AUSTIN, Texas — Swapping beef for a plant-based meat substitute changed breast milk composition in just six days — even when the rest of the diet was made up of whole, unprocessed foods — according to a first-of-its-kind study from researchers at The University of Texas at Austin .
The findings, published in the American Journal of Clinical Nutrition, reveal that even small shifts in a breastfeeding family's diet can quickly alter the types of fats their infant receives through breast milk. These changes could have implications for brain development and immune function. The study tested the effect of food processing by using meals that were nutritionally balanced and identical in every way except for the main protein: Either whole-food beef or a popular plant-based substitute classified as ultra-processed.
In the randomized clinical trial involving 24 families, researchers provided meals to local Austin mothers and their infants over a 25-day period. While total fat remained the same, the type of fats in breast milk changed with each diet. Mothers who ate the plant-based substitute had lower levels of long-chain polyunsaturated fatty acids, which are known to support infant neurodevelopment, and higher levels of saturated fats derived from tropical oils, commonly used in ultra-processed foods.
"We've known that breast milk reflects what moms eat, but we were surprised by how quickly and clearly we saw these changes, and from just one food swap," said Marissa Burgermaster , Ph.D., lead author of the study and assistant professor in the Department of Population Health at Dell Medical School . "This kind of research helps fill an important gap for breastfeeding families who want to make informed decisions, especially in a market full of ultra-processed plant-based alternatives that may not be nutritionally equivalent to whole foods like beef."
This is the first study to isolate the effect of an ultra-processed food on breast milk composition in a real-world postpartum diet, using meals made entirely of whole foods, except for the plant-based meat substitute.
"As a behavioral nutrition scientist, I'm always thinking about how food choices intersect with health, identity and access," said Burgermaster, who is also an assistant professor of nutritional sciences in the College of Natural Sciences . "We're not saying one food is 'good' or 'bad,' but we do want people to know that even foods with similar 'nutrition facts' have important differences. In the past few years, we have seen many epidemiological studies demonstrate relationships between ultra-processing and poorer health outcomes. Now we are starting to get closer to uncovering just how much they matter and why."
Although changes in the milk's fatty acid profile mirror those linked in past research to cognitive and immune development, the researchers emphasize that the study did not evaluate long-term health outcomes for infants and that the limited number of days for the intervention would be unlikely to produce long-term differences in outcomes specific infants in this study.
Beyond the scientific findings, the research team also observed strong enthusiasm among new mothers to contribute to nutrition research — a population often underrepresented in clinical trials. Participants expressed strong interest in learning how their diets affect their babies, highlighting a need for clearer, evidence-based dietary guidance during lactation.
The study was funded by the National Cattlemen's Beef Association, a contractor to the Beef Checkoff, and the Texas Beef Council. Neither organization was involved in the study design, data collection, analysis or publication of the findings.
Other authors of the study, all from UT Austin, were Summer Messer, Erin Hudson, Madalyn Rosenthal, Heather Leidy, Yan Ning Li, J. Thomas Brenna, Hui Gyu Park, Nitu Dahale, Lisa Kan, Jenna Lan Mai, Elizabeth M. Widen, Lorie Harper and Michele Hockett Cooper.
