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].

The sleep hormone melatonin, and its precursor tryptophan, are both present in breast milk and appear to fluctuate in a cycle that may help babies fall asleep or wake up. One study found that melatonin levels were, on average, nearly five times higher in breast milk produced at night than in milk produced during the day [4].

Furthermore, levels of the hormones cortisone and cortisol are higher in morning breast milk than in milk produced in the afternoon, evening, and at night. Both cortisone and cortisol are involved in the body's stress response, and cortisol helps to energise our physiology when it is time to wake up [5].

However, we do not know whether these variations in composition have a direct impact on the baby's sleep-wake cycle; all we know is that babies ingest these compounds and absorb them. 

To find out more, see our article on breastfeeding and sleep.

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].