We have all heard that ‘breast is best’, for a variety of reasons. It provides infants with an ideal source of nutrition, and breast-fed babies have fewer cases of illness than their formula-fed counterparts. Benefits extend to the mother as well; breast feeding helps reduce risk of post partum depression, aids in uterine contraction, helps mothers to return to pre-pregnancy weight, and promotes bonding with baby. As the research evolves and the benefits of breast milk keep adding up, there are even more remarkable things researchers are discovering about the white stuff. Here are five things you may not know about breast milk.
1. Breast milk may influence infant behaviour
Cortisol, a stress hormone that has been found in breast milk, can act on receptors in infants and trigger hormonal signaling cascades. In turn, these cascades have direct and indirect effects on behavior.
A study of 253 women found that mothers with higher blood cortisol concentrations, which reflects the levels of cortisol in breast milk, described their infants as more fearful than mothers with lower levels of cortisol. This effect was not observed in those mothers who formula-fed their babies. This suggests that the fearful behaviour is linked to ingestion of this stress hormone through milk, rather than maternal cortisol levels affecting infant care, (which could impact baby fearfulness), or the how mothers might report the fearfulness of their baby.
The same research team later assessed breast milk cortisol levels and found a positive correlation with the degree of ‘negative affectivity’ of the infant-the tendency towards sadness, fear, and discomfort. Interestingly, this association was only observed in breast-fed daughters, not sons.
Although these studies are certainly intriguing, the relationships between breast milk hormones and infant personality are still in their infancy. Given the complex nature of personality, which involves a wide variety of both genetic and environmental components, a great deal more research is needed in this emerging field of study before any conclusions can be drawn. So breast-feeding mothers, relax. You've got enough to think about as it is.
2. Breast milk contains bacteria-and lots of it.
But don’t worry, it’s the good kind. Studies suggest that a single millilitre of breast milk can contain 1000-10 000 colony forming units of bacteria, encompassing over 700 species.
Researchers believe these bacteria may contribute to the development of the infant's gut microbiome (all the commensal bacteria in the gut). When the species of bacteria in the breast milk of 16 women was analyzed, researchers discovered that half of the bacteria in each sample contained the same 9 species-representing a ’core’ microbiome within breast milk- while the remaining half varied from woman to woman.
Interestingly, the weight status of the mother (normal versus obese), and the mode of delivery (vaginal versus planned caesarean) influences the composition of the breast milk microbiome.
It’s hypothesized that specialized immune cells, called dendritic cells, transport bacteria from the mother’s gut to the mammary glands, where they are then passed on to the infant via breast milk.
3. Breast milk is full of 'good bacteria' food
Although it was originally believed that the guts of developing fetuses were sterile, many researchers now believe that the gut microbiome is first seeded in the womb. The microbiome, however, is anything but stable. It adapts to a variety of environmental factors, including diet.
Research suggests that breast milk can influence the infant microbiome. The third most dominant nutrient in human breast milk, after lactose and lipids, are human milk oligosaccharides. These oligosaccharides are the sugars in breast milk that are a food source for bacteria, but can't be digested by babies. Commensal strains of ‘good’ bacteria, such as Bacteroides and Bifidobacterium, are happy consumers of these sugars. Pathogenic strains-like Clostridium, E. coli, Enterococcus, and Streptococcus- are either poorly adapted to consuming these breast milk sugars, or unable to metabolize them altogether.
This is a positive feedback loop- the more ‘good’ bacteria that establish in the infant gut, the more difficult it is for pathogenic strains to compete for limited space and resources.
4. Breast milk contains decoys to fool bad bacteria
There are a whole slew of ways in which breast milk components protect infants from bad bacteria. Breast milk contains a variety of immune cells: neutrophils, macrophages, and white blood cells that can target and destroy bad bacteria. Or take lactoferrin, a protein that binds to iron, which makes this essential nutrient unavailable to the many kinds of pathogenic bacteria that thrive on iron.
A more recently discovered line of defence in breast milk are human milk oligosaccharides. Like using a key in a locked door, many harmful gut bacteria must first attach to receptors on the surface of the epithelial cells that line the gut in order to invade them. Among these pathogens include a strain of E. coli that is responsible for severe diarrheal diseases in infants. Studies on mice have determined that breast milk oligosaccharides serve as a decoy, as they possess the identical receptors that the pathogen uses to invade cells. These harmless sugar-bacteria complexes are then flushed from the intestine.
5. Caloric content of breast milk depends on infant sex. Maybe.
Is all breast milk created equal? Not so, say researchers at Harvard and Boston University. In a2010 study, the energy content of breast milk in 25 healthy mothers with babies aged 2-5 months was assessed. They found that the breast milk of mothers with baby boys had 25% more calories than the milk produced by mothers with baby girls. One possible explanation is that baby boys have higher energy requirements than baby girls, and their increased demand drives milk production.
However, it might be more complicated than that. A more recent study of 83 women similarly revealed differential breast milk quality for sons and daughters. In Northern Kenya, the sex of the infant that received the more fattier, richer milk depended upon the economic status of the mother. Wealthy mothers produced more calorically dense milk for their sons over daughters, while breast milk from poorer mothers was richer for daughters over sons.
The authors state that according to evolutionary theory, mothers should invest more in their sons than daughters, since sons can ultimately produce more offspring than daughters. In times when resources are scarcer, however, it may be prudent to for mothers to invest in the sex that will produce fewer offspring. In this case, the authors suggest that daughters have a greater chance of increasing their status through marriage than sons do, making daughters a more wise investment when times are rough. The biological mechanism driving this effect, however, remains unknown.
Author’s note: It is my intention to highlight some of the amazing features of breast milk. While breast milk is amazing stuff, not all women are able to breastfeed for a variety of reasons. Formula is a lifesaver in these cases, and its use as a supplement to, or in lieu of breastfeeding, shouldn’t be maligned or judged.