The Composition of Breastmilk, Part 2

by Dionna on January 13, 2010

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Most everyone knows that “breast is best” for baby. But why? What makes it so special? In this two-post series, we’ll take a look at what breastmilk is made of. In part 1, I shared information on colostrum and transitional milk, part 2 will present more on mature and involutional milk.

Mature Milk

Human breastmilk* contains more than two hundred recognized components, and each is specifically designed to the needs of infants. (1) These components include proteins, fatty acids, growth factors, vitamins, carbohydrates, and other substances. (2) Mature breastmilk contains different amounts of these components than does colostrum, transitional, or involutional milk.

Not only do the components of breastmilk change depending on the age and stage of the nursling, variations also exist within each nursing session, with the time of day or night, and to some extent with maternal diet. (3) And while maternal diet does have some effect on the composition and taste of breastmilk, “a mother’s breast milk is adequate in essential nutrients, even when her own nutrition is inadequate.” The volume of breastmilk is also relatively constant, regardless of maternal diet. (4)

2009-01-04 (2) (Medium)

Major Components of Mature Breastmilk

1. Nutrients

The major nutrients in mature breastmilk are lactose (a carbohydrate) and proteins. Lactose is essential for energy and brain development, while proteins provide nutrition and perform a vital function in an infant’s immune system. Both carbohydrates and proteins remain relatively constant in mature milk, regardless of maternal diet. (5)

Some proteins have a “direct antibiotic effect on bacteria such as staphylococci and E. coli” found in the child’s intestines. (6) Other proteins, which change in type and amount throughout the nursing relationship, provide specific immunities and other protection from infections to the nursling. For example, there are certain proteins that actually bind to bacteria and viruses in an infant’s intestines and eliminate them. (7) This awesome benefit is why breastmilk has been called “nature’s vaccine.” (8)

“Breastfed babies are protected in varying degrees from a number of illnesses including, pneumonia, botulism, bronchitis, staphylococcal infections, influenza, ear infections, and German measles. Furthermore, mothers produce antibodies to what ever disease is present in their environment, making their milk custom-designed to fight diseases their babies are exposed to as well.” (9)

2. Fats

Fats in breastmilk are “essential for neonatal growth, brain development, and retinal function . . . .” Milk fat composition varies significantly with maternal diet; it also “changes dramatically during each feeding . . . .” Interestingly, fat content in mature breastmilk is positively related to weight gain during pregnancy. (10)

During each nursing session, mothers produce foremilk and hindmilk. Foremilk (the milk that comes first) is thinner, higher in lactose, and quenches baby’s thirst. Hindmilk follows foremilk and is creamier and much higher in fat. Allowing baby to receive a good balance of foremilk and hindmilk (by allowing baby to empty a breast thoroughly) aids in digestion. (11)

3. Vitamins and Minerals

The amount of vitamins and minerals in breastmilk generally depends on maternal intake. Thankfully, women in developed countries generally do not need vitamin supplements as long as they eat a well-balanced diet. Many vitamin and mineral levels increase indefinitely as the mother increases her intake (e.g., vitamin B6), some reach a maximum level even if the mother exceeds an adequate intake (e.g., vitamin C), and some remain constant even if that means the mother’s own store of the substance is depleted (e.g. folate). (12)

4. Growth Factors and Hormones

Growth factors and hormones play a variety of interesting roles. Some growth factors promote the growth of cells in babies’ digestive tracts; others help seal up babies’ intestines, which protects babies from pathogens and harmful foreign substances. (13)

Each time a mother breastfeeds, she releases the hormone oxytocin. Oxytocin decreases heart rate and blood pressure and has a calming effect on both mother and child, which usually results in more sleep for both. The positive benefits of oxytocin last long after the nursing relationship ends: both mother and child are at a decreased risk for heart disease, and oxytocin “reduces the severity of the child’s lifelong reactions to stress.”  (14)

Breastmilk also contains endorphins, which are hormones that act as pain suppressors in an infant. It’s no wonder a baby with a bumped head is quickly soothed at her mother’s breast. (15)


Breastmilk is perfectly designed to nourish infants without supplementation. Even under circumstances where breastmilk contains certain nutrients in lower amounts than formula (e.g., with poor maternal diet), breastmilk’s “higher bioactivity and bioavailability more nearly meet the complete needs of [babies] than do even the best infant formulas.” (16)

In other words, breastmilk is specifically adapted to meet your child’s biological needs. Scientists will never be able to replicate it.

Involutional Milk

When I researched this post, I looked up the definition of “involution.” The results were pretty interesting:
entanglement; a spiraling inwards; intricacy”;
“when something turns in upon itself”;
“Involution of an organ is the shrinking or return to a former size.”

That third definition is closest to the involution of breastfeeding, which is what happens when your child stops nursing. When you are pregnant, your breast tissue changes, enabling your breasts to produce milk. When your breastfeeding relationship ends, your breasts will revert back to their pre-pregnant state. (17)

Not only will the actual structure of your breasts change, but the composition of your breastmilk will change too. (18) Involutional milk has a lower lactose content than mature milk, but higher levels of protein, fat, and sodium. (19) But what is most amazing to me about the weaning process, is that “some of the immune factors in breastmilk increase in concentration . . . during the weaning process.” (20) It’s like our bodies know to give our children an extra boost of goodness! Amazing.

Because your milk changes for the benefit of your child while you wean, it is better to make weaning a gradual, rather than an abrupt, process. Abruptly weaning will deprive your child of these important nutrients and antibodies. (21)


Throughout the nursing relationship, your breastmilk is uniquely suited to meet your growing baby’s needs.

If you are pregnant and want to give your newborn the best start by breastfeeding, I encourage you to educate yourself and to find support. Attend a La Leche League meeting (find groups in your area here), talk to a Lactation Consultant (most hospitals have certified LC’s), find a local attachment parenting group (try the Support Groups offered through Attachment Parenting International or the “Finding Your Tribe” forum at Mothering). All of these resources were invaluable to me and other breastfeeding mothers. And you are always welcome to email me – I will try to give you resources and support or point you in the direction of someone who can.


*The information and research available online about the components of breastmilk is overwhelming. In this post, I attempted to give you a synopsis of what I found most interesting, you can read through the cited material that follows to learn more.
1) Committee on Nutritional Status During Pregnancy and Lactation, Institute of Medicine, “Nutrition During Lactation” at 113, (citation omitted); Hamosh, Margit, PhD, “Breastfeeding: Unraveling the Mysteries of Mother’s Milk,”
2) Nutrition During Lactation at 114 (citations omitted)
3) Prentice, Ann, “Constituents of Human Milk,” (citation omitted); see also
“Foremilk and Hindmilk,”
(4) Breastfeeding: Unraveling the Mysteries of Mother’s Milk
(5) Foremilk and Hindmilk; Breastfeeding: Unraveling the Mysteries of Mother’s Milk
(6) “The Essential Components of Breastmilk,”
(7) Breastfeeding: Unraveling the Mysteries of Mother’s Milk; Wagner, Carol, MD, “Human Milk and Lactation,”
(8) “Nature’s Vaccine,”
(9) “101 Reasons to Breastfeed Your Child,” (citations omitted)
(10) Breastfeeding: Unraveling the Mysteries of Mother’s Milk
(11) Foremilk and Hindmilk
(12) Breastfeeding: Unraveling the Mysteries of Mother’s Milk; “Vitamins and Other Supplements for Breastfeeding Mothers,”
(13) “101 Reasons to Breastfeed Your Child,” (citation omitted); Newman, Jack, MD, “How Breast Milk Protects Newborns,”
(14) Palmer, Linda Folden, D.C., “Nursing: It’s More than Breastfeeding,”; 101 Reasons to Breastfeed Your Child (citations omitted); “Concentrations and Origin of Oxytocin in Breast Milk,”
(15) 101 Reasons to Breastfeed Your Child
(16) Breastfeeding: Unraveling the Mysteries of Mother’s Milk
(17) Langley-Evans, Simon, Nutrition: A Lifespan Approach at 105,
(18) Nutrition: A Lifespan Approach at 105
(19) Prentice, Ann, “Constituents of Human Milk,”
(20) Extended Breastfeeding Fact Sheet (citations omitted),
(21) Nutrition: A Lifespan Approach at 105

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Dionna (20 Posts)

Dionna writes at Code Name: Mama, where she shares information, resources, and her thoughts on natural parenting and life with a toddler/preschooler.

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