Why Composition Stays Remarkably Stable for Years
What if everything we thought we knew about how breastmilk changes over time was missing a crucial part of the story? For decades, research on human milk composition has focused primarily on the first few weeks or months of infancy, leaving a significant gap in our understanding of what happens during longer-term breastfeeding. This knowledge gap hasn't just limited scientific understanding—it has affected everything from milk bank donation policies to the quality of advice given to parents making infant feeding decisions.
Enter a groundbreaking citizen science collaboration that's rewriting the textbook on human milk science. A revolutionary study conducted in partnership with the "Parenting Science Gang" has revealed that human milk maintains remarkable compositional stability not just for months, but for up to two years of breastfeeding—and possibly beyond 1 4 5 . This discovery challenges long-held assumptions and has profound implications for how we support breastfeeding families.
To appreciate why this research is so transformative, we first need to understand the advanced technologies that made these insights possible. The scientists employed two cutting-edge approaches that go far beyond traditional milk analysis:
Think of this as creating a complete molecular portrait of breastmilk. Using a sophisticated technique called rapid evaporative ionisation mass spectrometry (REIMS), researchers can analyze thousands of metabolites simultaneously—including fatty acids, complex lipids, and small molecular weight compounds that play crucial roles in infant health and development 4 5 . The particular advantage of this method is that it requires no sample preparation, giving scientists a more accurate "snapshot" of the milk's natural state.
This approach maps the diverse microbial community present in human milk—what scientists call the milk microbiome. By sequencing the 16S rRNA gene, researchers can identify the types and relative abundances of bacteria that help establish an infant's gut microbiome and developing immune system 1 4 .
Together, these techniques provide an unprecedented window into the complex world of human milk composition, allowing scientists to ask questions that were previously impossible to answer.
What makes this research particularly innovative is its collaborative nature. The study was co-created with members of the "Parenting Science Gang," a citizen science group that contributed to everything from experimental design to participant recruitment and sample collection logistics 4 5 . This partnership allowed researchers to access a unique cohort of participants that would have been challenging to recruit through traditional means.
The study included 62 breastfeeding participants with infants ranging from 3 to 48 months old—nearly three years longer than most previous research on milk composition 1 4 . The participants were divided into five groups based on their nursling's age to enable comparative analysis.
| Age Group | Number of Participants |
|---|---|
| 3-6 months | 12 |
| 6-12 months | 12 |
| 12-24 months | 16 |
| 24-36 months | 14 |
| 36-48 months | 8 |
The research team implemented a rigorous protocol to ensure reliable results:
All milk samples were collected on the same day at Charing Cross Hospital in London, with participants allocated to hourly donation sessions between 9:00 and 16:00 4 5 .
Within 30 minutes of donation, each sample was divided for different analyses—metabolic fingerprinting, DNA extraction for microbiome analysis, and volume/fat determination 4 .
Metabolic fingerprinting was performed using LA-REIMS technology, while microbial analysis involved DNA extraction and 16S rRNA gene sequencing 4 .
This careful methodology allowed for direct comparisons across different lactation stages while minimizing technical variables that could affect the results.
The findings from this comprehensive analysis challenged conventional wisdom about how breastmilk changes over time. While volume decreased after 24 months—an expected adaptation as children consume more complementary foods—the fundamental composition of the milk remained remarkably consistent 1 4 .
One of the most surprising findings was the consistency of the milk microbiome across different stages of lactation. The bacterial load and community composition showed no significant differences from 3 to 24 months, suggesting that milk continues to provide a consistent source of beneficial bacteria throughout extended breastfeeding 1 4 .
At the macro level, the overall metabolomic fingerprint of human milk showed remarkable consistency across the first 24 months of lactation 1 4 5 . Neither the method of milk expression (hand vs. pump) nor the sex of the nursling affected the metabolic or metataxonomic profiles.
| Parameter | 3-24 Months | 24+ Months |
|---|---|---|
| Milk Volume | Stable | Significantly lower |
| Bacterial Load | No significant change | No significant change |
| Microbial Composition | No significant change | No significant change |
| Overall Metabolomic Fingerprint | Remarkably consistent | Remarkably consistent |
| Individual Metabolite Features | Minimal changes | ~14% showed altered abundances |
The research did identify some subtle changes—approximately 14% of individual metabolite features showed altered abundances when nursling age exceeded 24 months 1 4 . However, these changes represented fine-tuning rather than fundamental compositional shifts.
Additional validation comes from a related study on tandem feeding (breastfeeding siblings of different ages simultaneously). This research found no significant differences in milk composition between tandem feeding dyads and single nursling dyads—even when comparing milk from the same mother being fed to children of different ages 8 . This further reinforces the concept of compositional stability in human milk, even when feeding children with potentially different nutritional needs.
What does it take to conduct such sophisticated analysis of human milk composition? Here's a look at the key tools and technologies that made this research possible:
| Tool/Technology | Function in the Study |
|---|---|
| LA-REIMS (Laser Assisted-Rapid Evaporative Ionisation Mass Spectrometry) | Metabolic fingerprinting without sample preparation, reducing analytical bias |
| 16S rRNA Gene Sequencing | Identifying and quantifying bacterial communities in milk samples |
| FastDNA™ SPIN Kit for Soil | DNA extraction from milk samples for microbiome analysis |
| Qubit Fluorimeter 4.0 | Precise measurement of DNA concentration for accurate sequencing |
| Class IVb CO2 Laser | Sample heating for LA-REIMS analysis |
The implications of these findings extend far beyond academic interest—they have real-world significance for families, healthcare providers, and milk banking policies.
These findings offer scientific validation for extended breastfeeding, reassuring parents that their milk continues to provide valuable biochemical and immunological benefits throughout the natural breastfeeding period. The results also empower healthcare providers to offer evidence-based support for families who choose to breastfeed beyond infancy.
This study opens new avenues for exploring the functional significance of the subtle metabolic changes that occur after 24 months and investigating how individual variations in milk composition might relate to child health outcomes.
This citizen science collaboration has fundamentally advanced our understanding of human milk composition, revealing a biological reality far more complex and stable than previously recognized. The remarkable consistency of milk's metabolic and microbial profiles throughout extended breastfeeding highlights the dynamic, responsive nature of human lactation—a biological system precisely calibrated to support child development not just for months, but for years.
As we continue to unravel the mysteries of human milk, studies like this remind us that sometimes the most significant scientific discoveries come from asking simple questions about everyday human experiences—and having the humility to partner with the people living those experiences every day.
The next frontier? Longitudinal studies that track individual mothers over time and research exploring how these compositional elements directly impact child health outcomes. For now, though, we have a clearer picture than ever before: human milk remains a consistently valuable biological fluid throughout the natural course of breastfeeding.