Human Milk Composition: Milk is a highly complex biofluid that has evolved over millions
of years to nourish infants and protect them from infection while their immune system
matures. In addition to delivering complete nutrition (i.e. macronutrients and
micronutrients), milk provides bioactive components that further support infant growth,
development and health. These include immunoglobulins, antibodies, hormones, growth
factors, prebiotic oligosaccharides, and probiotic bacteria. Milk composition is
specifically adapted to each mammalian species depending on the growth requirements of
their young offspring. For example, the average energy content of human milk is around 70
kcal/100g, compared to 38 kcal/100g in donkey milk and 171 kcal/100g in mouse milk. Mice
produce just 2 milk oligosaccharides, while humans produce over 150. Even among humans,
milk composition is highly variable - for example, energy content can range from 57-83
kcal/100g and oligosaccharide concentrations range from 5-25 g/L.
Surprisingly little is known about the determinants and consequences of this variation.
The investigators will study the following milk components in the IMiC Consortium to
understand variability between individual women and across different geographic settings,
and their associations with infant growth. In addition, to guide these analyses, a review
of human milk components and infant growth will be undertaken by the IMiC members during
Year 1 of the Project.
Priority Components (to be analyzed in all samples):
Macronutrients include carbohydrates (primarily lactose), proteins and lipids. Lipids
provide about 50% of the energy content in human milk. The vast majority (98%) of milk
lipids are triacylglycerides, with the remainder consisting of diacylglycerides,
monoacylglycerides, free fatty acids, phospholipids and cholesterol. The fatty acid
profile of human milk varies in relation to maternal diet and genetics, particularly in
the long-chain polyunsaturated fatty acids (LCPUFAs), such as arachidonic and
docosahexaenoic acids, which contribute to immune function and neurodevelopment.
Micronutrient quality and concentrations can be compromised by maternal malnutrition.
Micronutrients in milk include minerals (e.g. Zinc, Calcium, Phosphorus, Magnesium,
Iodine, Selenium) and vitamins (A, B1, B2, B6, B12, C, D, E; folate, choline).
Immunoglobulins (Ig) are transferred in human milk, including IgA, IgM and IgG. Infants
are born with immature adaptive immunity, and rely on these maternal antibodies for
defense against pathogens. Soluble IgA (sIgA) is the predominant antibody of human milk;
sIgA-antigen complexes are taken up by intestinal dendritic cells, allowing for antigen
recognition.
Cytokines are multifunctional peptides can cross the intestinal barrier, where they
influence immune activity. Milk-borne cytokines include anti-inflammatory transforming
growth factor (TGF)-b, interleukins (IL)-10 and IL-7, and proinflammatory tumor necrosis
factor (TNF)-a, IL-6, IL-8, and interferon (IFN)-g.
Lactoferrin is an iron binding glycoprotein with antimicrobial activity against many
bacteria, viruses, and fungi. Osteopontin is an extensively phosphorylated acidic
glycoprotein that is present at high concentrations in human milk. It affects immune
functions, intestinal development, and brain development.
Growth factors and hormones in human milk have wide-ranging effects on the infant
intestinal tract, vasculature, nervous system, and endocrine system. Some act locally on
the neonatal intestine and many are absorbed into systemic circulation through the
'leaky' infant gut. Epidermal growth factor (EGF) is critical to the maturation and
healing of the intestinal mucosa. Insulin-like growth factor (IGF) promotes tissue
growth. The metabolic hormones leptin, insulin, adiponectin and ghrelin regulate energy
conservation, appetite and infant BMI.
Human milk oligosaccharides (HMOs) are the third most abundant component of human milk.
Over 100 different HMOs have been identified. These structurally diverse carbohydrates
are not digested by the infant, but are metabolized by the infant's gut bacteria,
providing a selective substrate to help shape the developing microbiome. In addition,
HMOs serve as soluble decoy receptors and prevent pathogen attachment to infant mucosal
surfaces, lowering the risk for viral and bacterial infections. HMOs may also modulate
epithelial and immune cell responses and provide the infant with sialic acid, an
important nutrient for brain development. In the CHILD cohort the investigators have
observed that, beyond genetic secretor status, HMO composition is associated with
ethnicity, lactation stage, parity, geographic location, season of collection, and
breastfeeding exclusivity.
Omics approaches will be applied to broadly assess the complete spectrum of peptides,
proteins, lipids, and metabolites in human milk. Targeted metabolomics analyses to be
conducted using the Biocrates platform (~500 metabolites), untargeted metabolomic
analyses to be conducted by Sapient Bioanalytics via mass spectrometry.
Microbes are present in human milk. Culture-dependent and independent (sequencing-based)
studies have confirmed the presence of bacteria and fungi in milk from healthy mothers.
In the CHILD cohort, the investigators have found that milk microbiota composition
differs by infant sex, method of feeding, maternal BMI, and maternal atopy. It is
estimated that breastfed infants receive 10^4-10^6 bacteria per day, providing a source
of live microbes to seed the infant gut, oral cavity and airways. Studies demonstrating
strain similarities between maternal gut, milk, and infant gut support this hypothesis,
and find that Bifidobacterium spp. constitute the majority of shared taxa between
maternal milk and infant stool. Given the central role of the gut microbiome in infant
growth, metabolism and protection from infectious disease, including in low to middle
income (LMIC) settings, it is critical to understand the origins of these fundamentally
important gut microbes early in life.
A secondary objective of IMiC will be to support data integration across sites to answer
important questions related to 1) the impact of maternal health and nutrition
interventions on breast milk composition, and 2) its relation to infant health, growth
and development. Each site will own its own data and will also be independently
addressing these same questions by site, as originally intended in their own
grants/studies.