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Research into the effects and benefits of HMOs is growing. In this article, we explore the unique properties of individual HMOs and how combining multiple types may have greater health benefits.
· Background to the development of commercial HMOs
o The different structural classes of HMOs
· What are the expected benefits of combining multiple HMOs?
· 1. Structurally different HMO degradation releases different monosaccharides
· 2. Supports a wider range of beneficial bacteria
· 3. Greater protection from a wider range of pathogens
o HMOs modulate the immune system
o HMOs can function as decoy receptors
o HMOs inhibit bacterial growth and toxin binding
· 4. Contribute to cognition and brain development through distinct mechanisms
o The effects of fucosylated HMOs on brain health
o The effects of sialylated HMOs on brain health
· Summary: Is combining structurally different HMOs the future?
The link between the importance of breastfeeding, infant gut health, and survival was discovered in thelate 19th century. This association was strengthened when the ‘bifidus factor’ in breast milk was defined as human milk oligosaccharides (HMOs).
Today, more than 200 different HMOs have been identified and some are now being added to infant formula, in an attempt to give bottle-fed infants the same nutrition advantages that breastfed infants get.
Despite gaining regulatory approval in 2015, many infant formulas only contain one or two types of HMOs. Although HMOs have numerous benefits for infants, they are also beneficial and well-tolerated in adults. In this article, we explore the potential advantages that can be gained by combining structurally diverse HMOs.
Five main building blocks make up HMOs. They are:
1. Glucose
2. Galactose
3. Fucose
4. N-acetylglucosamine (GlcNAc)
5. N-acetyl-D-neuraminic acid (Neu5Ac) also known as sialic acid
HMOs contain lactose at their reducing end and are covalently bound to one or more monosaccharides or disaccharides. They can either be fucosylated or sialylated to produce the smallest trisaccharide HMOs:
|
Fucosylated |
Sialylated |
|
· 2’-fucosyllactose (2’-FL) · 3’-fucosyllactose (3’-FL) |
· 3’-sialyllactose (3’-SL) · 6’-sialyllactose (6’SL) |
Lactose can also be elongated to produce more HMOs, such as lacto-N-tetraose (LNT) or lacto-N-neotetraose (LNnT). In other words, these structural differences mean that a wide range of HMOs can be produced that vary in size, composition, and shape (Table 1).
Table 1.The structural diversity and complexity of HMOs. The HMOs are organised by the number of monosaccharides and their composition and include the 10 most abundant HMOs in human breast milk. The HMOs highlighted in grey can be manufactured and are commercially available.
Fucosylated HMOs account for the largest proportion of human milk oligosaccharides, with2’-FL being the simplest and most abundant[i].
Fucosylated HMOs account for between 35% and 50% of the total HMOs found in breast milk[ii].
Fucosylated HMOs have a lactose core, a combination of glucose and galactose, and a fucose attached. For example, 2’-FL has fucose attached to the second position of the lactose core, hence the name 2’FL.
Figure 1.The structure of a fucosylated HMO. The fucose unit (red triangle) is attached to the lactose core. In the case of 2’-FL, fucose is bound to the second position and in 3’-FL it is joined to the third.
Non-fucosylated or neutral HMOs are made from four main sugar units:
1. Glucose
2. Galactose
3. N-acetylglucosamine (GlcNAc)
4. Another galactose
Their structures can differ. For example:
· LNT has its sugar units arranged in a linear chain
· LNnT has its sugar units arranged in a slightly branched chain, it is known as a structural isomer of LNT
Some of the benefits of non-fucosylated HMOs include:
· LNT: prebiotic effects, antiadhesive antimicrobials, antiviral protection, immune support[iii]
· LNnT: reduces the risk of allergies[iv], supports the growth of beneficial bacteria, immune modulation
Sialylated HMOs account for 12 to 14% of the total HMOs in human breast milk[v]. The simplest and perhaps most well-known sialylated HMOs are 3’-SL and 6’-SL. These HMOs are renowned for their prebiotic benefits, helping to modulate the gut microbiome, and also aiding cognition[vi].
Sialylated HMOs are comprised of three main sugar units:
1. Glucose
2. Galactose
3. Sialic acid (Neu5Ac)
Where the sialic acid attaches to the lactose core determines its name. For example:
· 3’-SL has the sialic acid attached to the third position of galactose
· 6’-SL has the sialic acid attached to the sixth position of galactose
Some of the benefits of sialylated HMOs include:
· Blocking harmful pathogens by stopping them from sticking to gut cells because sialic acid has a negative charge
· Sialic acid is essential for proper nerve formation and function and sialylated HMOs can support brain development and cognitive health
Because conducting clinical trials in infants is challenging, preclinical studies can give a good insight into how different HMOs affect things like gut microbiota composition, pathogen inhibition, and cognitive development.
It is suggested that different HMO structures can influence their biological effects on health outcomes. Therefore, combining several HMOs in infant formula could have greater health benefits than just using one or two HMOs.
Using a review by Wichmann (2024), we’ll explore some of the expected benefits of combining multiple HMOs on human health.
Here are some of the key potential benefits of combining structurally diverse HMOs:
1. Structurally different HMO degradation releases different monosaccharides: HMOs which are structurally different can produce different monosaccharides that can cross-feed various gut bacteria, supporting their growth and function.
2. Supports a wider range of beneficial bacteria: increasing the numbers and structural diversity of HMOs exposed to the infant's gut may support a wider range of gut bacteria.
3. Greater protection from a wider range of pathogens: HMOs can exert a range of antipathogenic effects and exposure to more structurally different HMOs could support the immune system to combat a broader array of pathogenic bacteria and viruses.
4. Contribute to cognition and brain development through distinct mechanisms: research shows that both fucosylated and sialylated HMOs can positively impact brain development and cognitive function through different pathways. Combining the two could mean the host benefits from all their brain benefits.
As we’ve seen, HMOs can be classified into one of three structural classes:
1. Fucosylated
2. Neutral core
3. Sialylated
Once the HMOs are broken down by your gut bacteria, the structural class is determined by the molecules or metabolites that are released, such as fucose, GlcNAc, and sialic acid. Colonic bacteria can use these molecules to produce secondary metabolites and have further health benefits.
When fucosylated HMOs are broken down by the gut microbiota, fucose is released and can be metabolised by specific gut bacteria, such as Bifidobacterium longum to produce 1, 2-propanediol.
When bacteria break down 1, 2-propanediol they can yield propionate, an important short-chain fatty acid (SCFA)[vii]. SCFAs like propionate are important energy sources and cell signalling molecules.
Overall, fucose supports the growth of diverse bacteria, suppresses the virulence of viruses, and lowers inflammation, making it a key player in maintaining gut microbiota function and supporting balance within the gut.
Neutral core HMOs, like LNnT and LNT, release GlcNAc or N-acetylglucosamine when they are degraded by bacteria, including B. infantis and B. bifidum, serving as an important energy source.
However, GlcNAc also has further health benefits. For example, a recent 2023 study found that oral supplementation with GlcNAc improves intestinal barrier function, can protect against chemically-induced colitis, and supports gut microbiota homeostasis[viii].
The breakdown of sialylated HMOs releases sialic acid. However, unlike fucose and GlcNAc, sialic acid isn’t used by the infant gut microbiota as an energy source, instead, it benefits the circulatory system and distal organs like the brain. For example, emerging research suggests supplementing sialylated HMOs or sialic acid enhances brain development and performance[ix].
Generally, the most abundant bacterial species in the gut of breastfed infants are Bifidobacterium species, including:
1. B. longum subsp. infantis
2. B. bifidum
3. B. longum subsp. longum
4. B. breve
While 1 and 2 can degrade a range of structurally diverse HMOs thanks to the possession of glycoside hydrolase enzymes, 3 and 4 can only degrade neutral core HMOs like LNT.
One study analysed the ability of certain bacteria, namely 21 B. longum subsp. infantis strains and 13 B. bifidum strains, to grow on five individual HMOs used as the sole energy source found that:
· B. longum subsp. infantis and B. bifidum grew well on all types of HMOs including fucosylated and sialylated types
· B. longum subsp. longum and B. breve strains thrived on neutral core HMOs but displayed limited growth on fucosylated and sialylated HMOs[x]
Bifidobacteria species are not the only bacteria that can utilize HMOs. Bacteroides and Akkermansia muciniphila can also use them as a carbon source. For example, A.muciniphila is also equipped with enzymes needed to degrade HMOs[xi].
While Lactobacillusisn’t renowned for breaking down HMOs, it can cross-feed on other HMO-derived metabolites. Therefore, as HMO diversity increases, more beneficial bacteria can be supported, enhancing gut and wider health.
Some studies have already begun to demonstrate this. Natividad and colleagues (2022) assessed the effect of 2'-FL, 2'-FL + LNnT, and a mixture of six HMOs (HMO6: consisting of 2'-FL, LNnT, difucosyllactose, lacto-N-tetraose, 3'- and 6'-sialyllactose) on the composition of the infant microbiota and gut barrier integrity. Their results found that all the HMOs have bifidogenic potential and increased SCFA levels, the combination of 6 HMOs promoted the greatest abundance and diversity of Bifidobacteria species.
The 6 HMO combination also increased the abundance of the prominent butyrate producer Faecalibacterium prausnitzii and significantly strengthened the integrity of the gut barrier.
Studies exploring theeffects of supplementing HMOs in infant formula milk have found that supplementation with5 HMOs can generate shifts in the gut microbiome that are closer to that of breastfed infants, and support their immune system and gut barrier function[xii].
Much research has shown that breastfeeding can reduce the risk of gastrointestinal and respiratory infections[xiii] and HMOs are a critical protective component. There are several ways thatHMOs can protect the body against pathogens.
HMOs structurally resemble blood group glycans and can bind to lectins on immune cells, including C-type lectins, galectins, and siglecs. Different lectin types have specificity for different HMOs, for example:
· C-type lectins like DC-SIGN for 2’-FL and 3’-FL
· Galectins bind to LNT and LNnT
· Siglecs have specific binding affinity for siallylactose
Structurally diverse HMOs can exert different immune responses. Boll et al (2024) demonstrated that sialylated HMOs (3’-SL and 6’-SL) stimulate the release of cytokines while fucosylated and neutral core HMOs enhance the integrity of the intestinal barrier [xiv].
Animal studies have found that supplementing HMOs increases the presence of natural killer cells and effector memory T cells[xv].
HMOs can also bind to pathogenic invaders stopping them from adhering to and infecting the host's cells, also known as a decoy receptor. Much of the research suggests that HMOs bind to specific pathogens and their toxins, based upon their structure.
· Fucosylated HMOs bind to norovirus strains
· Neutral core HMOs bind to rotavirus strains
· Sialylated HMOs bind to respiratory viruses including flu
HMOs can further protect against pathogenic infection by binding to specific pathogenic bacteria, bacterial toxins and parasites. Craft and Co (2019) demonstrated that a particular fucosylated HMO called DFL could reduce the growth of Streptococcus B by 50%[xvi].
Both 2’-FL and LNnT can exert antipathogenic effects against C. difficile and may even prevent it from returning after antibiotic treatment[xvii].
HMOs are bioactive molecules that have been extensively studied for their purported benefits for neurodevelopment. Both fucosylated and sialylated HMOs have established roles in the developing brain, but they have distinct mechanisms by which they do this.
While fucosylated HMOs can support brain function by modulating the gut microbiota and enhancing immunity, sialylated HMOs can directly influence neural development by supporting synaptic plasticity and brain cell communication.
Fucosylated HMOs, such as 2′-FL and 3’-FL, play an important role in brain health and development. These HMOs are thought to impact brain function by influencing the gut-brain axis.
Fucosylated HMOs may support cognitive function by modulating the gut microbiome, particularly by promoting the growth of beneficial bacteria such as Bifidobacterium species. These bacteria produce neuroactive metabolites like short-chain fatty acids, which have been shown to influence brain development and immune function.
Additionally, fucosylated HMOs can help modulate immune responses, reduce inflammation, and promote the integrity of the blood-brain barrier, essential for brain protection and function.
Sialylated HMOs, including 3′-SL and 6′-SL, are particularly important for the development of cognitive functions such as learning and memory. These HMOs are rich in sialic acid, a sugar molecule that plays a critical role in brain development by promoting synapse formation and neuronal communication[xviii].
Sialic acid is a key component of glycoproteins and glycolipids found in the brain[xix], especially in neural tissue, which aids cell signaling and synaptic plasticity. Additionally, sialylated HMOs may support brain development by enhancing immune modulation and protecting against inflammation, which is essential during critical periods of brain growth.
As a result, sialylated HMOs are likely to contribute to improved cognitive outcomes by supporting structural and functional brain development.
HMOs are undoubtedly an important component of human breast milk—they are also well tolerated and beneficial for adults—and have been the subject of many research trials, leading to their large-scale production and addition to infant formula milk.
Even though they share many characteristics and benefits, the structural diversity of HMOs means that individual types can have unique biological functions. When combined with other structurally different HMOs, greater health benefits can be achieved, and, in the case of infants, a microbiota that resembles that of breastfed infants is within reach.
Further research shows that in adults a daily combination of 2’-FL and LNnT can normalise bowel habits in people with irritable bowel syndrome (IBS), with patients reporting areduction in symptoms and an improvement in their quality of life[xx].
At Layer Origin, we are already ahead of the curve, combining fucosylated, neutral core, and sialylated HMOs in our following products:
You can explore our range of HMO products in ourshop.
Discover the unique benefits of our brand new MitiAging product. It restores the gut microbiome to kickstart it into naturally producing urolithin A, a postbiotic hailed for its anti-aging properties.