Predictors of milk cortisol in North American women.

OBJECTIVES: Human milk content varies across mother-child dyads, environments, and populations. Among the hormones in milk is cortisol, a glucocorticoid; its impact on the breastfeeding child is unknown. Milk cortisol may constitute a signal to the child's developing physiology which can shape characteristics (e.g., growth, temperament) to prevailing environmental conditions. This exploratory study evaluated the maternal, breastfeeding, and infant characteristics associated with milk cortisol. METHODS: We evaluated archived milk specimens for cortisol using enzyme immunoassay and employed an information-theoretic approach to assess associations between milk cortisol and participant characteristics with linear regression modeling. Because we employed secondary data, information for some variables likely to impact milk cortisol variation (e.g., time of day, socioeconomic status, maternal or infant body mass index, milk energy density) was unavailable. RESULTS: Participants were 48 lactating mothers from upstate New York, aged 21-40 years. Milk cortisol ranged from 0.098 to 1.007 µg/dL. Child age ranged from 1 to 26 months. In linear regression employing best fit modeling criteria, milk cortisol increased with child age (B: 0.069; p: .000; a 7.1% increase in milk cortisol for each month of child age), while child symptoms of illness (B: -0.398; p: .057; a 33% decrease) and consumption of complementary foods (B: -.525; p: .020; a 41% decrease) were associated with lower milk cortisol. CONCLUSIONS: We speculate that increasing milk cortisol with child age plays a role in signaling development (e.g., as increasing independence increases risk for injury and other negative health outcomes), independent of the maternal stressors we could capture.

Associations between milk cortisol and activity of the immune system of milk.

OBJECTIVES: Both the immune system of human milk and milk cortisol have complex short- and long-term effects on child health and development. As understanding continues to grow of the independent effects of each of these components of milk, it is also important to investigate their intersection, including how milk cortisol affects the immune system of milk. We began this important endeavor through secondary analyses of archived milk specimens. METHODS: Participants were 31 lactating mothers from upstate New York. We estimated milk cortisol concentrations via enzyme immunoassay. We assessed milk proinflammatory cytokine (interleukin-6, IL-6) responses to pathogenic (Salmonella) and commensal (Escherichia, Lactobacillus, Bifidobacterium) bacteria via in vitro stimulation. We estimated ordered logistic regression models to assess associations between milk cortisol and IL-6 responses to bacteria. RESULTS: Milk cortisol ranged from 0.098 to 1.007 µg/dL. Milk cortisol was positively associated with IL-6 responses to S. enterica (B: 4.035; 95% CI: 0.674, 7.395) and B. breve (B: 3.675; 95% CI: 0.426, 6.924); this association persisted after controlling for child age. Results were less clear for associations between milk cortisol and IL-6 responses to L. acidophilus (B: 2.318; 95% CI: -1.224, 5.859) and E. coli (B: 2.366; 95% CI: -0.960, 5.692). CONCLUSIONS: Complex interactions between cortisol and the immune system extend to milk. Milk cortisol was positively associated with proinflammatory responses to some bacteria in vitro. This may suggest that milk cortisol is causally upstream of protective immune activity.

In Vitro Stimulation of Whole Milk Specimens: A Field-Friendly Method to Assess Milk Immune Activity.

BACKGROUND: The immune system of milk protects against infections and guides immune system development. A system-level understanding of milk immune activity is critical for research into infant infectious disease risk and lifelong health. RESEARCH AIM: To describe a protocol to characterize immune activity in human milk via in vitro stimulation for use in population-based (rather than clinical) research. METHODS: This study proceeded in two phases, each with a cross-sectional design. Human milk specimens were incubated for 24 hr at 37 °C in mammalian cell culture medium with stimuli (e.g., Salmonella enterica) in a CO2-enriched environment. Immune responses to stimuli were characterized as the change in cytokine: [stimulated]/[baseline]. Predictors of cytokine responses were evaluated with generalized linear models. RESULTS: Patterns were detectable across mother-child dyads: Interleukin-6 responses to stimuli were generally positively associated with child age and with maternal autoimmune disease. CONCLUSIONS: Our method allows characterization of pro-inflammatory milk immune activity in vitro in population-based (rather than clinical) research settings. In vitro activity has a system-level interpretation and is likely to be of broad utility in global health research in settings with high infectious disease risk, where understanding the immune system of milk is critical to understanding maternal and child health.