The Composition of Breastmilk, Part 2

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)

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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. talks about This awesome benefit that is why breastmilk has been called “nature’s vaccine.”


“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.


*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

Author: Dionna

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

17 thoughts on “The Composition of Breastmilk, Part 2”

  1. i just wanted to know that even if we breastfeed our adopted babies..and we take meds to get the breastmilk…will it still contain all the nutrients?!?!will the meds make it like the breastmilk that comes naturally or will it be different?!

  2. That is a really good question Asma, and one I’m not sure I have all of the answers to. has some info/resources:
    But here’s my gut instinct: I would say the immunological protections will be present regardless of whether you are lactating after a pregnancy or whether you have induced lactation. Those are present because of your environment (you will pass on immunities of the things YOU are immune to b/c you are exposed to them).
    As far as the nutrients present in breastmilk – I wouldn’t see why induced lactation would produce milk that is any different from lactating after a pregnancy, but I don’t know for sure. I would suggest going on to’s Facebook page and asking that question, Kelly might answer you herself or find someone who can.
    Good luck!

    1. I worked with many lactation induced Mommas back in the day when I was an LLL leader. My district leader was one as a matter of fact. Just as different people are able to produce differing amounts of milk, mamas produce different antibodies in their milk. The body will adjust to whatever situation you ask of it. Unfortunately as usual these kinds of studies are not done on lactating women because the majority of medical science doe not think of smaller populations because there is no funding for such things. Unfortunately the body that does not go through the different stages of pregnancy has no way of knowing what is happening with hormone levels and all as far as what the baby needs and where they are in the growth chart. It is like being a nursemaid to the king where the babe gets whatever the Mom was fed to feed him and the other babies she was feeding. Diet is especially important and taking care of yourself while producing. It is good that you are giving this gift to your baby as neuroscience is finding that giving your baby the breast and closeness is what makes those neurons grow in relation to cortisol levels. I am a special needs educator now and follow this sort of thing. -Erin

  3. As a breastfeeding mother I want the best for my baby. But my fourth simpily will not nurse. Not real sure why although she has some tounge issues. So the reality is I can not commit to exclusively pumping again, I am weaning off the pump or down to a few times a day At 11 weeks what is the best formula? I do not want to but I see no other choice??

    1. Hi Debbie, I’m sorry you are having trouble! Have you talked to an LC or a breastfeeding-friendly doctor about the tongue issues? If it is truly tongue tie, I’ve heard a lot of good stories about a clip that can really help.
      Here are some resources from KellyMom:
      Here is a story from one of my friends: (who would be happy to talk to you – leave a comment on that post if you’d like to chat!)

      I do not have any experience with formula, so I do not feel qualified to answer that question. You might find some information here:

      Good luck!!

  4. Ha! Mine will never revert back to their pre-pregnancy state! Love reading this though, it makes me want to keep on pumping even though I thoroughly hate it.

  5. My baby was born tongue tied and at 1.5 wks had it fixed. It only took him a few feedings after that on the breast for him to get good at the whole process. My nips were extremely sore by the time we figured out what was wrong and got him in with a specialist, since he had been trying to suck with his gums, poor thing. However, once they healed, our breastfeeding experience was fantastic.

  6. I know this thread is pretty old but I have a question that I haven’t found the answer to. My son is 9 months old and since I went back to work, he is now bottle fed my expressed milk. I have a significant amount of frozen breastmilk that I would like to start giving him because I’m ready to stop lactating. Since the composition I breastmilk changes over time (say month to month), will milk expressed and stored properly in month 4 or 5 still be OK to meet his nutritional needs currently? I’m most concerned with fat content because he’s already on the small side (5th percentile, no medical concerns). Thanks 🙂

    1. From Katherine Wilson-Thompson, IBCLC, member of API’s Editorial Review Board:

      “Absolutely, your milk is appropriate for your baby at any time. Fat content in milk is so variable across time. There should be no worries about not meeting your babe’s fat needs — unless you skimmed the fat before freezing to increase the fat content when he was younger.”

      And from another IBCLC:

      “Yes, breastmilk composition changes over time; however, frozen breastmilk is still healthier to give baby than formula. As baby ages, the fat composition usually decreases, so her milk from 4 to 5 months probably has more fat content than it does now. And breastmilk composition changes from hour to hour, so I wouldn’t be too concerned with baby missing out on too much. Baby should be getting solids in addition to breastmilk for the extra iron that is needed at that age.”

  7. Dionna did you have an IUD inserted or use any new contraceptives? Even condoms? Years ago my MOM a longtime La Leche League leader, found that women with IUDs had trouble lactating. Just curious.
    pedullas AT aol DOT com

    1. Your breastmilk will naturally transition to colostrum, the first milk designed for a newborn, during pregnancy. This milk is perfectly OK for your older child to continue breastfeeding, if you’d like. About 4-7 days after childbirth, your milk will once again transition from colostrum to the mature milk. Your milk supply is based off how much milk is removed. So if you choose to tandem-nurse (breastfeed an older baby and younger baby at the same time), your body will adjust and build a supply to match the demand.

      ~ Rita Brhel, CLC, API’s Publications Coordinator

  8. I’m curious about freezing milk and giving it to you baby later in their develeopmental growth if the milk we are making is designed specifically for their growth at that point in time.

    1. While it’s true that breastmilk is made specifically for the growth and health factors of a baby at that moment, it is safe and healthy to give breastmilk expressed/frozen earlier in the later months. It is not “real-time” like breastmilk is during the act of breastfeeding, but expressed breastmilk still have innumerable benefits in it that make it much more preferable to formula if feeding at the breast is not possible.

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