According to research reviewed by Gram, urea in breast milk rises as lactation continues and may help shape the infant gut microbiome. A 2026 preprint analyzing three human cohorts and a mouse model found that infant formula contains more urea than human milk, though the findings have not yet been peer reviewed.
A new preprint from 2026 suggests that urea, a nitrogen-containing waste compound, plays a more active role in infant health than scientists once thought. According to research reviewed by Gram, urea levels in breast milk rise as lactation continues, and this pattern appears across many mammals. The researchers found that the mother’s blood levels and diet did not fully explain these changes. Instead, the mammary gland seems to regulate how urea moves into milk. Breast milk and stool also carry bacteria that can break down urea, which may help the baby build a gut community the researchers call the ‘ureabiome.’ Infant formula contained more urea than breast milk. Because this work has not yet been peer reviewed, it should be seen as an early step rather than a settled answer.
Key Statistics
A 2026 bioRxiv preprint that combined three human cohorts with a mouse model found that urea levels in breast milk increase across lactation.
A 2026 bioRxiv preprint reported that infant formula provides higher urea levels than human breast milk, according to analyses of the three human cohorts and mouse data.
A 2026 bioRxiv preprint found that breast milk and stool contain urea-metabolizing bacteria that may help establish the infant gut’s urea-using microbial community.
The Quick Take
- What they studied: Researchers wanted to know whether urea, a common nitrogen waste product, is present in breast milk on purpose, how the body controls its levels, and whether it affects the bacteria that live in a baby’s gut.
- Who participated: The study used three groups of human mothers and babies, along with a mouse model. The preprint does not report the exact number of participants.
- Key finding: Urea in breast milk increased over the course of lactation. The levels were not explained by the mother’s blood urea or diet, which suggests the body actively controls how much urea enters the milk. Infant formula contained higher urea levels than breast milk.
- What it means for you: This research is early and has not been peer reviewed, so it does not change how anyone should feed a baby. It does add to the picture of how breast milk supports infant gut development. Parents with questions about feeding should talk with a pediatrician or lactation consultant.
The Research Details
The researchers combined data from three human groups, which the paper calls cohorts. Each cohort tracked breast milk over time, so the team could see how urea changed as breastfeeding went on. This type of tracking helps separate natural patterns from one-time differences.
To understand the biology, the team also used a mouse model. In mice, they examined the mammary gland, the tissue that makes milk, and looked at how cells handle urea. They focused on a water channel protein called AQP3, which may help move urea into milk. They also studied stool and milk samples to identify bacteria that can break down urea.
By linking changes in the gland, the milk, and the infant’s gut, the team tried to build a complete picture. They also ran lab tests to see how urea affects both helpful and potentially harmful bacteria at different concentrations.
Using human cohorts shows what happens in real mothers and babies, while the mouse model reveals the cell-level mechanisms that human studies cannot easily measure. Combining both approaches makes it more likely that the findings are real and not a fluke of one group. Tracking milk over the whole lactation period also shows change over time, which a single snapshot could miss.
This is a preprint, which means it was posted on bioRxiv before formal peer review by outside experts. Preprints are useful for sharing findings quickly, but their conclusions can change after review. The paper does not state exact sample sizes in the summary, which makes it harder to judge how strong the evidence is. Readers should treat the results as promising but still preliminary.
What the Results Show
The main result is that urea in breast milk goes up as lactation continues. This pattern was seen in human mothers and appears to be shared by other mammals, which suggests it is a general biological feature rather than a quirk of one species.
The researchers also checked whether this rise simply reflected what was happening in the mother’s blood or what she ate. It did not. The levels in milk seemed to follow their own pattern, which points to active control inside the mammary gland. The team found changes in how mammary cells handle and move urea during lactation, including shifts in where the AQP3 protein sits in the cell. These shifts are consistent with urea being deliberately transferred into milk.
Urea in milk and stool also appears to carry bacteria that can use urea. The researchers describe these communities as building an infant ‘ureabiome,’ and the genes these bacteria carry suggest they have the ability to use urea as a food source. The effects depend on context. Infant formula contains more urea than human milk, and the lab tests showed that urea feeds both helpful bacteria and some opportunistic ones, with the effect depending on the amount present.
The team showed that urea’s effect on bacteria is dose-dependent, meaning the amount matters. Small and large concentrations do not necessarily produce the same outcome. This is important because it suggests that more urea is not automatically better for the infant gut. The finding that formula contains more urea than breast milk also raises questions about how formula composition might influence early gut microbes, though the preprint does not test long-term health outcomes.
Scientists have long known that urea is present in human milk, but most had treated it as a leftover waste product with no clear purpose. This preprint pushes that view forward by suggesting urea is carefully regulated and may serve a function. It also connects milk chemistry with the growing body of research on how early gut bacteria develop, which has been an active area of study in infant nutrition.
The study has not yet been peer reviewed, so its methods and conclusions have not been checked by independent experts. The exact number of participants is not reported in the summary, which makes the strength of the human evidence hard to judge. Mouse studies do not always translate directly to humans, so the cell-level findings may not work the same way in people. The research also does not show whether these urea-related changes affect a baby’s growth, immunity, or long-term health. Finally, the lab tests on bacteria were done under controlled conditions that may not match the real gut.
The Bottom Line
Breastfeeding remains recommended by major health organizations for infant nutrition, and this research does not change that advice. Based on this preprint alone, there is no evidence-based action for parents to take about urea levels. Confidence in the basic finding that milk urea rises during lactation is moderate, since it was seen in multiple groups. Confidence that urea shapes gut bacteria in ways that help babies is low to moderate and needs confirmation in peer-reviewed studies.
Researchers in infant nutrition, microbiology, and lactation science will find this work especially useful. Parents who are curious about how breast milk supports early gut development may also find it interesting. Parents who are formula feeding should not feel alarmed by the urea finding, since formula is designed to meet infant needs. Anyone making feeding decisions should speak with a pediatrician rather than rely on a single preprint.
Because this is early laboratory and cohort research, there are no practical benefits to expect in daily life yet. Peer review, follow-up studies, and clinical trials would be needed before any feeding guidance could change. That process typically takes several years.
Frequently Asked Questions
Does breast milk contain urea, and why does it matter?
Yes, breast milk contains urea, and its levels rise as breastfeeding continues. A 2026 preprint suggests urea may help build the baby’s gut bacteria, though these findings still need confirmation through peer-reviewed research.
Is there more urea in formula or breast milk?
Infant formula contains higher urea levels than human breast milk, according to a 2026 preprint analyzing three human cohorts and a mouse model. Researchers noted that the effect of urea on gut bacteria depends on how much is present.
What is the infant ureabiome?
The infant ureabiome is the community of gut bacteria that can break down urea. Researchers found these bacteria in both breast milk and stool, suggesting that the mother may pass them to the baby during breastfeeding.
Should I change my diet to change the urea in my breast milk?
Current research does not support changing your diet to alter urea in breast milk. Urea levels appear to be regulated by the mammary gland rather than by diet, so you should keep eating well and speak with your healthcare provider about feeding questions.
Want to Apply This Research?
- Users who are breastfeeding can log feeding sessions, including the time, duration, and whether the feeding was breast, formula, or both. Over several weeks, this log shows how a user’s feeding pattern changes as the baby grows, which is useful information to bring to a healthcare provider.
- Encourage users to keep up their usual hydration and balanced eating, which supports general health during breastfeeding. The app can offer a reminder to drink water at regular intervals, but it should not suggest changing the diet to alter milk chemistry, since this research does not show that is possible or safe.
- Over the long term, users can record feeding patterns, baby’s growth milestones noted by their pediatrician, and any changes in stool or digestion they observe. Sharing this summary at well-child visits helps the care team track progress. Users should also be encouraged to check back for peer-reviewed follow-up studies on this topic.
This article summarizes a preprint posted on bioRxiv that has not yet been peer reviewed, so its findings may change. It is for general education only and is not medical advice. Parents and caregivers should not change feeding practices based on this research and should talk with a pediatrician, lactation consultant, or other qualified healthcare provider about infant nutrition.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.