Let us examine lactation in the clear light of morning. I've talked about the mammary gland as a modified sweat gland, but there is another way to think of it: as a modified placenta. The placenta and the mammary gland have much in common. They are specialists, and they are temporary workers. They are designed to nourish a baby. No other organs are so fleeting, so single-minded, as the placenta-mammary dyad. They exist only for the baby, and if the baby does not call on them, they are retired. They are expensive organs, and they are not maintained unless absolutely necessary. That is why the suckling of the baby is crucial to the productivity of the mammary gland. The mammary gland will not continue making milk unless the mechanical sensation of suckling tells it that lactogenesis is necessary. In evolutionary terms, babies die too often to make automatic milk ejection a sane strategy. It would be terribly wasteful if, after the arrival of a stillborn infant, a woman's body were to generate milk automatically for anything more than a handful of days, at a cost of 600 calories a day. Lactation is a contingent function and a conditioned response, which is why it can be so frustrating to initiate and maintain. The body stands poised to flow, and to stop flowing. In a way, lactation is analogous to blood. Blood must course through your veins nonstop, yet it must be prepared to coagulate if the skin is breached, or else we would bleed to death at the brush of a thornbush. So too must milk be prepared to flow, but it is such a costly fluid, this ultimate cocktail, that the baby must beg for it and suck with the mouth of a divine pretender.
Milk production begins midway through pregnancy. The foliate lobules where milk is made thicken, proliferate, and surround the ducts until you can't see the ducts for the leaves. At the tips of the lobules, the alveolar cells quiver and distend and begin secreting a yellowish fluid of protein and carbohydrates, the colostrum. Some of it may make its way to the nipple and ooze out, but most gets reabsorbed in the ducts; there's no reason for it to go anywhere yet. The alveoli are simply making a dry wet run. Many hormones contribute to the glandular expansion, and to keeping the sequence sensible. Progesterone stimulates the division and maturation of the alveolar cells, but it also prevents them from getting ahead of themselves. If not for the high levels of progesterone (and to a lesser extent estrogen) that characterize pregnancy, the alveolar cells would pay heed to another hormone, prolactin, the breastfeeder's friend. During pregnancy, the pituitary gland, at the base of the brain, begins releasing evermounting quantities of prolactin. Prolactin urges the alveolar cells to synthesize milk. Progesterone counsels delay. For as long as gestation lasts, progesterone wins.
After delivery, progesterone and estrogen levels drop precipitously. For some women, the hormone crash leads to temporary depression and stasis, but for their mammary glands the change is bracing. The alveolar cells are freed to take up the circulating prolactin, and they absorb it greedily. At first they make what they are accustomed to making: colostrum, the sticky fluid of protein, carbohydrates, and other ingredients. But no fat—that comes later. Colostrum is yellow because it's rich in carotenoids, the same compounds that give carrots and squash their yellow-orange tint and that are needed to make the A and B vitamins. Colostrum is ten times richer in carotenoids than mature milk will be. If it looks like pus, it also acts like pus: colostrum contains a wealth of white blood cells and antibodies, as pus does, and it helps a newborn, whose immune system has yet to mature, resist the pathogens that will be more than happy to attack it. Colostrum is also rich in the loosened epithelial tissue that had kept the ducts plugged.
The infant suckles colostrum. It wants more than this thin carrot gruel. It suckles and suckles. The yanking and pulling on the nipple somehow is translated into neural idiom, suppressing the brain's production of dopamine. The less dopamine there is, the more the pituitary sends forth prolactin. The alveolar cells become sorcerers and start synthesizing that fascinating fluid milk, virgin white, the blank slate on which all hopes can be written. The cells become engorged with milk. The infant suckles and stimulates the pituitary to secrete another hormone, oxytocin. It is time for the great letdown. On oxytocin's signal, the muscle tissue surrounding the engorged alveoli contracts, forcing the milk out of the cells, down the ducts, out through the nipple, and into the mouth that has worked so hard to persuade the gland that it is here and it is famished.
What is milk? How does a fluid earn its milk stripes? By definition, milk is the product of the mammary gland, just as gastric juice is the product of the stomach and saliva the product of the salivary glands. But milk is chemically far more complex than many body secretions, because its assignment is so complex. The mammary gland accrues the subunits of milk through three avenues. Some components are taken directly from the mother's bloodstream and deployed unaltered. Others are extracted from the blood and then reworked and edited before being incorporated into the milk. Still others are invented in the alveolar cells themselves.
In keeping with its illustrious reputation, milk is often billed as "nature's perfect food," and in this case the ad copy is accurate. It is all that a newborn mammal needs to survive. Never again will an individual's menu be so easily planned as it is early in life. Every species' mammary gland offers a slightly different definition of the perfect food. All milk must give the basics that a neonate needs to survive and grow. Be it piglet, calf, kangaroo, or human, the body needs water, lipids, carbohydrates, and protein, and these are the core components of milk. But how much and what type of each varies from animal to animal. Animals that grow rapidly need milk that is high in amino acids, the constituents of protein. The milk of carnivores such as cats, hyenas, and canids is dense with amino acids. If Romulus and Remus, the founders of Rome, drank the milk of a wolf, as legend has it, they drank meat in solution. Animals that must put on a lot of fat in a short amount of time drink fatty milk. Perhaps the fattest milk in nature is elephant seal milk, which is fatter than butter. An elephant seal pup has but four weeks to suckle, and as it does it expands from its birth weight of seventy-five pounds to a weaning weight of three hundred pounds. For her part, the mother eats nothing the entire time, and so loses six hundred of her fifteen hundred pounds. As one scientist put it, she essentially slices a slab of blubber from herself and slaps it onto her young.
The milk of slow-growing animals has a comparatively low concentration of amino acids. Humans grow slowly, and our milk is among the least proteinaceous mammary product around. Rat's milk has twelve times the concentration of amino acids of human milk. Cow's milk is four times protein-heavier than our milk, which is the main reason that you can't give a baby cow's milk without first processing it into infant formula. A newborn's immature kidneys are not equipped to handle the high protein content of cow's milk. A human infant could handle the milk of a gorilla, chimpanzee, or orangutan, however. The milk of great apes is quite similar to our milk in every aspect that has been examined.
What human (and ape) milk lacks in protein it makes up for in lactose, the principal carbohydrate, or sugar, of milk. Lactose is second only to water as the major constituent of human milk. Our milk has twice as much lactose as cow's milk does. We scold ourselves for our sweet tooth, and we wonder why our children want to eat Ring-Dings, ice cream, and Froot Loops. We should be surprised by none of it. The milk we have evolved tastes as sweet as Kool-Aid. But lactose is not junk food. It is not a simple-minded sugar. It is formed in the alveolar cells through the combination of the simple sugars glucose and galactose, taken from the mother's blood, and it offers the newborn twice the energy content of glucose. Lactose also is important to the absorption of other nutrients in the milk, enabling the infant's gut to maximize its uptake of calcium, fatty acids, and the like. Cow's milk and human milk have roughly the same amount of fat, but the types of fat differ significantly. Human milk has comparatively more essential fatty acids, which are long chains of unsaturated fats that the body can't make on its own but must obtain through the diet—which for a baby means breast milk. Essential fatty acids are involved in the development of the eye, the brain, and the
peripheral nervous system. Formula makers are now debating whether to add certain fatty acids to their formulas, particularly docosahexaenoic acid, or DHA. But always there is a question about unexpected outcomes. In one study, infants fed a formula that had been fortified with fish oil, a good source of DHA, showed faster development of visual acuity, but on other measurements of psychomotor performance their progress lagged behind that of infants fed either breast milk or standard formula. Besides, how much fatty acid should be added? A woman who eats a lot of fish will have twenty times the amount of certain long-chain fatty acids in her breast milk as a woman who lives in the Sahara. Do you want to mimic the lipids in the breast milk of a fish eater, or of a vegetarian, or of an omnivorous American?
Apart from differences in some fatty acids and a few other components, human breasts secrete remarkably similar fluid despite wide variability in women's nutritional status. A malnourished woman in a developing country generates surprisingly nutritious milk, while a chubby woman in the Midwest does not have comparatively high-calorie milk. "One of the things about lactation that is an endless source of fascination to those of us who study it," says Peter Reeds, a professor of pediatrics at Baylor College of Medicine, "is the remarkable ability of lactating mammals, humans included, to preserve a narrow range of milk composition in the face of even a disadvantageous diet." If a woman is not eating what she needs to maintain that perfect formula, the mammary gland borrows from her body stores, the 7-Eleven that never closes. At the same time, the woman does not sacrifice quite as much as might be expected, for breast milk has evolved through compromise. The mother gives, but she does not give to the point of risking her future health and fertility. Breast milk is designed to be maximally exploited without maximally exploiting. A nursing woman does not need to lose her teeth or watch her spine shrink so that her baby can get enough calcium; the lactose in the milk ensures that every ion of calcium will be used instead of just peed away, as is much of the calcium that you get from drinking, say, fortified orange juice. The baby digests the proteins in the milk down to the last amino acid, which is why a suckling infant's used diapers hardly smell: there's very little waste matter, very little excreted protein, to lend a stench. A nursing woman does not have to become anemic to give her baby iron. Human milk has very little iron in it, but it has lactoferrin, a protein that allows the iron to be thoroughly absorbed. The same is true of other trace minerals, such as zinc and copper; they're rare in breast milk, but when they're there, the suite of milk proteins and sugars takes them up and guarantees they will not escape. Moreover, human babies in the past probably spent a bit of time rolling around in the dirt and picking up iron and minerals through an infantile form of pica. Babies put everything in their mouths, and they try to lick whatever they can. We see this as an unfortunate and risky little habit, but babies might do it for a reason: to lick up the occasional trace element their cells demand to perform and divide.
Infant formula cannot mimic breast milk, as we've learned, and as every new mother is told by every authority she encounters, and as every can of formula is required by law to warn, in the way that a pack of Marlboros must warn of cigarettes' lethality. Human milk is a solution of more than two hundred constituents, whose manifold roles have yet to be entirely understood. Nothing does just one thing. Milk sugars offer calories, and milk sugars allow other nutrients to be fully metabolized. The sugars of human milk and of infant formula are quantitatively similar but qualitatively different. Lactoferrin permits the scarce iron in milk to be "bioavailable" to the baby, and it also prevents pathogenic bacteria from getting their maws on the metal, which they need to survive. There is no lactoferrin in infant formula. The immune properties of breast milk are legion, and most of them are missing from formula, because the preparation of formula destroys their counterparts in cow's milk. There are B cells, T cells, macrophages, and neutrophils in breast milk, there are antibodies, there is gamma interferon, which stimulates the activity of the immune cells. The fatty acids in milk disrupt the membranes surrounding viruses, while lysozyme does the same to the cell walls of bacteria. Bifidus factor encourages the growth of benign flora in an infant's gut, the better to outcompete insidious strains.
Much research on breast milk over the past ten years has concerned hormones and growth factors. Here is where the human mammary gland is portrayed as a kind of brain, a self-replicating mind, feeding the developing brain of the neonate the proteins that its neurons require to differentiate. The mammary gland synthesizes gonadotropin-releasing hormone, for example, and deposits it in milk. Gonadotropin-releasing hormone is a protein best known as the product of the hypothalamus, located in the midbrain. In adults, it stimulates the gonads and may play a role in sexual behavior. We don't have a clue what, if anything, this hormone might do to a suckling infant, but there it is, swimming in the milk, at a concentration ten times that seen in the mother's bloodstream. Breast milk also has nerve growth factor, and thyroid-stimulating hormone, and factors given vague names like "mammotrope differentiating peptide." The human newborn is a weak, altricial little creature, a postpartum fetus, and it is possible that some of the factors in mother's milk are "obligate differentiation factors," required to orchestrate the full maturation of the infant's brain and other organs. Infant formula has some of these factors, but again, most of the equivalent peptides in cow's milk are destroyed by the processing necessary to make cow's milk digestible. In the absence of these factors, can the infant realize happiness, healthiness, and peak brilliance? We don't know yet. We don't know what these supposed differentiation factors and neuropeptide equivalents are for. It's logical to assume that they're necessary, or at least good for an infant, but logic is not evidence and biology is not always logical.
The more we look at breast milk and the more we find within it, the more we are driven to marvel that anybody can survive, much less thrive, on its wretched artificial substitute. Yet many have. The majority of baby boomers were reared exclusively on infant formula, and they're all around us, and are us, occupying space and every occupation. Today almost 40 percent of infants in this country are still bottle-fed from birth. Among babies in the United States who are breastfed to start, only half still receive breast milk at the age of six months. By a year, only 10 percent of babies still breastfeed. Researchers don't know what to make of it. They wonder if they are asking the wrong questions or neglecting subtle cues, or if it's simply a matter of general ignorance about growth and development, an ignorance reflected in scientists' inability to parse out and pin down all the deficiencies in bottle-fed babies. "As a scientist, I can't help but notice that millions of babies have never seen human milk, and they apparently have not been harmed," said Dr. Reeds. "At the same time, I can't escape the feeling that nature went to enormous lengths to produce a particular food, and that must mean something."
In Third World countries where breast milk may offer the only sterile fluid around, breastfeeding can be imperative to an infant's survival. In the developed world, the advantages of breastfeeding over bottle feeding are less obvious, but they exist. Breastfed infants have fewer infections of the middle ear, gastrointestinal tract, and upper respiratory system than bottle-fed babies do. They suffer less from diarrhea and constipation. When breastfed infants get sick, they recover sooner than those on formula do.
Of the many other benefits that have been ascribed to breastfeeding, though, not all have been proved. Breastfeeding supposedly helps prevent obesity later in childhood, but the evidence is equivocal, and complicated by socioeconomic factors. Breastfeeding is thought to lower the risk of allergies and asthma among children, yet the rates of chronic respiratory diseases have been climbing in recent years right along with an increase in breastfeeding. Some studies have suggested that breastfed children have higher IQs than their bottle-fed cohorts, but other studies that took maternal IQ into account found no connection between breast milk and intelligence. Perhaps the most questionable and philosophically disturbing benefit that has been
pinned to breastfeeding is that it improves the emotional bond between mother and infant. Not only is such a bond impossible to quantify, but it pretty much sweeps aside all efforts to engage fathers as full and legitimate participants in child care. If it takes suckling to feel the most intimate and profound love for your baby, then a man with a bottle—even one that holds expressed breast milk—will always be as feeble a substitute for the well-teated mother as formula is for her milk.
Women know they should breastfeed their babies, and many are more than delighted to give it their best shot. But what is their best shot, and what of those who will not? In Scandinavia, it is considered tantamount to child abuse to feed an infant anything except human milk. Scandinavians have milk banks to supply milk when mothers can't or won't. In this country, fears of viral contamination have prevented the institution of a similar network of milk banks. Viruses, like the one that causes AIDS, can be transmitted through milk, and though it's possible to screen milk for the virus as blood is screened, the existence of a reasonable milk facsimile—infant formula—has kept down the demand for human milk that has been put through an expensive series of tests.
Breastfeeding is considered natural, an extension of pregnancy. The mammary gland is an extension of the placenta. All the products found in the placenta reappear in breast milk, including the immune factors, the growth factors, the hormones. Yet pregnancy takes care of itself; lactation doesn't. Pregnancy lasts 240 days. Suckling can last as briefly or as long as you please, or as it pleases others. Various authorities have sought to determine the true, basal human breastfeeding interval, but there is no such animal, and there probably never has been. The Koran advocates that a woman suckle her child for two years, but adds that if the husband and wife wish to wean the infant sooner they may do so, suggesting that many people in the past did so. The World Health Organization and UNICEF recently have recommended that women breastfeed for two years "and beyond," but only among contemporary hunter-gatherers like the !Kung, who nurse an average of 2.8 years per child, do we see much of that "beyond."
Woman Page 20