Just What Is Breastmilk, Anyway?

All mammals have mammary glands, and in females, these glands have evolved to produce milk. Milk is so much more than simply a food source for newborn offspring.

In addition to containing the most appropriate combination of fats, proteins, and sugars to sustain species-specific infant, growth, milk contains innumerable maternally-derived signals that contribute to many aspects of juvenile growth and development – hormones, immunological factors, growth factors, and so much more.

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It is highly digestible, variably concentrated, and nutritionally balanced.

Compounds in Breastmilk

Many of the compounds found in mammalian milk are not found elsewhere in nature.

There are unique milk proteins called caseins (alpha, beta, and gamma caseins) and whey proteins, which provide amino acids to infants for growth and immunological provisioning against invading pathogens. They also play a role in fat digestion and uptake of nutrients from other milk components. Milk proteins play other physiological roles that scientists have yet to fully understand. For example, there are over 100 proteins in human milk, each serving its own biological purposes.

Minerals are another important component: key minerals provided are calcium, phosporous, sodium, magnesium, and potassium.

There are also milk fat globules (MFGs) which are essentially membrane-enclosed lipid drops secreted as unique structures, and these are an evolutionary phenomenon of mammalian lactation. Milk fats provide support for metabolic energy as well as immune and developmental signaling. For example, large-brained animals exhibit a heavy requirement for polyunsaturated fatty acids for normal brain development.

Milk sugars are another key component, and the main sugar in placental mammals is lactose. In monotremes and marsupials, the main sugars are oligosaccharides.

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Hormones in Breastmilk

In addition to the nutritional components of milk, there are hormones that can act individually or synergistically to regulate infant growth, physiology, and behaviour. These are hormones such as insulin-like growth factor, glucocorticoids, leptin, and adiponectin.

Bioactive molecules, growth factors, and enzymes are other milk components whose specific roles are being investigated but are likely impactful on things like infant growth, behaviour, metabolism, and even sexual maturation.

Research on Breastmilk Composition

The complexities of breastmilk composition are illustrated by work done on rhesus monkeys. Although most work has been done on its nutritive components, scientists are just now starting to unravel the additional resources, like bioactive molecules, provided in breastmilk. Many of these factors are as fluid as the milk itself.

Bioactive molecules are those that have an active function in the body of the ingesting animal (in this case, the infant).

For example, in rhesus monkey milk there are bioactive cytokines that bind to receptors in the infant gut. Instead of being digested, these molecules have a direct role in the further development of the infant gut. They are just one of a suite of bioactive factors that is being investigated.

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Rhesus macaque mothers have different combinations and concentrations of milk bioactives that are influenced by factors like social rank, mother’s weight, and parity (how many babies a mother has had previously). The fact that these important messages are being transmitted to baby macaques based on both physiological and ecological conditions of individual mothers is something that is likely important in other primate species as well.

This post is an adapted excerpt from my book “Wild Moms: Motherhood in the Animal Kingdom

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