Understanding HMO Abundance in Human Breastmilk: What It Means for Parents

As a researcher who has spent much of my career studying how early-life nutrition affects infant health, I find Human Milk Oligosaccharides (HMOs) to be one of the most fascinating and important components of breast milk. They are complex carbohydrates that make up the third-largest solid component of milk, right after lactose and fat, and they play a crucial role in shaping your baby’s gut microbiome, immunity, and even brain development.

What Are HMOs?

HMOs are unique to human milk. Unlike lactose, which your baby digests for energy, HMOs pass through the digestive system largely intact until they reach the colon. There, they act as prebiotics, feeding beneficial bacteria such as Bifidobacterium. This helps create a gut environment that favors healthy bacteria and keeps potentially harmful microbes in check.

But HMOs do more than nourish gut bacteria. They also block pathogens from attaching to the intestinal wall, lowering the risk of certain infections, and they provide building blocks like fucose and sialic acid that are critical for brain development and nerve myelination.

In short, HMOs help your baby build a healthy gut, a resilient immune system, and a strong foundation for cognitive growth.

Why HMO Levels Vary

One of the first things I tell parents is that no two mothers’ breast milk is exactly the same, and that’s completely normal.

The biggest factor influencing HMO levels is genetics. About 70% of women are what we call “secretors,” meaning they produce higher levels of HMOs, especially 2’-fucosyllactose (2’-FL).

Non-secretor mothers produce very little of certain HMOs like 2’-FL, which may slightly change how their baby’s gut microbiome develops.

HMO levels also change over time. Most decrease during the first six months, but some, like 3’-sialyllactose (3’-SL) and 3’-fucosyllactose (3’-FL), actually increase during extended breastfeeding and these provide nutrients important for ongoing brain development.

We are still learning about how maternal diet influences HMO levels, but some studies suggest that higher fruit and vegetable intake is linked to slightly higher HMO production, while higher added sugar intake may reduce levels of 2’-FL.

Supporting Your Baby — Whether Breastfeeding or Formula-Feeding

Because HMOs are so effective at shaping the gut microbiome, we generally see more beneficial bacteria like Bifidobacterium in breastfed babies compared to formula-fed babies. This is one reason why many formula companies have started adding HMOs, most commonly 2’-FL, to more closely mimic the benefits of breast milk.

It is impossible to fully replicate the complexity of human milk because there are over 100 different HMOs, and each mother’s unique combination is like a fingerprint. However, these formula innovations are an important step toward narrowing the gap.

I also encourage parents who are formula-feeding to choose lactose-based formulas rather than those made with corn syrup solids, as lactose better supports the development of beneficial gut bacteria.

Begin Health's Editor's Note

For babies born via C-section, exposed to antibiotics early in life, or if you know you are a non-secretor, adding an HMO-containing prebiotic + probiotic can help during early infancy. Products like Beginbaby Prebiotics + Probiotics provide HMOs alongside beneficial bacteria. Always speak with your pediatrician first before starting any new supplement routines. 

Practical Tips for Parents

If You Are Breastfeeding:

• Eat a balanced diet rich in fruits, vegetables, and fiber to support your milk quality.

• Consider probiotics that contain Bifidobacterium infantis, which work synergistically with HMOs.

• Take care of yourself — hydration, rest, and stress management all contribute to healthy lactation.

If You Are Formula-Feeding:

• Look for formulas that contain HMOs, ideally 2’-FL, and are lactose-based.

• Talk to your pediatrician about whether additional prebiotic/probiotic supplementation might be helpful.

Final Thoughts

HMOs are one of nature’s most powerful tools for supporting infant health. Whether you are breastfeeding, formula-feeding, or doing a combination of both, there are ways to ensure your baby reaps the benefits of these remarkable molecules. My advice to parents is simple: focus on nourishing your baby’s gut, because doing so lays a foundation for lifelong health.