Understanding human milk composition
By Ardythe L. Morrow, PhD, MSc, Professor Emerita, University of Cincinnati and Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA
Pediatricians and other health professionals worldwide recognize breastfeeding as the gold standard for infant nutrition.
What we do know is that human milk is unique. Scientists have sampled milk from hundreds of mammalian species. Comparative analysis indicates that the milk of each species supports the particular growth and developmental needs of its own offspring. For example, bovine milk has more than three times the quantity of protein found in human milk, which is too high for human infants but supports the rapid growth of calves. Whale milk has about ten times the quantity of fat than human or bovine, which supports efficient transfer of milk in the aquatic environment, and provides the dense energy that whale calves require. Interestingly, human milk has the most abundant and diverse forms of bioactive carbohydrates (oligosaccharide), which supports infant brain, gut, and immune development.
Moreover, mother’s milk provides a unique mix of bioactive components distinct from other milks that help manage the infant’s microbiota and enhance infant development. The most abundant of the bioactive components are the human milk oligosaccharides. In addition, mother’s milk provides antibodies that are specific to the infant’s environment, as well as innate defense proteins, peptides, cells, and other components. These bioactive components appear to contribute to human milk outcomes. For example,breastfeeding lowers infant risk of death and a wide range of infectious, immune disorders and improves infant cognitive development.
Human milk is a dynamic fluid that varies systematically in composition over the course of lactation, by the baby’s gestational age, within a day, and within a feed. Overall, these differences in milk composition are thought to support the stage-specific growth and developmental needs of the infant. Colostrum is the first secretion that the mammary gland produces after delivery. Produced in low quantity for the first few days after birth, colostrum is high in defense proteins. It is followed by transitional milk, and then by mature milk. Over lactation, changes in milk composition depend on the component. For example, levels of protein and oligosaccharide, but not lipids, decline over the course of lactation. Maturity at birth also influences milk composition. For instance, mothers whose infants who are born preterm have higher milk protein levels in the first few weeks of life. Milk composition also varies within a feed. Milk expressed early in a feed (“fore milk”) is lower in fat and cell content than the milk given towards the end of the feed (“hind milk”).
Some milk components differ between mothers and populations, influenced by different causal mechanisms. Differences in composition of fatty acids, vitamins, and phytochemicals reflect maternal dietary patterns. Human milk oligosaccharide levels differ by maternal genetics, while levels of immunoglobulins and other defense proteins vary in relation to the mother’s microbial exposures. But the core composition of human milk is generally consistent among human mothers and observed differences may be “tailoring” milk composition to the circumstances of the infant.
Variation in human milk composition has implications for the medical management of high-risk infants, and should be considered to assure adequate growth. Breastfeeding mothers should be aware that their own milk, produced at each stage of lactation, is the best match to the infant’s developmental needs. Given the variations in composition of milk within a feed, mothers should fully empty one breast before starting to feed using the other. When a mother cannot breastfeed, her choices are donor milk or an infant formula that is designed to approximate human milk composition and function. No formula replicates mother’s own milk, which is a personal, complex and dynamic fluid. But as the science of human milk advances, the findings are helping to improve infant nutrition.