Researchers uncover how breast milk acts like a biological clock, delivering timed hormonal and microbial signals that may influence a baby’s circadian rhythm, immunity, and long-term development.
Study: Day/night fluctuations of breast milk bioactive factors and microbiome. Image credit: comzeal images/Shutterstock.com
A new study published in Frontiers in Nutrition demonstrates day-night fluctuations in human breast milk composition, which may impact infant maturation, particularly when expressed breast milk is fed at a different time from when it is originally produced.
Background
Breast milk is considered the optimal source of infant nutrition, providing numerous immunological and developmental benefits. Breast milk contains various bioactive compounds, including human milk oligosaccharides, hormones, immune factors, and microorganisms that help establish the infant's gut microbiota.
Several factors can influence the composition of breast milk, including maternal diet, maternal health, infant age, geographical location, and timing of milk production. Circadian rhythms, the 24-hour biological cycles driven by an internal body clock synchronized with the environmental day-night cycle, also play a crucial role in modulating breast milk composition.
The circadian system matures in infants during the initial years of life, and several factors, including gestational age and duration of light exposure, influence it. Breast milk bioactive compounds also facilitate circadian system maturation by providing circadian cues.
Given the significant impact of breast milk components on the maturation of biological systems in infants, the current study aimed to investigate the day-night fluctuations in breast milk composition and microbiome dynamics. Understanding these fluctuations is particularly important for assessing the impact of expressed breast milk on infant development and maturation, since there is an asynchrony between milk production and infant feeding time.
Study design
The study included 38 lactating mothers who expressed breast milk over a period of 24 hours at four consecutive timepoints (6:00 am, 12:00 pm, 6:00 pm, and 12:00 am).
Breast milk samples were analyzed by Enzyme-linked Immunosorbent Assay (ELISA) to quantify the levels of bioactive compounds, including cortisol, melatonin, immunoglobulin A (IgA), lactoferrin, and oxytocin.
DNA extracted from breast milk samples was sequenced to assess microbiome composition.
Key findings
The study analysis revealed significant day-night fluctuations in breast milk hormonal and immune compounds, with melatonin and cortisol showing more substantial fluctuations. Melatonin levels peaked at midnight, while cortisol levels peaked in the morning.
The levels of oxytocin, IgA, and lactoferrin did not change significantly over 24 hours. However, lactoferrin levels showed significant day-night fluctuations for mothers with infants aged 1 to 6 months. Moreover, lactoferrin and IgA levels exhibited variations across timepoints only when separated by infant age, maternal body mass index (BMI), or infant sex. Mothers of female infants showed stronger day-night variations in IgA and lactoferrin than mothers of male infants.
Regarding microbiome composition, overall alpha- and beta-diversity remained stable across the 24 hours. Alpha diversity increased mainly with infant age, with only a small change detected between two timepoints in the <1-month age group.
Notably, breast milk exhibited an enrichment of skin-associated bacteria during nighttime and an enrichment of environmental soil-associated bacteria during daytime. These changes were associated with maternal BMI and infant age. Network analysis also revealed that different bacterial taxa were most interconnected at different times of day. For example, Bacteroides and Prevotella in the morning, and Bifidobacterium and Megasphaera at midnight, indicating temporal shifts in microbial interactions even when overall diversity was stable.
Study significance
The study found significant changes in breast milk bioactive compound levels, notably cortisol and melatonin, over 24 hours. This highlights the significance of preserving natural time-related dynamics in breastfeeding, as disruptions may impact infants’ development, metabolism, and immune regulation.
According to the study findings, cortisol levels in breast milk decrease with infant age, whereas melatonin fluctuations become more pronounced over time. This highlights the potential influence of infant age in regulating breast milk composition.
The study finds the highest IgA and lactoferrin levels in mothers' breast milk with infants aged less than one month. The levels start declining as the infants mature. Given the roles of IgA and lactoferrin in immune defense and microbiome development, these findings suggest a shift in infant immune needs with the stabilization of gut microbial colonization and highlight the importance of breast milk in immune system maturation during the first year of life.
The study highlights a potential impact of maternal BMI on circadian fluctuations in breast milk hormones. The blunted fluctuations in cortisol and melatonin levels observed in obese mothers suggest possible long-term metabolic implications, though the authors caution that this interpretation remains speculative.
The study observed a nighttime increase in skin-associated bacteria and a daytime increase in environmental bacteria, which may reflect differences in maternal activity patterns, feeding behavior, and infant oral contact.
The microbiome analysis reveals distinct changes in microbial composition over a 24-hour period, despite a stable alpha and beta diversity. These fluctuations may influence infant gut colonization and immune priming.
The study findings have important implications for breastfeeding practices for mothers who express and feed milk via bottle at a time different from the original milk production time. The study also notes that while oxytocin levels remained stable across the day, expressed milk may not reflect the natural oxytocin peaks triggered by direct breastfeeding, raising questions about potential effects on maternal–infant bonding and stress regulation.
Since breast milk composition is naturally timed to support infant circadian rhythms, altering feeding schedules could disrupt biologically timed cues for infant sleep, metabolism, and immune development.
Future studies are required to understand the long-term impact of expressed breast milk feeding on infant’s circadian development.
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