Human Milk Oligosaccharides: The Fiber of Breastmilk

By Victoria Daniels, M.S. in Nutritional Sciences

According to the CDC, 75% of US infants are not exclusively breastfed at 6 months of age. Breastmilk is the gold standard for infants because it is full of essential macro and micronutrients for healthy growth and development. Breastfeeding has a multitude of benefits for both the mother and baby. One important component in breastmilk is human milk oligosaccharides (HMOs): a complex, fiber-like sugars that aids in the gut and immune development of the infant.

Throughout the lifespan, fiber and oligosaccharides play an important role in the human diet. Toddlers, teenagers, and adults consume oligosaccharides  from plant sources such as grains, fruits and veggies. These fibers function as prebiotic fuel for the bacteria in the gut. But what does this look like for babies who only consume milk? Infants obtain different fiber-like, indigestible sugars through breastmilk or infant formula, which offer a multitude of benefits to the infant.

Human milk oligosaccharides are the third most abundant component of breastmilk behind fat and lactose. Over 150 HMOs have been identified so far!  Concentrations in breastmilk vary during lactation, highest early in lactation, around 20-25g/L in colostrum  5-15g/L in mature milk. Because these concentrations are so variable and abundant, the synthesis of HMO in mammary tissue is not yet well understood. Genetic variation causes differential expression of enzymes involved in the synthesis of HMOs. For example, women with the Secretor gene have the  α1-2-fucosyltransferase enzyme, which produces the most abundant HMO in breastmilk (2’-fucosyllactose). Although variable, HMO concentrations in human milk are highly beneficial to the infant and is part of the reason breastmilk is the gold standard.

Oligosaccharides in infant formula look a bit different than HMOs. Rather, infant formula contains fructooligosaccharides (FOS) and galactooligosaccharides (GOS). These molecules are added to formula because they stimulate growth of Bifidobacteria. However, they are also fuel for other gut bacteria like Streptococcus, which could reflect a shift towards a more adult-like gut microbiome earlier in life than an exclusively breastfed infant. Therefore, HMOs from breastmilk may be better suited for the infant gut because they can serve as a more selective fuel source for the developing infant gut microbiota. 

Food science allows for the synthetic production of formula, and may enable HMOs to be incorporated into infant formula to support microbiota development. There are many methods to synthesize HMOs using enzymes and bacteria, so they can be added to infant formula. For example, E. coli can be genetically modified to express α1-2-fucoslyltransferase which allows food scientists to synthesize 2’-fucosyllactose. Adding this HMO to formula has its advantages. Due to the variable nature of HMO content in breast milk, researchers also try mixes of different HMOs in formula. Babies who received formula with the mix of HMOs had a microbiome closer to that of exclusively breastfed infants. 

    The American Academy of Pediatrics recommends exclusive breastfeeding to 6 months of age but 75% of babies across the country rely on formula to fulfill their nutritional needs. Oligosaccharides support infant health making them a necessary component for infant formula. Although infant formula has yet to fully mimic breastmilk, researchers continue to work towards  making infant formula more like breastmilk utilizing prebiotics and HMOs. 

Peer-edited by Malik Tiedt, BSPH Student in Nutrition

Image credit: Shutterstock

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