‘Breast is best’ is the rallying cry of many who want to encourage mums to breastfeed their children, though some think the slogan should be banished. To those who wish to do away with the slogan, it’s not a matter of what is ‘best’ but a matter of ‘biologically normal’. I’ll admit to being rather nonplussed about slogans, but I am rather adamant that one should put trust in basic biology when possible.
Over the years, studies have shown correlations suggesting many benefits of breastfeeding, including increased IQ, decreased obesity, and a decrease in several pediatric ailments such as gastritis and ear infections in the offspring (I think those last two are particularly interesting). While these studies are interesting, they are just correlations, and I hate to put too much weight on correlative studies (though they can be fun to look at).
Biology is marvelously complex. As a result, studying it and then mimicking it is time consuming and tricky. I think breastfeeding is a wonderful example of a fascinating bit of biology that shows us how tricky it is to properly and fully study complex biology, and also shows us how humans have evolved to thrive on the ‘evolutionary norm’ (breastfeeding).
Breast milk is interesting stuff- though to be honest, some species have much more interesting milk than humans, with some species producing milk that changes in composition from day to day (and even different compositions from different nipples to feed different offspring!). While breast milk provides sustenance for an infant, it also helps develop the immune and metabolic systems of the offspring. The composition of breast milk can pass along information from mother to child, relaying information about the relative abundance or paucity of the environment in which the offspring will develop. This transfer of information, and the relative role of this communication in growth and development, is a fascinating area of research.
If people think about breast milk in the context of immunity, they generally think of the active transfer of antibodies from mother to child. The earliest milk produced by a mother, colostrum, is particularly rich in antibodies, and imparts great benefits to an infant. Even mothers that don’t wish to breastfeed extensively are encouraged to at least nurse in the early days of a neonate’s life so as to transfer these important antibodies. But is that it on the breast milk/immune system link?
In my last few posts I’ve touched on the importance of the gut in a number of immune mediated responses. The gut is actually rather rich with immune tissue, and fosters a number of bacteria (hopefully helpful and health-promoting ones) that help keep out nasty pathogenic invaders (and also helps keep the gut healthy in other ways, such as producing short chain fatty acids to nourish cells and producing Vitamin K). In my last few posts, I’ve talked about the appendix, and how it appears to have evolved as a safe house for our native microbiota in times of duress. But where do these bacteria come from?
Before birth, the intestinal tract of the fetus is thin and immature- lacking any significant lymphoid tissue. The intestine of a newborn develops substantial lymphoid tissue, which indicates the active bacterial colonization of the gut, altering its structure and function. There appear to be three phases of normal gut colonization in an infant starting with whatever flora are picked up during a normal vaginal delivery. The process of breast-feeding ushers in a second phase of colonization, with the third and final stage occurring at weaning. During colonization, it appears that infants generally inherit their mother’s microflora , and if all goes well- this is what will populate your appendix!
Any one with an eye to evolution might suspect that breast milk is exquisitely effective at producing healthy offspring- we’ve been making this stuff for hundreds of thousands of years… it is nuanced in ways we’ve yet to even think about. One thing we are starting to understand is that the composition of breast milk fosters a healthy microbiome in infants. In addition to the protein, fats, and carbohydrates that make up breast milk and nourish the child, there are a number of products present in breast milk that can’t be actively digested by the infant, but that are excellent fodder for a blossoming gut microbiome. These compounds are known as human milk oligosaccharides (HMOs- not the annoying health insurance type), and we’re nowhere close to identifying all the different types, though we know there are more than 100 different molecular structures. These molecules are resistant to gastric acid and aren’t absorbed or metabolized by the baby, and instead make it to the large intestine where they can be fermented by intestinal microflora.
The gut microbiome of infants that are breast fed is quite a bit different from that of formula-fed babies. A number of studies have looked into this, and I don’t want to go into details of different studies, but there appear to be significant differences in the type and number of bacteria in the gut of these populations. Depending on the study, findings showed babies that had been breast-fed tended to have the same amount or more ‘good’ bacteria (Lactobacillus sp. and Bifidobacterium sp.) and less ‘bad’ bacteria (E. coli and C. diff (remember that guy from my last post?)). Breastfeeding probably helps set the microbiome up for success in a number of ways, including the direct transfer of bacteria from mother to child and also the transfer of maternally produced prebiotics that encourage appropriate bacterial growth. There’s also evidence that there is a significant difference in the pH of the large intestines of breast-fed vs formula-fed infants (an acidic pH in breast-fed infants vs a fairly neutral pH in those fed formula). I’m not sure how much of this is a result of appropriate bacterial growth, and how much that acidic pH then encourages more appropriate bacterial growth, but it’s fun to think about (it’s probably a nice little positive feedback loop).
Setting the neonatal microbiome up for success is just one of the many roles that breast milk has evolved to excel at. As mentioned above, breast milk is incredibly complex, and we are only starting to understand the many ways in which is sets an infant up to thrive (and not just survive). In the ideal world, all mothers would breast feed exclusively for 6 months, and then continue to breast feed while foods are introduced, but unfortunately, reality is not this easy. For some, formula is a necessity. So how can we take what we know about breast milk and the infant microbiome and put this into practice?
In addition to the digestible micronutrients that are important for the proper growth of an infant, breast milk contains a vast array of non-digestible (but fermentable!) oligosaccharides that promote the development of a robust commensal microbiome. Knowing this, it seems appropriate to supplement formula with prebiotics that might equally foster the growth of appropriate microflora. There are a number of well-known prebiotics (perhaps the best known being inulin), but as we have yet to identify all the complex molecules in breast milk, we are unlikely to recreate the full spectrum of prebiotics offered by the ‘evolutionarily appropriate’ diet of an infant. Nonetheless, it appears that formula fortified (gosh I hate that word- if something needs to be fortified, I tend to think there’s a better, naturally more nutritious, option you should be consuming instead) with prebiotics might improve the gut microbiome in comparison to that of a regular formula-fed baby. A number of researcher groups have studied the effects of supplementation, with mixed results. Some have found positive effects of supplementation  (I’ll admit I’ve just read the abstract as I can’t read Chinese!) and some showing no difference . This is not surprising, considering the complex nature of maternally produced prebiotics, but it does suggest that this is an avenue of investigation that should be further explored.
We are a long way off from making a formula that adequately and completely replaces breast milk. Like the gut microbiome (and growing infant!) that it nourishes, breast milk is extraordinarily complex, and we are only just exploring the tip of the iceberg of this massive subject. Nonetheless, realizing that breast milk helps promote a healthy microbiome (that you will hopefully keep for life!), which in turn helps develop a robust and appropriately directed immune system, should encourage researchers to further pursue this avenue of exploration. The more we learn about modern diseases, the more it appears that the integrity of the gut, and the functionality of the associated immune tissue, should be a central point of exploration for understanding disease.
1. Kulagina, E.V., A.N. Shkoporov, L.I. Kafarskaia, E.V. Khokhlova, N.N. Volodin, E.E. Donskikh, O.V. Korshunova, and B.A. Efimov, Molecular genetic study of species and strain variability in bifidobacteria population in intestinal microflora of breast-fed infants and their mothers. Bull Exp Biol Med, 2010. 150(1): p. 61-4.
2. Cai, J.W., Y.D. Lu, and X.M. Ben, [Effects of infant formula containing galacto-oligosaccharides on the intestinal microflora in infants]. Zhongguo Dang Dai Er Ke Za Zhi, 2008. 10(5): p. 629-32.
3. Xia, Q., T. Williams, D. Hustead, P. Price, M. Morrison, and Z. Yu, Quantitative Analysis of Intestinal Bacterial Populations From Term Infants Fed Formula Supplemented With Fructo-Oligosaccharides. J Pediatr Gastroenterol Nutr, 2012.