Previous parts: Part 1 , Part 2
In previous posts, we discussed the discovery of Human Milk Oligosaccharides (HMOs) and their chemical and biological properties, from the late 19th century to the middle of the 20th century. By now we know that HMOs have clinical benefits to infants, providing protection from gastrointestinal infections.
HMOs may benefit humans directly, meaning, certain HMOs may have a direct effect on cellular signaling or indirectly, by interacting with the human microbiome.
We also highlighted the significant implications of HMOs on gut bacteria populations.
In this post, we’ll explore those hypotheses in depth. Specifically, we’ll explore the direct mode of clinical benefit of HMOs and the indirect mode, which involves the cultivation of desired populations of gut bacteria.
Let’s start by discussing what effects are exerted directly by HMOs on their consumers.
All the cells in our body (and in all living organisms) have proteins anchored to the cell membrane that faces the cell’s external environment. Those proteins are attached to the outward-facing part of the cell’s external membrane and serve many functions, from receiving signals from other cells, informing immune cells of potential infectious processes inside the cell, to nutrient uptake, and much more. Those proteins can be “hijacked” by various types of pathogenic bacteria and viruses, which use them as a gateway to penetrate the cell’s interior where they can exploit the cell’s resources and evade the immune system. Glycosylation is an important type of modification added to many types of proteins after their synthesis by the ribosomes. Glycosylation is a process in which oligosaccharides are covalently attached to one or more of a protein’s residues.
Glycosylation¹ serves several biological roles, like helping proteins fold to their three-dimensional shape and remain stable in the harsh extracellular environment, as the environment outside the cell can be harsh for proteins and lead to the destabilization of the protein structure².
Glycosylation also plays a major role in cellular signaling, adding another layer of complexity to signaling molecules. An example of how glycosylation can modify signaling molecules is the functional switch of Gonadotropins hormones. Gonadotropins are a family of hormones that are secreted by the anterior pituitary and act on the gonads (testes and ovaries) to increase the production of sex hormones and stimulate the production of either sperm or ova. Members of this family have several isoforms that differ only by their glycosylation profiles, meaning, that one isoform can have the exact amino acid composition as another different set of oligosaccharides attached to it, and on different positions on the protein. Intriguingly, different isoforms fulfill different biological roles and can interact differently with the same receptor³, and all of this diversity is encoded by the carbohydrates and not the protein itself!
Some enteric pathogens take advantage of glycosylated proteins on the epithelial lining of our gut. These bacteria exploit the attached oligosaccharides on cell surface receptors and use them as an anchor to penetrate cells or perform other malevolent downstream actions⁴, and in many cases help the bacteria identify and find the ‘right’ kind of cell that is most suitable for them.
Now, what does all this long discussion about glycosylation has to do with HMOs?
Leveraging their chemical similarity to the oligosaccharides on cell surface proteins, some HMOs serve as decoys to pathogenic bacteria, disabling their ability to bind to cell surface proteins.
For example, Studies in mice have revealed that a type of HMO called 2’FL attenuates the invasion of a bacteria named C. jejuni by 80%⁵. Certain HMOs were also found to directly inhibit the growth of the neonatal pathogen Streptococcus B⁶ and decrease the number of Streptococcus pneumonia cells in the lungs of animal subjects.
HMOs can fend off parasitic infections too. In one example, a protozoan named Entamoeba histolytica, which causes amoebic dysentery starts its attack on intestinal cells by attaching to one of the cell surface receptors. Parasites that cannot attach are excreted in feces and do not cause disease.
HMOs, being only minimally digested and absorbed in the small intestine, reach the colon at the same site as a budding E. histolytica infection. Interestingly, Some HMOs were found to significantly reduce the binding and cytotoxicity of E. histolytica in-vitro. Although this finding requires further validation it aligns with epidemiological findings that demonstrated that infants are significantly less likely to be infected with E. histolytica than formula-fed infants⁷.
The scientific literature has many more such examples that show how HMOs can directly thwart pathogens from infecting our cells. The interested reader is encouraged to read the review in the following article⁸
Moving on to discussing the indirect effects exerted by HMOs on their consumers, those can be mapped to several mechanisms, all have the common theme of interacting with the bacterial populations in the infant’s gut.
The human microbiome has been co-evolving with us for the past several millennia in a symbiotic relationship. Our gut provides the bacteria with a stable environment in which they can grow and thrive, and in return, those bacteria provide us with beneficial compounds that have positive effects on our health, protection from pathogens, and maintenance of the immune system. Our microbiome can also act as a double-edged sword; Emerging data have demonstrated that an aberrant gut microbiota composition is associated with several diseases, including metabolic disorders and inflammatory bowel disorder (IBD).
The composition of gut microbiota is affected by several factors, such as the host immune system and general health, and most importantly — the host’s diet. For example, human populations with a diet enriched in complex carbohydrates, such as the Hadza hunter-gatherers from Tanzania, have increased diversity of the gut microbiota⁹. On the other hand, long-term intake of a high-fat and high-sucrose diet can lead to the extinction of several taxa of the gut microbiota¹⁰
The infant microbiota develops over the first 3 years of her life. At this time, the digestive tract is colonized by bacteria, mainly from the Enterobacteriaceae family, especially from the Escherichia coli, Klebsiella, Enterobacter, Bacteroides, and Clostridia groups. In infants who are breastfed, bifidobacteria are the most common. There are fewer bacteria of the genera Clostridium and Enterococcus, while the least is Klebsiella and Enterobacter. In those infants that are fed artificially, the microflora resembles the digestive tract of adults and hence its composition is more complex than those infants who are breastfed.
The infant nutrition in her first years of life has been shown (and also discussed in previous chapters of this series) to directly cultivate the genus of bifidobacteria. By fermenting the oligosaccharides that are present in breastmilk, this genus of bacteria produces acetic acid, which increases the acidity in the intestine and as a result, inhibits the growth of harmful, pathogenic bacteria. In addition to acetic acid, bifidobacteria also produces butyric and propionic acids which also have an important function. Butyric acid is an important source of energy for colonocytes, a type of cell that lines the large intestines. Bifidobacteria also produce Short Chain Fatty Acids (SCFA) which are another type of rich source of energy for enterocytes and are key molecules for maintaining intestinal health.
Butyrate/butyric acid is an interesting and relatively well-researched example of the effect the synergy of HMOs and the bifidobacterial populations in the infant gut play in human health.
Butyrate, as mentioned, is metabolized very well by healthy colonocytes, and it also protects against colorectal cancer and inflammation, at least partly by inhibiting a class of enzymes that are involved in epigenetic control¹¹ altering the expression of many genes with diverse functions, some of which include cell proliferation, apoptosis, and differentiation. In contrast to colorectal cancer cells, butyrate does not inhibit cell growth when it is delivered to the healthy colonic epithelium or when it is added to noncancerous colonocytes in vitro. Instead, butyrate has either no significant effect or the opposite effect of stimulating cell growth under these conditions by acting as an energy substrate¹². This may be explained by the fact that butyrate is the preferred energy substrate for normal colonocytes, whereas cancerous colonocytes prefer to metabolize glucose since this molecule is much easily digestible in an oxygen-poor environment that is often encountered inside a tumor.
In that sense, butyrate gives a selective advantage, cultivating healthy colonocytes over cancerous ones.
Butyrate also acts as an immune system modulator. In one recent example, it was demonstrated that butyrate and other SCFAs were able to attenuate human eosinophils¹³(a type of cell that’s usually involved in allergic reactions and asthma) at several functional levels, including adhesion to the tissue lining (endothelium), migration around the body, and survival.
Indeed, there is a considerable body of evidence that SCFAs and butyrate, in particular, have not only a protective effect against various types of allergies¹⁴ but also a therapeutic benefit¹⁵ when used in combination with desensitization treatments for allergy¹⁶.
Taking into account older observations that showed that breastfed babies are less susceptible to different types of allergies¹⁷, it is safe to say that HMOs play a major role in preventing such conditions by modulating the immune system through the cultivation of certain types of gut bacteria.
The effects of HMOs are probably more far-reaching than what we currently know. Long-term studies and even multi-generational studies are being conducted to demonstrate the benefits of breastfeeding long into adulthood. Getting back to the question we started this series with — why would the human body produce molecules such as HMOs that its consumers can not digest — by now, the answer is hopefully clear, or at least clearer.
HMOs act in many different ways to benefit the human body, either directly by inhibiting the pathogenic capabilities of certain types of bacteria or, more intriguingly, indirectly by modulating the immune system and leveraging beneficial bacteria to produce a wide range of chemicals that in turn, act as a potent regulator for various important functions.
However, one must consider that the ability to fortify breastmilk with fermentable oligosaccharides has evolved millions of years ago, in an era when our ancestors faced an unimaginable risk from a host of pathogens — from parasitic worms to viruses and bacteria. Infant mortality was estimated to be anywhere from 25% to 50%¹⁸, thus, every small factor that could have improved the odds of survival to puberty was rapidly spread throughout the population.
Therefore, with access to modern medicine, the survival rate of infants who are breastfed compared to those who are not, has practically vanished.
That does not mean that breastfeeding is considered obsolete or redundant. Breastfeeding and HMOs have many other benefits, some of them were mentioned in our series as well.
We hope you enjoyed this series!
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