Although breast milk is a baby’s first food, it also contains certain compounds – especially some fibres and oligosaccharides –that are not meant for the infant but for an army ofbacterial diners that start colonising the child’s gut at birth.

“Breast milk’s first impact is to favour the colonisation of the gut by specific bacterial groups that can digest these sugar molecules,” said Thierry Hennet, a researcher at the Institute of Physiology at the University of Zurich, in a telephone interview with Gut Microbiota for Health. “Infants do not have the machinery to digest these sugars so they are literally there for the gut bacteria. It’s like a seeding ground and breast milk is the fertilizer,” he adds. Bacteria therefore use these molecules as nutrients, with a prebiotic effect.

This is one of the main conclusions of a comprehensive review carried out by Hennet and his colleagues of more than 70 scientific papers, published in Trends in Biochemical Sciences.

The authors also highlighted another important role for human breast milk: laying the foundations for the infant’s immune system (which is also related to gut microbiota). The very first milk, called colostrum, which mothers typically produce just before and after the baby’s birth, is extremely rich in maternal antibodies, proteins and beneficial carbohydrates.

“It protects the infant’s gastrointestinal tract and, in the case of premature babies, it is a lifesaver, as it protects them from having a very severe disease called necrotising enterocolitis,” said Hennet. The antibodies and molecules slow down the growth of harmful bacteria and coordinate the activity of white blood cells, or leucocytes–the cells involved in protecting the body against infectious diseases and invaders.

After a month or so of being exposed to the outside world, children start to develop their own immune system and that is when breast milk changes its composition. The levels of maternal antibodies drop by 90% and the diversity of milk sugars (which come to more than 200 different sugar molecules) also decreases. Instead, the milk starts increasing the amount of lipids and other nutrients that support infant growth.

As the researchers highlight in the paper, human breast milk is the most complex one of all mammals’ milks and its chemistry and the role of some of its components remain a mystery for science, which we are only beginning to understand. The amount of sugar molecules alone, as mentioned above, is huge in comparison with the average 30 to 50 found in cows or mice. And it is perfectly designed for the baby’s first need–the development of his or her brain.

The World Health Organization (WHO) recommends exclusive breastfeeding until children are at least 6 months old.




Hennet, T and Borsig, L: Breastfed at Tiffany’s. Trends in Biochemical Sciences. 2016 March 17. DOI: