5 mysteries of breastfeeding

It is well established that human milk (as well as colostrum, the first milk produced after birth) contains protective factors such as antibodies that help the baby fight infections.

But that's not all! Research suggests that the immune components of breast milk may intensify when babies need them most. One study showed that when both mothers and babies had a cold, the levels of white blood cells in the milk were multiplied by 64! Even when only the babies were ill, white blood cell levels were still multiplied by 13[1].

Research suggests that the immune components present in breast milk may intensify when babies need them most.

A second study revealed that lactoferrin — an immune molecule that performs various protective functions, such as perforating the walls of harmful bacteria — was elevated in the weeks before and after an infant's illness[2].

The most likely explanation is that the child's saliva, which travels back into the mammary ducts during feeding, transmits a report on the child's health status. The pathogen is therefore transferred via the baby's saliva, and breast milk adapts its composition accordingly[3].

L'hormone du sommeil, la mélatonine, et son précurseur, le tryptophane, sont tous deux présents dans le lait maternel et semblent fluctuer selon un cycle qui pourrait aider les bébés à s'endormir ou à se réveiller. Une étude a révélé que les niveaux de mélatonine étaient, en moyenne, près de cinq fois plus élevés dans le lait maternel produit la nuit que dans celui produit le jour [4].

Par ailleurs, les niveaux des hormones cortisone et cortisol sont plus élevés dans le lait maternel du matin que dans le lait produit l'après-midi, le soir et la nuit. La cortisone et le cortisol sont tous deux impliqués dans la réponse au stress de l'organisme, et le cortisol sert à donner un coup de fouet à notre physiologie lorsqu'il est temps de se réveiller [5].

En revanche, on ne sait pas si ces variations de composition ont un impact direct sur le cycle veille-sommeil du bébé, tout ce que nous savons c’est que les bébés vont ingérer ces composés et les assimiler. 

Pour en savoir plus, allez voir notre article sur allaitement et sommeil.

Breast milk is not only food for babies — it is also nourishment for the billions of bacteria that establish themselves in their digestive system, a community known as the gut microbiome. Recent research suggests that breast milk may promote the growth of bacteria that help keep babies healthy [6].

Breast milk contains nearly 130 different oligosaccharides. These are complex chains of sugars found in breast milk. These complex sugars support the types of gut bacteria capable of digesting compounds into short-chain fatty acids, which babies need in order to develop [7].

No two breast milks are alike. They can vary in terms of proteins, fats, sugars, hormones and other components. But breast milk does not only differ from one mother to another. It can also vary when the same mother breastfeeds different babies, and throughout the child's development.

It can also vary depending on the sex of the baby. Working with monkeys, a researcher found that mothers produced more milk for their female offspring, but richer, fattier milk for their male offspring [8]. She found similar sex-based differences when studying the lactation records of more than one million cows [9].

If these differences are found in human milk, understanding them could help optimise formula milk or donor milk for babies who do not have access to their own mother's milk, explains Hinde. But scientists are only just beginning to characterise these variations and the factors that determine them.

In 2007, scientists discovered an unexpected ingredient in human breast milk: stem cells. These cells retain a flexibility that most adult cells have lost and can form a wide variety of tissues. They pass through the walls of the infants' stomachs, circulate through the body and settle in the developing tissues throughout. In a study on baby mice, researchers observed that as the babies grew, the mother's cells were still present and had transformed into mature tissues alongside the babies' own cells [10].