Written by: Lauren Milligan Newmark, Ph.D. | Issue # 125 | 2025
- A study of over 600 one-year-old infants identified significant associations between the concentration of five serum immune proteins and infant diet.
- At one year of age, infants consuming human milk had higher concentrations of bioactive proteins associated with immune tolerance and suppressing inflammation and a lower concentration of a protein associated with increased inflammation compared with infants no longer consuming human milk.
- These results fill a gap in research by identifying potential mechanisms by which human milk feeding helps the infant immune system learn to tolerate harmless antigens and suppress inflammation.
The human newborn’s immune system has the challenging task of learning to distinguish harmful pathogens from harmless environmental antigens. Mounting the appropriate response—attack or tolerate—without prior experience is like building the airplane while flying it. But here’s where human milk comes in. In addition to providing infants with mom’s experienced antibodies, human milk contains a multitude of ingredients that provide on-the-job training for the infant’s naïve immune system.
Previous research on how this “training” influences the activity or function of infant immune cells has largely focused on identifying an association between human milk intake and infant health outcomes (e.g., comparing the incidence of asthma between infants fed human milk or formula) or investigating the function of specific milk components in animal models. Surprisingly, research on the association between human milk intake and human infant immune profiles— the molecules and mechanisms that regulate immune responses—has received less attention [1].
A new observational study [1] addressed this blind spot by investigating the association of human milk feeding practices with the concentration of 76 serum immunological markers. The study participants were part of the long-running Canadian CHILD Cohort Study, which followed (and continues to follow) over 3,000 infants from birth through adolescence. From this larger cohort, 605 infants had sufficient data to be included in the final analyses. Thorough data on environment, health, family, and diet for each participant were collected from questionnaires and clinical visits during the second trimester and at birth, three months, six months, one year, and three years of age. Blood samples were drawn at the one year visit. This large and comprehensive dataset allowed the research team to control for factors other than human milk that can influence serum biomarkers, such as maternal age, household pets, parental allergy history, and age at introduction to formula or solid foods [1]. After taking all these other factors into account, does human milk feeding explain the variation among infants in serum biomarkers?
Current milk feeding status was significantly associated with the concentration of seven of the 76 biomarkers. Six of the biomarkers (CCL20, CD244, CXCL6, FGF-21, IL-10, and TRAIL) were significantly higher and one (EN-RAGE) was significantly lower in infants that were receiving human milk at the time of the blood draw (n=258) compared with those that were not receiving human milk (n=347). Five of these seven biomarkers (CCL20. CD244, CXCL6, FGF-21, and EN-RAGE) were also found to be significant predictors of current infant human milk feeding status [1].
One obvious explanation for the significantly higher quantities of serum CCL20, CD244, CXCL6, and FGF-21 in infants currently receiving human milk is that the biomarkers came directly from human milk. Although they are found in human milk, this study was not able to determine if the biomarkers measured in infant serum were made by the mother (and passed along in milk) or by the infants [1]. It also is unknown whether these biomarkers can cross the intestinal barrier (where they would end up if provided by milk) and enter the infant’s circulation (where they were measured). Moreover, numerous biomarkers measured in infant serum also present in milk did not have a significant association with milk feeding.
An alternative (and not mutually exclusive) explanation is the production (whether it be to make more or make less) of these biomarkers by the infant was regulated by other ingredients provided by human milk. For example, the researchers highlighted how milk fatty acids can stimulate liver cells to produce FGF-21 (a peptide hormone) [1].
Similar biological functions among these five biomarkers further suggest that their increased (or decreased) levels are not solely related to their inclusion (or exclusion) in the infant’s diet. Higher levels of FGF-21 in animal models have been associated with promoting maturation of B and T cells and reducing the production of pro-inflammatory molecules. CD244 is a surface protein expressed on many types of immune cells, including T cells, and has immunoregulatory functions. Depending on which type of immune cells they are expressed, CD244 protein receptors can either activate or inhibit an immune response. CCL20 and CXCL6 both help to promote a more anti-inflammatory gut environment. EN-RAGE—the only biomarker that was found to be significantly lower in infants receiving human milk at the 12-month blood draw—is a pro-inflammatory cytokine. Lower concentrations of EN-RAGE along with higher concentrations of CCL20 and CXCL6 suggest a lower degree of gut inflammation associated with human milk feeding [1]. Taken together, these findings suggest infants currently receiving human milk have more anti-inflammatory immune cells than those not receiving human milk [1].
Human milk feeding history, specifically the duration of human milk feeding, was only significantly associated with one biomarker, IL-7, but this protein is also functionally similar to the previously highlighted biomarkers. Physiologically, IL-7 influences the function and activity of naïve T cells (one type of white blood cell), including increasing their proliferation (T cells dividing and increasing in number) and differentiation (T cells becoming specialized cells, such as CD8+ cells). The researchers suggested that IL-7 provided by human milk may therefore be responsible for the previously reported positive association [2] between human milk feeding duration and the quantity of different types of T cells, including regulatory T cells (T regs), CD4+ T cells and CD8+ T cells [1]. The association of this biomarker with feeding duration but not feeding status suggests that this biomarker may remain elevated in infants for months after human milk feeding ends. This hypothesis is supported by this study’s finding that IL-7 concentrations were lower among infants who stopped receiving human milk more than five months prior to the blood draw [1].
The biomarkers highlighted by this study represent potential mechanisms by which human milk “trains” the infant immune system to suppress inflammation and tolerate harmless antigens. Whether this training happens by directly providing critical bioactive proteins as milk ingredients or by increasing (or suppressing) their production by infant cells through the presence of other milk ingredients, or both, is not known. Indeed, this study [1] was the first to identify an association between these biomarkers (except for FGF-21) and current human milk feeding.
And that is the purpose of observational studies—to look for associations among variables and generate hypotheses to test in experimental studies. This study does that in spades, identifying at least five potential pathways for regulating the development of the infant immune system. Expanding this research with experimental studies will not only help bolster the current recommendations by the World Health Organization to encourage human milk feeding up to two years but also can potentially identify mechanisms that can improve health outcomes of formula-fed infants.
References
- Ames SR, Lotoski LC, Rodriguez L, Brodin P, Mandhane PJ, Moraes TJ, Simons E, Turvey SE, Subbarao P, Azad MB. Human milk feeding practices and serum immune profiles of one-year-old infants in the CHILD birth cohort study. The American Journal of Clinical Nutrition. 2025 Jan 1;121(1): 60-73.
- Wood H, Acharjee A, Pearce H, Quraishi MN, Powell R, Rossiter A, Beggs A, Ewer A, Moss P, Toldi G. Breastfeeding promotes early neonatal regulatory T‐cell expansion and immune tolerance of non‐inherited maternal antigens. Allergy. 2021 Aug;76(8): 2447-60.