Woman: An Intimate Geography (28 page)

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Authors: Natalie Angier

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BOOK: Woman: An Intimate Geography
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does not fade over time. Milk is homeopathic: it holds the memory of every mouth it feeds.
The practical breast is a modified sweat gland, and it is meant to be used, as the pancreas, the liver, and the colon are there to be used. Lactation is a basic biological function. Milk is a body fluid. Yet metaphorically, breastfeeding and breast milk have been and remain in a class by themselves, the class of metaphysiology. They have been accorded a magical status, breathless, declarative, absolute. They have been the source of endless exhortation, celebration, guilt, joy, and pain. We think of breastfeeding as natural and good and lovely, yet throughout history, in a variety of permutations, it has occasioned spleen and hectoring. Nobody has to beseech us to let our heart pump, our neurons fire, or our menstrual blood flow. Breastfeeding is another matter. It may be natural for a woman to nurse her baby, but it is not guaranteed, and so it has been variously mandated by prophets, legislated by politicians, and hoisted onto a sociomedical pedestal that brooks no excuses or complaints. Lactation has not been allowed to be what it is, the business of the body. The mammary gland often has been underrated, which is why in the middle of this century infant formula was thought to be not merely a passable substitute for breast milk, but an improvement on it. Now the gland is overrated. We believe that it can make every baby into Izzy Newton or Jane Austen. Now breast milk is seen as the quintessential female elixir. Through it, we give more than a part of ourselves to our children, we give ourselves purified and improved. Our breast milk is better than we are.
We know ourselves too well, alas but we don't know our breast milk. It is mysterious. Scientists continue to analyze it and continue to find unexpected ingredients in it. Is it getting better with time? Is it evolving in advance of us? You might suspect as much if you read comments by breastfeeding advocacy groups. It is "the miracle substance," in the words of Lee Ann Deal, the executive director of La Leche League. Even scientists set aside their dustiness and qualifiers when praising mother's milk, calling it "the ultimate biological fluid," "a cocktail of potency," "a truly fascinating fluid,'' "a human right," "more, so much more, than just food." In believing that breast milk conveys an almost supernatural power to an infant, we echo ancient medical authorities, who posited that a woman's personality, her humors, would

 

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mold the character of any infant who supped at her breast. They cited famous examples, such as the emperor Tiberius, a pitiful drunkard, who was said to have been suckled by an alcoholic wet nurse, and the savage Caligula, suckled by a wet nurse who supposedly rubbed blood on her nipples.
Think of your own feelings about breast milk. If you're a woman who has lactated, you've surely tasted your own milk, enough to know that it's sweeter and waterier than cow's milk. But if you saw a full glass of human milk in the refrigerator, would you drink it? The idea is disturbing. It feels almost cannibalistic. We don't know the half of what's in it, or why. Human milk tastes thinner on the tongue than cow's milk, but it is so rich, so pregnant with meaning and attribute, that like a bubbling solution in a mad scientist's beaker, it practically crackles to life and beyond. If an adult were to drink human milk every day, might that person grow huge, like Alice nibbling from the left side of the mushroom, or become immortal, like Hercules, say, or the vampire Nosferatu?
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

 

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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 ever-mounting 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

 

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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,

 

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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 ques-

 

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tion 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,

 

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