Key Sugars in Breast Milk Support Toddler Gut and Brain Development After the First Year

• Systematic review and meta-analysis design — Researchers included only studies that measured HMOs at or beyond the one-year mark. Thirteen studies qualified, representing 4,786 milk samples collected across multiple countries. Concentrations were standardized into grams per liter (g/L), and when totals were missing, they were calculated by summing measured sugars.

This harmonization allowed comparison across analytical methods such as high-performance liquid chromatography and mass spectrometry.

• Forty-two sugars were detected, but six stood out as “core” — Across all studies, 42 HMOs appeared at least once, yet only six consistently showed up in 10 or more publications. These were 2′-fucosyllactose (2′-FL), 3-fucosyllactose (3-FL), lacto-N-tetraose (LNT), lacto-N-neotetraose (LNnT), 3′-sialyllactose (3′-SL), and 6′-sialyllactose (6′-SL).

Together, they accounted for more than 70% of the total HMO pool at every stage of lactation, marking them as the dominant structures persisting into toddlerhood.

• Overall HMO concentrations decreased, then stabilized — Total levels fell from 7.72 g/L in colostrum (the first nutrient-dense milk produced by the body in the days after birth) to 5.34 g/L at 12 months. However, measurements between 13 and 24 months averaged 8.47 g/L, ranging between 7.2 and 10.1 g/L. Although later data relied on fewer samples, the findings showed that breast milk continues to deliver substantial amounts of HMOs well into the second year.

• Individual sugars followed distinct trajectories — 2′-FL remained the most abundant at all stages, averaging 3 g/L in colostrum and staying higher than 3-FL at 12 months (1.59 g/L vs. 1.45 g/L). 3-FL was unique in steadily rising, nearly quadrupling from 0.38 g/L in colostrum to 1.45 g/L at one year. By contrast, LNT and LNnT steadily declined, 6′-SL dropped sharply, and 3′-SL remained relatively stable.

These shifting trajectories suggest that while the total HMO pool remains steady, the balance of individual sugars evolves over time.

• Persistence points to ongoing biological importance — HMOs are already linked to microbiome maturation, immune training, and neurodevelopment in infants, but little was known about their role after the first year. By showing that core HMOs remain abundant beyond 12 months, the study highlights a need to explore how ongoing exposure continues to shape gut and brain outcomes during toddlerhood.

• Implications for feeding practices — For children who continue breastfeeding into the second year, HMOs remain available in biologically meaningful quantities. For those not breastfed beyond infancy, donor milk, fortified formulas, or HMO-enriched supplements, such as those containing 2′-FL and LNnT, provide alternate sources.

While long-term impacts require more research, the persistence of these sugars emphasizes their ongoing relevance during a period of rapid microbiome and brain development.

• Gut health — One of the most important roles of HMOs is their prebiotic effect. They selectively feed beneficial bacteria such as Bifidobacterium, supporting their growth while limiting less favorable strains. This selective feeding leads to higher production of short-chain fatty acids (SCFAs) like butyrate, which have been associated with strengthening the intestinal barrier, reducing inflammation, and creating a balanced environment that discourages harmful microbes.

HMOs also help protect your child against infections by acting as “decoy receptors.” Many pathogens, including Campylobacter jejuni and Pseudomonas aeruginosa, attach to sugar structures on the intestinal lining to initiate infection. Fucosylated HMOs mimic these structures, blocking pathogens from binding and reducing the risk of illness.⁵ Together, these actions give your child a healthier microbial community and stronger defenses during a time of rapid growth.⁶

• Brain and cognitive health — HMOs also play a role in shaping brain development through the gut-brain axis. By fostering a healthy microbiome, they promote the production of metabolites like SCFAs that influence neurotransmitter activity and protect developing neurons. Research shows that bacteria supported by HMOs produce gamma-aminobutyric acid (GABA), a neurotransmitter important for mood and learning.

Sialylated HMOs provide sialic acid, which influences synapse formation and myelination. These processes form the wiring of your child’s brain, supporting memory, learning, and communication between neurons. Specific HMOs, such as 2′-FL and 6′-SL, have been associated with improved scores in cognitive, motor, and language development in infancy. Higher exposure to 30-sialyllactose (30SL) has also been linked to improved early learning scores by 10 months of age.⁷

• Immune system development — HMOs influence immune cell activity by promoting balanced signaling between Th1 and Th2 responses, which helps train the body to respond appropriately to challenges. They also reduce inflammatory cytokines, which protect against excessive immune reactions.⁸

Early exposure to HMOs helps establish immune tolerance, lowering the likelihood of allergies and autoimmune responses later on. Studies in both breastfed and formula-fed infants show that those receiving HMOs have lower rates of respiratory and gastrointestinal infections, and even reduced use of medications.⁹

• Metabolic programming and long-term health — Beyond immediate growth and defense, HMOs appear to influence long-term metabolic health. Their ability to shape gut bacteria and regulate immune activity contributes to healthier energy balance and reduced inflammation. Some evidence suggests protective effects against obesity, Type 2 diabetes, and allergic disease.¹⁰

• Consistent lactation pattern across populations — Researchers at Yale School of Medicine and their collaborators investigated this molecule by analyzing breast milk samples from mothers in Cincinnati, Mexico City, and Shanghai, China.

Across all three regions, myo-inositol followed the same pattern — levels were highest early in lactation and gradually declined as months passed. The identical trajectory across countries suggested that its production is tightly regulated by the mother’s body rather than shaped by diet or environment.

• Boosts synapse formation and connectivity — Using cultured human neurons and brain tissue, the team found that myo-inositol significantly increased synapse abundance. Synapses are the contact points where neurons exchange signals, so higher numbers translate to more opportunities for communication.

Synapse formation peaks in the first months of life, providing the foundation for sensory processing, learning, and memory. Myo-inositol also strengthened neuronal connectivity, meaning that brain regions were more effectively linked into working networks that enable learning, memory, and information processing.

• Dynamic alignment with developmental needs — The study also emphasized the broader message that breast milk is a complex and dynamic fluid. Its composition changes over time, adjusting to meet your child’s developmental needs at different stages. The decline of myo-inositol levels as lactation progresses mirrors the brain’s developmental timeline, in which the initial burst of synapse formation gives way to later phases of refinement and optimization.

1. Breastfeed exclusively for at least six months — The World Health Organization (WHO) recommends exclusive breastfeeding for the first six months, with continued breastfeeding alongside solid foods up to two years or longer.¹³ When breastfeeding is not possible, donor milk from a human milk bank is the next best option. Introducing formula milk too early disrupts gut health and weakens many of the benefits of breastfeeding.

2. Prioritize your own nutrition while breastfeeding — Your diet plays a direct role in the composition of your breast milk. To give your baby steady support, emphasize whole, nutrient-dense foods and avoid seed oils and excess omega-6 fats. Rely on stable sources of saturated fat such as grass fed butter, tallow, and ghee to help maintain milk quality.

It is also important to eat enough carbohydrates (around 250 grams per day for most adults) to prevent metabolic stress that could reduce milk supply. When you nourish yourself well, you provide your baby with the strongest nutritional start.

3. Avoid unnecessary antibiotics and medications early on — Antibiotics destroy both harmful and beneficial bacteria, which can slow the development of your baby’s gut microbiome.¹⁴ Unless truly necessary, they should be avoided during the first year of life. Other common medications, such as acetaminophen and reflux drugs, can also disturb gut balance and are best used cautiously.

For minor illnesses, turn first to natural approaches whenever possible and allow your baby’s immune system to strengthen through good nutrition and consistent feeding. Keep in mind that some medications taken by mothers may also alter the composition of breast milk, making it important to review all prescriptions and over-the-counter drugs with a trusted healthcare provider. Learn more in this article, “How Maternal Medications Alter Breast Milk Nutrients.”

4. Reduce your exposure to “forever chemicals” — Studies have detected polyfluoroalkyl substances (PFAS) in breast milk, showing that they can be passed from mother to infant. These chemicals are widely used in consumer products such as nonstick cookware, food packaging, and water-resistant fabrics.¹⁵

You can lower your family’s exposure by reducing contact with items that contain PFAS and choosing safer alternatives whenever possible. Taking these steps helps decrease your overall toxic burden and limits the amount that may appear in breast milk. For more information about this, read “Scientists Uncover How Forever Chemicals Pass from Mother to Child.”