Mother’s Milk


Human milk is potent stuff. The Greek for milk was gala, and as you might be able to see if I hadn’t had to reduce this Tintoretto so much, the galaxies were created from the spray of milk from Hera’s breasts. Modern astronomers might quibble with that explanation of the origins of the extrasolar universe, but what do I know…I’m a biologist. I’ll stick to biology now, and with that, here’s a short summary of the biology behind mother’s milk.


A recipe:

Lactose 7.3 g/dl
Oligosaccharides 1.2 g/dl
Milk Lipids  
Triglycerides 4.0 %
Phospholipids 0.04 %
Caseins 0.2 g/dl
α-Lactalbumin 0.2 g/dl
Lactoferrin 0.2 g/dl
Secretory IgA 0.2 g/dl
Sodium 5.0 mM
Potassium 15.0 mM
Chloride 15.0 mM
Calcium 8.0 mM
Magnesium 1.4 mM

Milk is a rich (but not too rich) food source for suckling infants. It contains sugars and fats enough to keep a baby alive; cow’s milk is roughly similar in fat content at about 5%. That fat content is not particularly high (marine mammals hold the record: some seal milk has a fat content of 50-60%), though, so infants need to nurse at frequent intervals during the day and night, as all parents know.

A little-appreciated fact is that we mammals are generally fairly slimy creatures. Our skin is a glandular organ that is constantly secreting fluids, oils, fats, and proteins, many with a defensive function. Our sweat and tears contain a protein called lysozyme, an antibiotic agent that digests bacteria. We also dribble out a particular kind of antibody called immunoglobulin-A, which can bind to and coagulate bacterial invaders. Note that human milk contains the same agents, in higher concentration; α-lactalbumin is a protein related to lysozyme. Milk also contains lactoferrin, a protein that carries iron and also has antibiotic and fungicidal properties. During the first few days of lactation, the breast secretes an immunological cocktail called colostrum exclusively, and only later adds the fats to the mix. Milk is thought to have evolved from simple, sweaty secretions from the skin that were lapped up by the infant, and we see something similar in monotremes that just secrete milky fluids from nippleless glands on their chest.

Several hormones regulate milk production. The glands of the breast are normally quiescent, until triggered by estrogen to mature and begin actual milk production. The elevated levels of estrogen in the later months of pregnancy prepare the breast for secretion; the breast doesn’t actually produce anything because a second hormone that is maintained at high levels during pregnancy, progesteron, suppresses secretion. After birth, when progesterone levels decline, the breast is then ready and free to begin making milk.

Another signal is needed to actually trigger secretion by the cells of the gland: prolactin. The production of prolactin by the anterior pituitary stimulates milk synthesis and production by the glands that have been prepared by estrogen. What triggers prolactin? Another hormone, oxytocin. Stimulation of the nipple triggers the release of oxytocin from the posterior pituitary, which has multiple effects. Oxytocin directly causes the contraction of smooth muscle cells surrounding the ducts, forcing milk from the nipple (it is also the hormone that stimulates contractions of the smooth muscle of the uterus, initiating labor). Oxytocin inhibits the release of hypothalamic factors that suppress prolactin, and so causes a prolactin surge to ten times the resting level; that prolactin increase maintains the production of milk. Not nursing for a few days allows prolactin levels to decline, and milk secretion will end.

Males have the capacity to produce milk as well. They don’t, usually, because their estrogen levels aren’t high enough, so the glandular tissue of their breasts are not fully differentiated. Men who have been treated with estrogen as part of cancer therapy will develop functional breasts, and even without estrogen they possess the circuitry to produce prolactin surges in response to nipple stimulation. Diamond, in his book Why is Sex Fun, has a nice story about that:

My favorite case is the chauvinist husband who kept complaining about his wife’s “miserable little breasts”, until he was shocked to find his own breasts growing. It turned out that his wife had been lavishly applying estrogen cream to her breasts to stimulate the growth craved by her husband, and the cream had been rubbing off on him.

One last hormonal effect of nursing is that it suppresses the production of yet another hormone, gonadotropin releasing hormone (GnRH), by the hypothalamus. The reduction in GnRH reduces levels of two other hormones, luteinizing hormone and follicle stimulating hormone, that are essential in inducing ovulation. Reduced likelihood of ovulation reduces fertility while lactating.


  1. PaulC says

    The computer scientist Alan Turing reportedly complained that he was growing breasts as a result of estrogen treatments required by British anti-gay laws of the time.

    This was obviously disconcerting to him and may have been one of the factors leading to his suicide.

    I had no idea that males could actually produce milk, though. I wonder if anyone has ever done this intentionally to split child-care duties more equitably.

  2. Greco says

    Modern astronomers might quibble with that explanation of the origins of the extrasolar universe

    We should Teach the Controversy, then.

  3. Buffalo Gal says

    Thanks, PZ – I didn’t know that the galaxy came from Hera’s milk. Way cool! Nursing my baby was wonderful – feeling nourishment go from my body directly to his, and seeing him grow like a weed in the rain.

  4. thwaite says

    Fun factoid to know & tell:
    Linnaeus called the group mammals to emphasize the mammaries … whose use by the human mammal he thought needed emphasizing, as he was an activist in the 1750’s equivalent of the La Leche league.

    Wikipedia background discussion (which I see is now absent from their main article):

    PBS background article by Jonathan Marks, a readable though often tendentious anthropologist:

  5. aiabx says

    The Milky Way was the only object created from Hera’s milk, since no other galaxies were known at the time. It is *the* Milky Way, rather than *a* Milky Way. Presumably, the other galaxies were created by local deities, or in the absence of life elsewhere, made of non-dairy artificial creamer.

  6. Niko says

    Actually, the Greek for milk still is gala, or as they prefer γάλα. Not completely OT. Worth wondering why the word gala has been stable for several thousand years in Greece, while in about 1000 years Latin lactem evolved into (among others) latte in Italy, and the almost parasite-like simplified form lait (pronounced, more or less, leh) in France.

  7. Kagehi says

    I wonder if anyone has ever done this intentionally to split child-care duties more equitably.

    Cases of male milk production are rare, but not unheard of, nor is estrogen treatment needed to induce it. Some known cases are sympathetic reactions, similar to odd cases like the one on one of those crime shows awhile back (CSI maybe?) where the teen girl was lactating, because she “wanted” to be pregnant.

  8. plover says

    Milk is thought to have evolved from simple, sweaty secretions from the skin that were lapped up by the infant

    Not to mention baby koalas which also, IIRC, eat their mother’s feces for immune purposes. (Do other marsupials do this too?)

    has anyone ever made seal cheese? (or any other “weird” sort of cheese)

    Okay, any volunteers for milking a cheetah?

    I understand they do make yak cheese in Nepal.

    There are a few obvious candidates for weird cheese: pigs, horses, camels, llamas. Do they all just get too cranky if someone tries to milk them?

    Also, especially now in the age of breast pumps, I’d actually be pretty surprised if nobody had tried making human cheese.

  9. says

    While reviewing the literature when my wife was breastfeeding, I was amazed to learn of the other beneficial compounds present in breast milk beyond the nutrients you listed here. Breast milk contains TGF-beta1 and 2, the latter of which has been associated, but not yet causally, with decreased incidence of atopic dermatitis and other atopic disorders in breast-fed infants. Interestingly, (and I have yet to understand the mechanism) ingestion of probiotic supplements by nursing Moms increased the TGF-beta2 concentration of their breast milk more than two-fold.

    Mind you, I would be extremely cautious about nursing Moms taking any kind of supplements beyond multivitamins, but this was done under the auspices of a clinical trial and is simply just like eating a good helping of yogurt. I’m also amazed that the TGF-beta2 would have oral bioavailability to permit systemic absorption and distribution, but all bets are off in the neonate.

    Finally, folks at the University of Arizona have even found that the breast milk of Moms with extremely premature infants have elevated levels of EGF and TGF-alpha and these could certainly have enough local bioavailability to prevent the gastrointestinal disorders associated with extreme preterm births. Unfortunately, these babies are off in the PICU away from their lactating mothers. Of course, they wouldn’t survive without intensive care, but there is incredible biological beauty in the attempt by the mother’s body to provide greater concentrations of helpful peptides to their premature offspring.

  10. says

    Penguins, pigeons, and flamingos feed their young with a kind of milk, as does the discussfish. More at my URL, along with some speculations about prolactin and nurtuting behavior.

  11. daver says

    _Ghost Soldiers_ relates that some WWII POWs in the Philippines started to grow breasts and that masectomies were performed on some of them. This book was copyrighted in 2001, which might be too recent for it to be where Davis read it.