Because milk is the sole source of food for young mammals, it must provide all the nutrients essential for their growth. Therefore, the mammary gland must synthesize and secrete sufficient milk of appropriate composition to nourish the young, and the young must be able to remove the milk to sustain their growth and ensure continued milk production. This article reviews the synthesis and secretion of breast milk, the changes in the volume and composition of breast milk after birth, the breastfeeding patterns of babies, and the methods by which babies can regulate the milk production of the mother.
Breast milk is synthesized in the lactocytes that line the alveoli of the mammary gland. Blood capillaries surround the alveoli. Substrates (glucose, amino acids, fatty acids, minerals, and vitamins) diffuse from the blood to the basement membrane of the lactocytes and are used for the synthesis of milk components. These are then secreted across the apical membrane of the lactocyte into the lumen of the alveolus.
The milk that is secreted is composed of nitrogenous components (proteins including caseins, alpha-lactalbumin, immunoglobulins, albumin, lactoferrin, nonprotein nitrogen, enzymes, hormones, growth factors, and nucleotides), fats (triglycerides and fatty acids), carbohydrates (lactose, glucose, galactose, and oligosaccharides), minerals, electrolytes, trace elements, vitamins, and water.2 Although all mammals produce milk containing similar components, the concentrations of these components vary markedly between species. This supports the common wisdom that the milk of one species is specifically adapted to the growth of the young of that particular species,3,4 and breast milk is ideally adapted to the needs of the developing infant. Therefore, milk from other species is unsuitable as a substitute.
How Breastfeeding Works