Breast milk contains hundreds of nutrients and bioactive compounds, many of which scientists are still working to understand. A new study suggests one fatty acid found in human breast milk may help shape early immune development, but the strongest evidence so far comes from experiments in mice.
In a study published in Science, researchers at the University of Chicago focused on trans-vaccenic acid, or TVA, a fatty acid found in human breast milk and in meat and dairy products from grazing animals such as cows and sheep. The body cannot make TVA on its own, so it must come from the diet.
The researchers found that nursing mice fed a TVA-enriched diet passed the fatty acid to their pups through breast milk. Those pups developed changes in immune cells involved in adaptive immunity and responded more quickly to later infections with influenza and Salmonella.
The study also included human breast milk and blood samples from nursing mothers and infants. Higher TVA levels in breast milk were closely linked to higher TVA levels in infants’ blood. In preterm infants, TVA levels were associated with immune-response patterns similar to those seen in mice. Higher TVA levels in human breast milk were also associated with a lower risk of bronchopulmonary dysplasia, a chronic inflammatory lung disease that can affect premature infants.
Those human findings are important, but they do not prove that TVA has the same immune effects in babies that it had in mice. The study does not show that pregnant or breastfeeding people should take TVA supplements, change their diets to increase TVA or make decisions about infant feeding based on this research.
“It’s common knowledge that breastfeeding is important for neonatal immune development and overall health, but breast milk is so complex that it seems almost impossible that one single molecule would be sufficient to change a baby’s immune development,” said Jing Chen, PhD, the Janet Davison Rowley Distinguished Service Professor of Medicine at the University of Chicago and one of the senior authors of the study. “So, it was very surprising to see that during this crucial stage of development, one nutrient derived from the mother’s diet and delivered through breastfeeding has such a tremendous effect.”
The immune system develops rapidly early in life. During that time, immune cells learn how to respond to infections while also avoiding unnecessary reactions. The researchers found that TVA exposure during breastfeeding appeared to influence that process in mice by changing the development and programming of T cells.
T cells are immune cells that help the body recognize and respond to threats. The study found effects on CD4+ T cells, which play an important role in coordinating immune responses. Genetic analyses suggested that TVA helped shift these cells in ways that favored responses to microbes and pathogens.
The timing appeared to matter. In the mouse experiments, the immune benefits were seen only when pups were exposed to TVA through breastfeeding after birth. Pups exposed to TVA during pregnancy but then nursed by a foster mother who was not on a TVA-rich diet, did not show the same improved responses to infection.
“We saw that only postnatal exposure to TVA through breastfeeding is important to train the neonatal T cells, and this can have long-lasting imprinting effects,” Chen said. “Even in adulthood, when we challenged the mice with influenza, the ones that were exposed to higher TVA levels during breastfeeding responded better when battling the infection.”
The researchers also reported that mice exposed to TVA-enriched milk responded more quickly when challenged with influenza or Salmonella, with better survival rates than control mice. That finding adds to the idea that early nutrition can influence immune development beyond infancy, at least in animal models.
But mouse studies are not the same as human trials. Animal research can help scientists understand biological mechanisms, especially in early development, but it cannot by itself establish dietary recommendations for human pregnancy, breastfeeding or infant care.
The human part of the study points to questions worth studying next. The researchers found that TVA levels in breast milk and infant blood were connected, suggesting maternal diet may influence how much of the fatty acid reaches infants. They also found immune and health associations in preterm infants, a group especially vulnerable to inflammatory and respiratory complications.
Those findings are not enough to show cause and effect. Preterm infant health is shaped by many factors, including gestational age, medical care, infection risk, feeding needs and underlying complications. More research would be needed to determine whether TVA itself can reduce disease risk in human infants, and whether any intervention would be safe or useful.
Chen said the team hopes to study whether TVA supplementation during pregnancy or breastfeeding, or addition to infant formula, could eventually have benefits. For now, those are research questions rather than clinical recommendations.
“There are close to 40 fatty acids in total in breast milk, along with hundreds of other components,” Chen said. “So, I think it's safe for us to say that we believe there could be additional fatty acids and nutrients that can do something similar.”
The study adds to a growing body of research showing that breast milk is not simply a source of calories. It is a complex biological fluid that can carry signals from mother to infant, including nutrients, immune factors and other compounds that may help shape development.
It also shows why early-life nutrition research needs care. Discovering a promising compound in breast milk does not mean one nutrient explains the benefits of breastfeeding, and it does not reduce infant health to a single dietary factor. Breastfeeding, formula feeding, donor milk, medical needs and family circumstances all exist in the real world, where infant feeding decisions are often complex and deeply personal.
For scientists, TVA may offer one more clue about how diet, breast milk and immune development are connected. For now, the finding opens a door to future research rather than closing the case on what parents or clinicians should do next.
The study was supported by the National Institutes of Health, the National Cancer Institute, the Ludwig Center at the University of Chicago, the Sigal Fellowship in Immuno-oncology and the Harborview Foundation Gift Fund.