Project Summary/Abstract Human milk provides significant health benefits for infants. Beyond nutrients, breastmilk contains antibodies for immunity, growth factors associated with gut epithelial maturation, bacteria for establishment of the gut microbiome, and metabolites that modulate inflammation. These important elements of human milk support strong immune system development within the infant gut and better infant health. Unfortunately, many parents aiming to breastfeed will supplement or wean earlier than planned and frequently report perceived low milk supply as the reason. Increased mammary epithelium permeability (IMEP), as indicated by elevated milk sodium, is a physiologic condition that could have significant implications for milk secretion, composition and infant health. Lactation physiology studies have established that closed paracellular pathways, i.e., low permeability, are essential for the establishment and maintenance of adequate milk secretion. Human studies on permeability have focused on the period of secretory activation and persistent mammary epithelium permeability at day 7 postpartum has been associated with perceived low milk supply. However, little is known about IMEP during established lactation. We recently reported that IMEP among US women is more common across lactation than previously recognized. Longitudinal US studies are needed to quantify mammary epithelium permeability during established lactation and to investigate the extent to which increased permeability influences lactation insufficiency. Our overall goal is to establish a clinically relevant robust IMEP threshold, based on our analysis of sodium as a continuous variable, and determine the extent to which IMEP is associated with suboptimal lactation outcomes and reduced infant health. We propose that IMEP is associated with low milk supply and earlier than desired weaning. We also posit that IMEP results in altered milk nutrient content, shifted inflammatory profiles and milk microbiome composition impacting the infant gut immune profile and microbiome. We will enroll a diverse cohort of 400 mother-infant dyads from New Mexico and Massachusetts. Participants will provide bilateral milk and infant fecal samples at four timepoints spanning transitional, early and mature milk. Understanding the extent of IMEP linkage to suboptimal lactation outcomes and infant intestinal health is of major importance because IMEP can be measured, treated and prevented. Interventions to limit IMEP could include behavioral and dietary changes, as well as medications or supplements. This will be the first study to determine the prevalence of IMEP and associated changes in milk throughout the first five months postpartum, and the only large US cohort study of IMEP. Results from this study could lead to a major change in clinical practice. Assessment of IMEP in milk could become routine, especially among women seeking lactation counseling, and this may lead to improved outcomes. We expect that our geographically and ethnically diverse cohorts will provide novel data on differences in risk factors for IMEP with implications for lactation outcomes.

Project Narrative Maintaining a healthy breast microenvironment that supports optimal lactation, as well as the ideal nutrient composition, is important for the growth and development of the human-milk-fed infant. Subclinical mastitis (SCM), an asymptomatic inflammation of mammary tissue during lactation, has been linked with low milk supply, reduced nutritional composition and reduced microbial diversity, yet the prevalence of SCM in the US is unknown. We propose to fill this significant gap by following temporal changes in milk in two lactating US populations to determine the prevalence, as well as the impact on milk composition and infant health, as it relates to the occurrence and severity of SCM.