The many functions of human milk oligosaccharides: A Q&A with Prof. Ardythe Morrow

Human milk is the ‘gold standard’ of infant nutrition—and some scientists have set their sights on working towards that standard to improve the health of infants who are not breastfed. Among the many important components of human milk are human milk oligosaccharides (HMOs): complex carbohydrates that are 3-32 sugars in length. Over 200 different HMO molecules have been discovered, but a mother typically has between 12 and 20 in her milk. Some types of HMOs are affected by genetic polymorphisms – for example, only those who have the FUT2 (secretor) gene have breast milk containing HMOs called 2′-fucosylated (2’-FL) glycans.

ISAPP held a webinar in October, 2022 featuring Prof. Ardythe Morrow, University of Cincinnati College of Medicine, speaking about the latest research on HMOs and their health effects in both infants and adults.

HMOs as prebiotics

Prof. Morrow emphasized that research to date on HMOs shows they clearly fit the scientific consensus definition for prebiotics: a “substrate that is selectively utilized by host microorganisms conferring a health benefit”. HMOs are utilized by bacteria in the infant gut—mainly bifidobacteria, but also other genera (Yu, Chen & Newburg, 2013)—producing end-products that benefit infant health. B. longum subsp. infantis are the quintessential bacteria that grow on HMOs; pathogens do not typically grow on them.

Within the prebiotic category, HMOs are unique. Unlike other prebiotic substances they are structurally similar to gut oligosaccharides, which populate the surface of mucosal surfaces of the GI tract and are abundant in the mucin layer. They also can function via mechanisms that do not require utilization by gut microbes.

Beyond prebiotic function

Prof. Morrow emphasized that HMOs are multi-functional agents: in addition to their prebiotic functions, they have direct functions in the infant gut that are not mediated by microbes. First, individual HMOs have been shown to bind pathogens and inhibit infections and bind to immune cells to optimize their function (Triantis, Bode & van Neerven, 2018). Further, they can enhance neurodevelopment and brain function (Furness, Kunze & Clerc 1999; Sharon et al, 2016). The latter is a more recent domain of research, but so far it is known that basic neurodevelopmental processes are modulated in animals that are germ-free or have a depleted gut microbiota.

Certain HMOs (notably 2’-FL) can be produced synthetically and are being tested in infant formulas, and more recently for healthy adults (Elison et al., 2016). Prof. Morrow noted HMOs also have potential as novel therapeutics for various indications, such as inflammatory bowel disease (IBD). Determining which specific HMOs are most effective in these outcomes, and the dose needed, is an active area of research.

The webinar participants generated some interesting questions, some of which Prof. Morrow answers below.

Are 2’FL and LNnT (Lacto-N-neotetraose) found in cow’s milk?

2′-FL is not found in cow’s milk. Other oligosaccharides, especially sialyl oligosaccharides, are present but generally at very low levels.

How similar to HMOs are the glycosylation patterns on gut mucin?

Mucin glycosylation is not identical to human milk. But there are structural motifs that recur in both milk and gut mucin.

Do the more abundant HMOs have more potential for health benefit, compared with those at lower abundances in human milk?

We do not know that more abundance means more functionality or importance. But it is a reasonable place to start with the research. Also, several of the most abundant HMOs are trisaccharides (2’FL, 3FL, 3′-SL, and 6′-SL), and these are the most manageable to synthesize and start with.

For non-secretors, HMO complexity in milk is around 30% lower than for secretors. Does this factor affect the beneficial functions of non-secretor HMOs?

Having lower HMO content might be an issue in some circumstances. But we cannot say that it is a general problem. Furthermore, if non-secretors have more sialyloligosaccharides and 3-FL instead of 2′-FL, for example, perhaps this helps protect against viruses that bind to sialic acid epitopes (for example, influenza). Or perhaps this helps with increasing sialic acid to the brain (see Mudd et al., 2017). So, my argument is that at this point in our knowledge, we should avoid any idea of “superior” or “inferior” milk for the general healthy public. More likely, there are situation-specific benefits or disadvantages for different milk oligosaccharide phenotypes.

What do you think is more important for infant formula, more HMO complexity or more structure-function relations?

A set of HMOs for normal infant nutrition will be important, and these include fucosyllactoses, sialyllactoses, and neutral oligosaccharide with neither sialic acid nor fucose. Structure-function orientation is important to guide use in special populations with specific health needs.

Long term, will HMOs replace FOS and GOS in infant formulas?

All of the efforts in making infant formula have the goal of doing the best possible job of mimicking the physiological function of breastmilk, but cost and function are also relevant factors to consider in this process. It’s important that babies get some form of prebiotic. GOS is structurally more similar to HMOs, but it’s not enough on its own. Ideally, we’d hope for a rational mixture of different oligosaccharides backed by research confirming their combined functions.

Can we really replicate HMOs with synthetic formula, given the large number of diverse HMOs present in human milk?

I do not foresee ever achieving full replication, no. But getting closer to mother’s milk, yes, over time.

How is the dosing of HMOs in clinical trials for adults being determined? Should it be based on human milk concentration?

Elison et al. published a dosing study based on tolerance and shift of microbiota. A dosing study is now underway in Cincinnati, too.

Since it is fairly difficult to manufacture HMOs, do you think they provide sufficient advantages compared to GOS to justify their use as prebiotics in adults?

We do not yet know whether HMOs might have enough advantage over GOS in some situations, or whether prebiotic combinations might be best. This is research in progress! The reason for testing 2′-FL in IBD is because of the structure-function evidence. IBD is increased in non-secretors, and is associated with dysbiosis, inflammation, and so on. We will learn from the ongoing research.

Do you think adults will differ in response to HMOs therapeutically, possibly based on genetic differences?

I don’t yet have data on this, but have a study ongoing that I hope will be able to address this very question.





Human milk oligosaccharides as prebiotics to be discussed in upcoming ISAPP webinar

Human milk oligosaccharides (HMOs), non-digestible carbohydrates found in breast milk, have beneficial effects on infant health by acting as substrates for immune-modulating bacteria in the intestinal tract. The past several years have brought an increase in our understanding of how HMOs confer health benefits, prompting the inclusion of synthetic HMOs in some infant formula products.

These topics will be covered in an upcoming webinar, “Human milk oligosaccharides: Prebiotics in a class of their own?”, with a presentation by Ardythe Morrow PhD, Professor of Pediatrics, Cincinnati Children’s Hospital Medical Center, University of Cincinnati College of Medicine. The webinar will provide an overview of what HMOs are, how they are breaking new ground with the types of health benefits they can provide to infants and the recent technological innovations that will facilitate their translation into new infant formulas.

Dr. Karen Scott, Rowett Institute, University of Aberdeen, and Dr. Margriet Schoterman, FrieslandCampina, will host the webinar. All are welcome to join this webinar, scheduled for Wednesday, Oct 19th, 2022, from 10-11 AM Eastern Daylight Time. Registration is free of charge. Spaces may be limited.

Register here.

A pediatrician’s perspective on c-section births and the gut microbiome

By Prof. Hania Szajewska, MD, Medical University of Warsaw, Poland and Kristina Campbell, MSc, ISAPP Consulting Communications Director

The decision to have a Cesarean section (C-section) should always depend on whether this is the best choice for the mother and baby, and it is never made by pediatricians. However, pediatricians are often asked about the consequences of C-section delivery for a child later in life and whether potential C-section-related harms may be reduced.

The data show that delivery by C-section is now more common than ever globally. The World Health Organization estimates the  C-section rate is around 21% of all births, and predicted to continue increasing. Although C-section rates are increasing both in developed and developing countries, Korea, Chile, Mexico, and Turkey have the highest rates in the world, with C-sections constituting 45% to 53% of all births. C-sections outnumber vaginal births in countries that include Dominican Republic, Brazil, Cyprus, Egypt, and Turkey.

Cesarean delivery is a medical procedure that can of course save an infant (or a mother) in a moment of danger, making birth less risky overall. But analyses have shown not all C-sections are initiated for safety reasons—some are driven by convenience and other non-medical factors. In areas with the highest C-section rates, only around half of the time are they required for life-saving reasons. Although the rate of medically necessary C-sections globally is difficult to establish, the WHO estimates it is between 10-15% of all births.

Non-essential C-sections would be perfectly reasonable if the health risks later in life were negligible. But are they? Scientific work in the past decade has shown that, in fact, there may be downsides to being born by C-section—and these health risks may manifest later in a child’s life.

By now, many observational studies have associated Cesarean births with an increased risk of various chronic health conditions that appear long after birth. C-section is associated with a higher risk of asthma and allergy, as well as obesity and type 2 diabetes. A systematic review and meta-analysis (incorporating 61 studies, which together included more than 20 million deliveries) also linked C-sections with autism spectrum disorders and attention deficit hyperactivity disorder (ADHD). Type 1 diabetes is also more prevalent in children born by c-section.

Since association is not the same as causation, scientists have looked at possible biological correlates of C-section and how they could be tied to future health problems. A leading hypothesis is that C-section deliveries cause health problems by disrupting the infant’s normal gut microbiota (i.e. the collection of microorganisms in specific ‘habitats’ on the infant’s body, such as the gut) within a critical time window for immune system development.

An altered microbiota in C-section births

One of the main clues about whether C-section births affect health via the microbiota is the consistent observation that infants born by C-section have a different collection of microorganisms in their digestive tracts and elsewhere on their bodies immediately after birth, compared with vaginally-born controls. Newborns delivered by C-section tend to harbor in their guts disease-causing microbes commonly found in hospitals (e.g. Enterococcus and Klebsiella), and lack strains of gut bacteria found in healthy children (e.g. Bacteroides species). Because it is known that gut microbiota are in close communication with the immune system, this difference in birth microbes may set the immune system up for later dysfunction.

However, an important confounding factor exists. Antibiotic administration is a recommended medical practice for C-section births in order to prevent infections. Antibiotics are potent disruptors of microbial communities – in this case the mother’s, or perhaps the infant’s if antibiotics are administered prior to umbilical cord clamping. It is not yet clear whether the timing of antibiotic administration can prevent such disruptions. (See conflicting evidence here and here; also see here.).

Gut microbiota disruption is associated with C-sections, but since C-section and antibiotics nearly always go together (with potential exposure of the infant to these drugs), it is not clear to what extent C-section and/or antibiotic treatments drive increased risk of chronic disease later in life. Antibiotic treatments within the first 2 years of life are independently associated with an increased risk of several conditions: childhood-onset asthma, allergic rhinitis, atopic dermatitis, celiac disease, overweight / obesity, and ADHD.

Options for microbiota ‘restoration’

If mechanistic studies continue to support the idea that the C-section-disrupted gut microbiota is the trigger for chronic diseases later in life, strategies could be proposed for ‘restoring’ or normalizing the infant gut microbiota after such births. Already some microbiota modifying interventions have been evaluated:

  • Probiotics: Undesired changes in microbiota composition and function caused by antibiotic treatments and/or caesarean birth may be addressed by probiotics—i.e. “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host”. In one study, a mixture of several probiotic strains along with at least partial breastfeeding shifted the infant gut microbiota toward a more favorable profile. Intriguingly, probiotics (of various strains) prevent IgE-associated allergy until age 5 years, specifically in cesarean-delivered children but not in all children.
  • Synbiotics: A study showed supplementation with scGOS/lcFOS and B. breve M-16V appeared to compensate for delayed Bifidobacterium colonization in the guts of C-section delivered infants.
  • Maternal vaginal microbial transfer (otherwise known as ‘vaginal seeding’ or ‘microbial bath’: This is a procedure in which infants born by C-section, immediately after birth, are swabbed with gauze that contains microbes from the mother’s vaginal tract. After the media attention given to early studies, there is increased demand from parents for this procedure. Although some studies have found it effective for normalizing the infant gut microbiota, safety is not completely established and it is too early for routine use of this procedure. Parents wishing to try this approach are advised to participate in a study as part of an institutional review board-approved research protocol.
  • Maternal fecal microbiota transplantation: This procedure involves fecal microbes from the mother, orally administered to C-section infants after birth. A proof-of-concept study showed that after this intervention the gut microbiota of C-section-born infants looked more similar to that of vaginally born infants. But for this procedure, as above, the risk of transmitting harmful microbes is a concern, making it too early to recommend the procedure unless it is part of an institutional review board-approved research protocol.
  • Breastfeeding: Breastfeeding is the gold standard for infant nutrition, and breast milk contains live microorganisms as well as other components that interact with the gut microbiota. Exclusive breastfeeding for about 6 months of C-section infants helps the gut microbiota shift toward a profile seen in vaginally born infants.

So far, probiotics, synbiotics, and microbiota ‘restoration’ are not sufficiently reliable solutions for correcting the microbiota disruptions that accompany C-section births. Further studies are needed to develop these approaches.

A leading strategy

At present, breastfeeding is the main strategy for supporting the infant gut microbiota after C-section for the greatest chance of avoiding negative health consequences. Breastfeeding has multiple benefits, but may be of increased importance after C-section birth. Mothers should be supported after giving birth by C-section to breastfeed the infant during this critical period of early life and immune system development.