Breast milk biology: Hundreds of different sugar molecules boost baby immunity

Breast milk's unique composition is key to boost babies' microbiota and immune system, scientists have said. The 200 different sugar molecules it contains sets it apart from other mammals' milk, like from cows or mice.

Many previous studies have investigated how breast milk benefits young children. Their results often divide the scientific community. Indeed, while breast-feeding seems to reduce infant mortality and decrease newborns' risk of gut and airway infections, there is only limited evidence regarding longer-term effects. In fact, many children appear to be in good health, even when they have not been exposed to breast milk.

The latest study on the topic, published in Trends in Biochemical Sciences, reviews what is known so far about human breast milk's properties and how it impacts on babies's health in the short term. In particular, it attempts to make sense of previous findings regarding immunity and gut bacteria.

Colonisation of the gut and immunity

Among the 200 sugars molecules present in the milk, not all of them are intended to feed the baby. Rather, their role is to stimulate the growth of gut bacteria, which play a crucial role in digestive health, protection against allergies, general immunity or even in influencing body weight.

When babies are born, they go from having no bacteria in their guts to having billions, in just a week. The sugar molecules are used to feed these bacteria and to culture specific species crucial to the child's good health.

"The first impact breast milk is favouring is the colonisation of the gut by specific bacterial groups that can digest these sugar molecules," says review co-author Thierry Hennet. "Infants don't have the machinery to digest these sugars, so they are literally for the bacteria − it's like a seeding ground, and breast milk is the fertiliser."

The second key benefit of breast milk identified by the scientists concerns the baby's immune system. It has been argued that the milk lays the foundation for a solid immunity because it is full of antibodies that slow the growth of harmful bacteria, and coordinate white blood cell activity.

Clear recommendations

Interestingly, the composition of breast milk also evolves as the infant grows. When, after a month, he or she develops an adaptive immune system of his or her own, the milk was shown to contain less sugars and antibodies, and more fats and varied nutrients to accompany the infant during his growth. But when does this evolution of the breast milk cease to be beneficial for the child in question?

"On the one hand, breast milk is the product of millions of years of evolution and certainly possesses the optimal nutrients for a newborn, but the question is how long does the newborn really need this supply?" Hennet explains.

To clarify this, he points out it is necessary to come up with more research about the specific role of each of the hundreds of molecules contained in the milk. With the progress of DNA sequencing, scientists will probably be able to solve these interrogations in the coming years.