.
The researchers found that some bacteria appear to have a special advantage during this transition because they can use both HMOs from breast milk and fibers from solid foods. In other words, these bacteria may be well suited to bridge the gap between an infant’s milk-based diet and a more varied diet that includes plant foods.
To study that process, the researchers analyzed changes in gut bacteria and their genes during weaning. They also grew bacteria in the lab under conditions designed to resemble the human gut, allowing them to identify bacteria that could metabolize both breast milk sugars and dietary fibers.
The findings point to weaning as a potentially important window for gut microbiome development. But they should not be read as a source of pressure or guilt for parents. Infant feeding is shaped by health needs, milk supply, work schedules, medical care, family circumstances and many other realities. The study helps explain one biological mechanism, but it does not show that children who are not breastfed have harmed gut health or that breastfeeding during weaning prevents disease.
The research may be especially relevant in clinical settings where feeding support is already a priority, including care for premature or critically ill infants.
“The findings are important in daily clinical practice as an additional justification to the already strong emphasis on promoting the mother’s own milk production and breastfeeding when infants and young children are admitted to a neonatal intensive care unit due to preterm birth or critical illness,” said Lise Aunsholt, a consultant from the Department of Intensive Care for Newborns and Young Children at Rigshospitalet.
The study also points toward future nutrition research. If scientists can better understand which bacteria thrive during weaning and what they need to grow, that knowledge could eventually help guide infant nutrition products or targeted feeding strategies. But that work is still developing.
The study was funded by Independent Research Fund Denmark, Natural Sciences, through an FNU Project 2 grant to Maher Abou Hachem. One author was partially employed through a Novo Nordisk Foundation Interdisciplinary Synergy Programme grant.