Key takeaways:
- Switching from beef to a plant-based meat substitute for just 6 days significantly altered breast milk fatty acid composition.
- Changes occurred even though the rest of participants’ diets consisted of whole, unprocessed foods.
- Major and minor fatty acid profiles differed between diets, including reductions in DHA during the substitute phase.
- No differences were observed in maternal weight, satiety, or postprandial appetite between the 2 dietary conditions.
- Findings suggest that replacing a single whole food with an ultraprocessed substitute may influence nutrients vital for infant brain and immune development.
Researchers from the University of Texas at Austin have reported changed breast milk composition when switching beef with a plant-based meat substitution within 6 days, publishing their findings in The American Journal of Clinical Nutrition.1
These changes were found even in patients with the remainder of their diet being made up of whole, unprocessed foods. Investigators concluded this data may have implications for brain development and immune function.
“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, PhD, lead study author and assistant professor at Dell Medical School.
Comparing beef vs substitute
Investigators conducted a double-blind, randomized crossover-controlled feeding trial to evaluate how human milk fatty acid is influence by swapping beef with a substitute.2 Pregnant women were recruited between May 2023 and August 2023 and followed through 6 weeks postpartum.
Additional eligibility criteria included speaking English and having an infant aged 6 to 12 weeks during the study period. Mothers also had to be willing to feed infants exclusively with their milk, consume both beef and a substitute during the study period, and limit caffeine intake to under 1 cup of coffee or black tea per day.
For 6 days before the intervention period, participants were asked to continue their habitual diet. Then, they were randomized to take either beef or a substitute, only consuming the meals and snacks provided during this time. The diet was followed for 6 days, during which patients were provided 3 meals and 3 snacks per day.
Dietary compliance
Meals were designed following recommendations from the United States National Academies of Science, Engineering and Medicine, and all ingredients except the “main” food were identical between groups. These were whole, non-ultraprocessed food ingredients.
Menu checkoffs completed after meals and snacks were used to monitor dietary compliance. Baseline characteristics were obtained during the study orientation, and milk samples were collected using a handheld breast pump and 4-oz food safe, human milk storage bags, with participants collecting 3.5 mL human milk samples per day.
Additional procedures included maternal weight, infant intake, satiety, and glucose metabolism. Human milk docosapentaenoic acid (DHA) was the primary outcome, though 27 total fatty acids were quantified.
Major differences in fatty acid profiles
There were 17 participants included in the final analysis, most of whom were non-Hispanic, White, in their mid 30s, with a normal body mass index, high education attainment, and high household income. All patients met the 80% threshold to be considered compliant with the diet interventions, with only 3 reporting deviations over 5% on any day.
Approximately 72 human milk samples were provided by each participant. Significant variations in both major and minor fatty acid categories were reported between the beef and substitute groups. Overall, 43.97 ± 1.25% of the fat in beef was comprised of saturated fatty acids (SFAs), vs 53.01 ± 1.69% in substitute.
The substitute samples had greater concentrations of the major SFAs capric acid, lauric acid, and myristic acid, with a concentration of 43.6% of total fat content. In comparison, beef had greater concentrations of palmitic acid, palmitoleic acid, and stearic acid, with a concentration of 42.7%.
Human milk during the substitute diet also had reductions in the following minor acids vs the beef diet:
- Myristoleic acid
- γ-linolenic acid,
- Arachidic acid
- Dihomo-γ-linolenic acid
- DHA
Implications
Significant changes were not observed in maternal weight, postprandial appetite control, or satiety between the 2 diets. Overall, the results highlighted changes in human milk composition from replacing a single whole food with an ultraprocessed plant-based substitute.
“Given that milk lipids provide nearly half of an infant’s energy intake and serve as essential building blocks for brain development, even modest compositional shifts may have physiological consequences,” wrote investigators.
References
- Beef vs. plant-based meat: UT Austin study finds diet alters breast milk composition in under a week. University of Texas at Austin. November 19, 2025. Accessed December 4, 2025. https://www.eurekalert.org/news-releases/1106733
- Messer S, Hudson E, Rosenthal M, et al. The effect of consuming diets containing beef compared with plant-based beef substitute on human milk composition in the study of nutrition in postpartum and early-life (SUPER) randomized crossover feeding trial. The American Journal of Clinical Nutrition. 2025. doi:10.1016/j.ajcnut.2025.11.001
