M.D. Candidate University of Washington, University of Idaho Moscow, Idaho, United States
Introduction/Rationale: Breast milk is a dynamic fluid that provides daily variation in bioactive signals to support infant neuro and immune development. In contrast, formula is a compositionally static nutritional replacement, lacking daily bioactive rhythms. Understanding how early-life feeding shapes infant health is imperative. We investigated the temporal variation of cortisol in breast milk and saliva as well as formula and milk alternatives. We compared selected bioactive factors between human milk and infant formulas.
Methods: Breastfeeding dyads (infants aged 3–4 months; n=15) from Moscow, Idaho, were enrolled. Before each feeding over two study days (8–10 feeds per 24 h), maternal and infant saliva were collected, and breast milk was collected from the same breast. Infant formulas and milk alternatives were obtained locally or from donors. Cortisol, IgA, and cytokines were quantified by ELISA; melatonin in breast milk was measured by LC-MS. All assays were run in duplicate; analyses were performed in GraphPad Prism.
Results: We validated cortisol and melatonin quantification in full-fat breast milk. All targets showed inter-individual variability. Normalized breast milk cortisol tracked maternal (not infant) saliva rhythms. Infant saliva cortisol was phase-shifted by ~12 h relative to breast milk. Melatonin was undetectable in daytime milk and present at low pg/mL in some evening/night samples. Cortisol in infant formulas (n=25 brands) ranged 4.78–21.45 ug/mL; products with corn syrup solids trended higher (p=0.1732).
Conclusion: Breast milk exhibits daily rhythms in the bioactives we measured (e.g., cortisol, melatonin, cytokines), whereas infant formula and milk alternatives provide single, static concentrations that vary across products. Such temporal signals may be important for infant neuro immune development. Prospective, longitudinal studies should investigate whether dynamic versus static exposure differentially shapes long term neuro and immune outcomes.
