Ecologies, synergies, and biological systems shaping human milk composition-a report from "Breastmilk Ecology: Genesis of Infant Nutrition (BEGIN)" Working Group 2

Abstract

Human milk is universally recognized as the preferred food for infants during the first 6 mo of life because it provides not only essential and conditionally essential nutrients in necessary amounts but also other biologically active components that are instrumental in protecting, communicating important information to support, and promoting optimal development and growth in infants. Despite decades of research, however, the multifaceted impacts of human milk consumption on infant health are far from understood on a biological or physiological basis. Reasons for this lack of comprehensive knowledge of human milk functions are numerous, including the fact that milk components tend to be studied in isolation, although there is reason to believe that they interact. In addition, milk composition can vary greatly within an individual as well as within and among populations. The objective of this working group within the Breastmilk Ecology: Genesis of Infant Nutrition (BEGIN) Project was to provide an overview of human milk composition, factors impacting its variation, and how its components may function to coordinately nourish, protect, and communicate complex information to the recipient infant. Moreover, we discuss the ways whereby milk components might interact such that the benefits of an intact milk matrix are greater than the sum of its parts. We then apply several examples to illustrate how milk is better thought of as a biological system rather than a more simplistic "mixture" of independent components to synergistically support optimal infant health.

Keywords: breast milk; composition; ecology; human milk; nutrition; systems biology.

FIGURE 1

Human milk constituents provide myriad benefits to infants, including nourishment (supporting physical growth, development, and metabolism), communication (relaying complex information as to how to best thrive in a particular environment and culture and with a specific genetic background), and protection (shielding the infant from infection and supporting the infant when the infant’s immune system is naive). Whereas some components have a single function, most play multiple roles.

FIGURE 2

Milk composition varies greatly among individuals and populations, and it is possible (albeit understudied) that this variation might differentially support optimal health and survival of infants living in varying ecological contexts, including those related to lactating parent/infant genetics, food/nutrient availability, and environmental exposures (e.g., pathogens). This concept of a shift in “normal” to support health in a particular ecosystem is referred to as ecohomeorhesis (9).

FIGURE 3

Illustration of 3 possible scenarios whereby milk components might “interact” in the mammary gland (type 1 interaction) or in the infant (type 2 and type 3 interactions) and thus impact milk composition and/or infant health.