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.

New publication co-authored by ISAPP board members gives an overview of probiotics, prebiotics, synbiotics, and postbiotics in infant formula

For meeting the nutritional needs of infants and supporting early development, human milk is the ideal food—and this is reflected in breastfeeding guidelines around the world, including the World Health Organization’s recommendation that babies receive human milk exclusively for the first six months of life and that breastfeeding be continued, along with complementary foods, up to two years of age or beyond. In certain cases, however, breastfeeding is challenging or may not even be an option. Then, parents rely on alternatives for feeding their infants.

A group of scientists, including three ISAPP board members, recently co-authored an article in the journal Nutrients entitled Infant Formula Supplemented with Biotics: Current Knowledge and Future Perspectives. In the review, they aimed to highlight the new technologies and ingredients that are allowing infant formula to better approximate the composition of human milk. They focused on four types of ingredients: probiotics, prebiotics, synbiotics, and postbiotics.

Co-author Gabriel Vinderola, Associate Professor of Microbiology at the Faculty of Chemical Engineering from the National University of Litoral and Principal Researcher from CONICET at Dairy Products Institute (CONICET-UNL) in Santa Fe, Argentina says, “Modern technologies have allowed the production of specific microbes, subtrates selectively used by the host microbes, and even non-viable microbes and their metabolites and cell fragments—for which scientific evidence is available on their effects on infant health, when administered in adequate amounts. Thus, this current set of gut modulators can be delivered by infant formula when breastfeeding is limited or when it is not an option.”

The authors say a well-functioning gut microbiota is essential for the overall health and proper development of the infant, and components of human milk support the development of this microbiota. They list important human milk components and the novel ingredients that aim to mimic the functions of these components in infant formulas:

  • Human milk oligosaccharides (HMOs)

HMOs are specialized complex carbohydrates found in human milk, which are digested in the infant colon and serve as substrates for beneficial microbes, mainly bifidobacteria, residing there. In recent years, prebiotic mixtures of oligosaccharides (e.g. short-chain GOS and long-chain FOS) have been added to infant formula to recapitulate the effects of HMOs. But now that it’s possible to produce several types of HMOs synthetically, some infant formulas are enriched with purified HMOs: 2’-fucosyllactose (2’FL) or lacto-N-neotetraose (LNnT). Even 3′-galactosyllactose (3′-GL) can be naturally produced by a fermentation process in certain infant formulas.

  • Human milk microbiota

Human milk has a complex microbiota, which is an important source of beneficial bacteria to the infant. Studies support the notion that the human milk microbiota delivers bioactive components that support the development of the infant’s immune system. Probiotic strains are sometimes added to infant formula in order to substitute for important members of the milk microbiota.

  • Bacterial metabolites

Human milk also contains metabolic byproducts of bacteria called “metabolites” in addition to the bacteria themselves. These components have not been fully studied to date, but bacterial metabolites such as butyrate and other short-chain fatty acids may have important health effects for the overall development of the infant. A future area of nutritional research is likely to be the addition of ‘postbiotics’ — non-viable cells, their metabolites and cell components that, when administered in adequate amounts, promote health and well-being — to infant formulas. (ISAPP convened a scientific consensus panel on the definition of postbiotics, with publication of this definition expected by the end of 2020.)


The precise short- and long-term health benefits of adding the above ingredients to infant formula are still under study. One pediatric society (the ESPGHAN Committee on Nutrition) examined the data in 2011 and at that time did not recommend the routine use of infant formulas with added probiotic and/or prebiotic components until further trials were conducted. A systematic review concluded that evidence for the health benefits of fermented infant formula (compared with standard infant formula) are unclear, although improvements in infant gastrointestinal symptoms cannot be ruled out. Although infant formulas are undoubtedly improving, review co-author Hania Szajewska, MD, Professor of Paediatrics at The Medical University of Warsaw, Poland, says, “Matching human milk is challenging. Any alternative should not only match human milk composition, but should also match breastfeeding performance, including how it affects infant growth rate and other functions, such as the immune response.”



Probiotics and D-lactic acid acidosis in children

Prof. Hania Szajewska PhD, The Medical University of Warsaw, Department of Paediatrics, Poland and Prof. Seppo Salminen PhD, Faculty of Medicine, Functional Foods Forum, University of Turku, Finland

See related post ‘Brain Fogginess’ and D-Lactic Acidosis: Probiotics Are Not the Cause

In their recent study, Rao and colleagues1 incriminated probiotics in the induction of D-lactic acidosis (1). Many who benefit from probiotics could be frightened—on the basis of this report—into stopping them, with potentially negative impacts on their health (2). Some probiotic bacteria, including some specific components of the intestinal microbiota, may produce D-lactic acid. Indeed, if plasma D-lactic acid rises sufficiently, it is clinically relevant, causing D-lactic acidosis. D-lactic acidosis has mainly been observed in subjects with short bowel syndrome. However, some authorities have regulated the use of D-lactic acid producing bacteria in infant and weaning foods, but the reasoning for normal infant population has been debated. Even in adults, the safety of D-lactic acid producing bacteria has been challenged, but apart from short bowel patients no evidence on clinical problems has been reported (3).

For this reason, we conducted a review and examined whether D-lactic acid-producing bacteria, acidified infant formulas and fermented infant formulas were potential causes of paediatric D-lactic acidosis (4).

We identified five randomised controlled trials conducted between 2005-2017 with 544 healthy infants. Additionally, some case reports and experimental studies were considered. No clinically relevant adverse effects of D-lactic acid-producing probiotics or fermented infant formulas in healthy children were identified. The only known cases of paediatric D-lactic acidosis were observed in patients with short bowel syndrome (4). It is of importance that human milk also contains lactic acid bacteria and bifidobacteria, some of which may produce D-lactic acid. Some stress situations, such as exercise, may elevate human milk lactate concentrations.  Thus, breast milk D-lactate content needs to be analysed more carefully to compare with fermented infant formulas.

Taken together, our results suggest that neither the probiotics that were evaluated in the studies we reviewed nor fermented infant formulas cause D-lactic acidosis in healthy children.


  1. Rao, S. S. C., Rehman, A., Yu, S. & Andino, N. M. Brain fogginess, gas and bloating: a link between SIBO, probiotics and metabolic acidosis.  Transl. Gastroenterol.9, 162 (2018).
  2. Sanders, M. E., Merenstein, D. & Merrifield, C. A. Probiotics for human use.  Bull.43, 212–225 (2018).
  3. Quigley E.M.M, Pot B., Sanders M.E. ‘Brain fogginess’ and D-lactic acidosis: probiotics are not the cause. Transl. Gastroenterol.9, 187 (2018).
  4. Łukasik, J., Salminen S., Szajewska H. Rapid review shows that probioticsand fermented infant formulas do not cause D-lactic acidosis in healthy children. Acta Pediatrica 107, 1322-1326 (2018).