How Does Human Milk Change Over Time?

Human Milk


Babies are constantly changing from the day they’re born – and their nutritional needs change along with them. As a newborn baby grows and develops, their newly formed stomach expands and their body requires more calories from new sources.

That’s why human milk changes – to accommodate the fluctuating demands of a growing baby. Human milk has evolved to be the most effective form of nourishment at every stage of a baby’s development.

Unfortunately, infant formulas simply do not capture this dynamic quality.

What Are The Stages of Human Milk?

Human milk goes through three main stages: colostrum, transitional milk, and mature milk..

Colostrum is produced in a mother’s mammary glands before and shortly after birth, and will continue to be produced several days after birth. This early milk is rich in proteins, vitamins, minerals, and immune-supporting components necessary for a newborn’s uninitiated immune system.

Transitional milk begins to form in a few days to a week following birth. Transitional milk lasts for approximately two weeks after birth. The volume of milk increases as the colostrum becomes more watery during this transitional stage. The milk formed is light yellow in color, and continues to be rich in fats, carbohydrates, and fat-soluble vitamins. The protein concentration of this milk also begins to decrease compared to colostrum.

Mature milk is created by the mammary glands two weeks after birth. Mature milk contains the most water of the three milks, with protein levels dropping off even further until the composition of the milk plateaus. This milk is even lighter in color than transitional milk.

Two forms of mature milk are produced during feeding, foremilk and hindmilk. Foremilk is watery and provides calories in the form of carbohydrates and protein, while hindmilk is rich in fat and helps to satisfy the hunger of the newborn.

Why is Colostrum Called “Liquid Gold”?

Colostrum is known as liquid gold because it is a highly nutritious form of human milk. This human milk must be very dense in calories and nutrients because newborns have a very small stomach and cannot accommodate large volumes of liquid.

Colostrum is easy for infants to digest. Infants are highly sensitive due to their underdeveloped immune system and gastrointestinal tract. The whey-to-casein protein ratio at this stage is 90:10, which is significant because whey proteins are much easier for a baby’s stomach to handle. Colostrum also contains a higher concentration of protease enzymes compared to the later stages of milk, which helps a baby break down and absorb these proteins.

The main function played by colostrum is to support immunity. As such, colostrum contains high levels of immune-boosting factors, such as lactoferrin and immunoglobulins, and low levels of lactose as a source of calories. Colostrum possesses the highest concentration of human milk oligosaccharides (HMO) at 25 grams per liter, which help seed the growth of beneficial bacteria in the young gastrointestinal tract.

Other components that contribute to the beneficial effects of colostrum for a baby’s growing body are the bioactive molecules found in this milk.

For example, colostrum contains the following growth factors: transforming growth factors (TGF-alpha and TGF-beta), insulin-like growth factors, fibroblast growth factors, epithelial growth factor (EGF), platelet-derived growth factor (PDGF), and vascular endothelial growth factor (VEGF). All of these growth factors help promote the development of essential tissues and organs, such as the circulatory system, the gastrointestinal system, and the skeletal structure, as these functions are vital to the early days of the infant

Proline-rich polypeptides (PRP) are another class of bioactive molecules found in colostrum. These compounds are known to play roles in neurodevelopment, immunity, and metabolism. Another benefit to colostrum is that it serves as a mild laxative to help rid the infant body of excess blood cells built up before birth, which helps reduce the risk of infant jaundice caused by the breakdown of these blood cells.

What Happens During The Later Stages of Lactation?

Transitional milk has some characteristics in common with colostrum, however, this stage of human milk represents a ramp up in milk production to help the baby grow and develop. The protein concentration of transitional milk drops relative to colostrum as the milk becomes more watery to quench the thirst of a growing baby. The levels of lactose, calcium, and potassium increase while sodium, chloride, and magnesium decrease to maintain balance in osmolarity.

The whey-to-casein ratio declines to 70:30, with a decrease in alpha-lactalbumin, lactoferrin, and other albumins. This increase in casein content allows the infant to slowly adapt to more complex and difficult to digest milk proteins. The incomplete breakdown of caseins by specific enzymes released by the infant’s gastrointestinal system also produces bioactive peptides known as casomorphins, which contribute to the formation and structure of the human brain.

Transitional milk also contains a different set of immunological proteins that begin the transition from the direct pathogen-killing proteins found primarily in colostrum to proteins that promote an independent immune system found in mature milk.

Mature milk contains 90% water, with the other remaining 10% made up of carbohydrates, proteins, fat, and minerals. Of the three stages of human milk, mature milk contains the lowest concentration of protein, with a whey-to-casein ratio of 60:40, and HMOs at 10 to 15 grams per liter. Protein synthesis also declines in favor of fatty acid synthesis, with the lipid content of the milk increasing to accommodate the rising caloric needs of the baby.

As the final stage of human milk, mature milk is relatively stable, with minor fluctuations in composition over the first year of an infant’s life. And of course, a baby will continue to be nourished through this mature milk until they are prepared for new sources of food.

Human evolution has already precisely engineered a dynamic source of nutrition that contains everything a baby requires to thrive, at precisely the right time. The changing nature of human milk is perfectly aligned with the first few weeks of a newborn’s life, arguably the most important time to give a growing baby exactly what his or her body needs.