The story of infant nutrition begins with mom. The major nutrients that keep mothers healthy are broken down and then transformed into a liquid food that is delivered to their infants, who will, in turn, break down and re-assemble these nutrients in their own growing bodies. Breast milk must support the infant’s extraordinary growth and development as well as provide energy for cell division, tissue growth, and motor and neural activities. Milk builds babies.

But isn’t milk just milk? Sure, all mammals produce it and their babies drink it, but there’s a lot more to the story than that. First of all, each type of mammal, whether mouse or moose, makes their own version of milk, specially suited to the nutritional needs of their own offspring. These nutritional needs reflect how fast and big the offspring will grow, the environment in which the offspring grow, and even the size of the animal’s brains. All of these things are mirrored in the milk that nourishes mammal babies.

The basic components of milk are fats, proteins, carbohydrates, vitamins, minerals, and water. That goes for cow’s milk as well as human milk. But again, there are differences. Cow’s milk, for example, has approximately three times more protein than human milk. Both a baby and a calf need proteins as building blocks for growth and development. But while human babies can triple their birth weight in the first year, a calf’s birth weight can increase fivefold in just 6 months.

Analyzing Breast Milk

Researchers began to scientifically analyze the major components of milk (fats, proteins, and carbohydrates) more than a century ago. But determining precisely the complete list of all ingredients found in milk has been very difficult. Milk is an extraordinarily complex fluid, rich in vitamins, minerals, and other nutrients as well as a wide range of trace elements, antibodies, growth factors, and digestive enzymes. In a very real sense, milk is alive, because it also contains living immune cells.

The quest to unravel the mysteries of human breast milk has been an important challenge in public health. An international community of researchers has spent decades analyzing the components of milk, determining their function, and deconstructing and reconstructing the physiology of infant nutrition.

Although milk is a difficult substance to work with, some researchers note that, given our scientific sophistication, it’s surprising we know as little about breast milk as we do. Part of the problem is the limitations inherent in studying babies and nutrition. In the 1970s and 1980s, explains Sharon Donovan, professor of nutrition and pediatrics at the University of Illinois, “we developed better analytical chemistry and we started discovering lots of different growth factors in other secretions, like blood and tears and other biological fluids, and we also began to identify those in breast milk. So we started to compile lists and tables of different growth factors, hormones, cytokines, even live immune cells in human milk.”

That was certainly a critical first step, but the real challenge was to understand the function of breast milk within the context of infant physiology. You can only get so much information doing experiments in test tubes, says Donovan. What has changed dramatically in recent years, however, are the improved technological means scientists have to understand biochemical pathways and the genetic underpinnings of function.

“There have been a lot of changes,” says Bo Lönnerdal, professor of nutrition and internal medicine at the University of California, Davis, “and fortunately scientists have been following new developments and exploiting advances in other disciplines. Breast milk studies are by nature very interdisciplinary. It’s a fascinating time to be working in this area and I think most of the breakthroughs are still to come.”

One of the major challenges that remain, however, will be to design experimental approaches that will enable researchers to study not just the impact of a single ingredient in breast milk, but the combined interaction of breast milk components. How does a given nutrient act in the presence of other nutrients? When does it enhance, inhibit, or otherwise modify the action of nutrients? “As scientists,” says Donovan, “we often take a reductionist approach, so we’re just looking at one hormone or one growth factor. But it’s likely that all these things are working together in different ways because they are all present as a package and that’s how the infant experiences it.”

The complexity of breast milk has other important dimensions as well. “Typically, we think of breast milk as nutrition, fuel to be completely digested and absorbed,” explains Donovan. “But there are a number of components in breast milk that are not digested.” That doesn’t mean they aren’t important, however. Far from it. What researchers are finding is that many components in breast milk perform specialized “functional” roles that go well beyond providing the infant with a source of calories or building blocks. These functional components actively “do” something. For example, breast milk contains indigestible carbohydrates that encourage the growth of beneficial gut bacteria and stimulate the maturation of the infant’s intestinal lining. Other functional components include proteins that first battle pathogens in the baby’s body, and are then, in turn, digested as fuel and used as building blocks for the baby’s body.

Scientists are increasingly trying to decipher the functional roles played by various substances in breast milk. This knowledge will help researchers deepen their understanding of the mysteries of growth and development as well as improve nutrition for all infants. Meanwhile, the list of benefits of breastfeeding continues to grow longer as scientists unlock the secrets of just exactly why mother’s milk is the best way to support the health and well-being of babies.

Lactation 101

Biologists believe the evolutionary origins of lactation may go back some 200 million years. And although mammals are named for the glands that are central to lactation, the physiology of lactation involves the entire body, including the digestive, neurological, and endocrine systems.

“The mammary gland is made up of a variety of cells,” explains Sharon Donovan, “including fat, blood vessels, lymph vessels, ligaments, and nerves. Most of the mammary gland size is related to the amount of fat in the breast. What you have embedded within the fat are basically the milk-synthesizing units,” says Donovan, who adds, “The perception that small-breasted women can’t make enough milk is not true; breast size has very little to do with the amount of milk produced.”

In terms of architecture, each breast is made of multiple lobes arrayed around the areola like petals of a flower. Ducts from the lobes drain into the nipple. According to Susan Tucker Blackburn in Maternal, Fetal, & Neonatal Physiology, each lobe consists of 4-18 lobules and each lobule contains 10-100 alveoli with their respective ductules, smaller branches that lead into the larger ducts. The alveolus is the site of milk production and consists of clusters of specialized epithelial cells called lactocytes.

Picture it this way, suggests Donovan. “The alveoli are round, balloon-type structures surrounded by the epithelial cells, or lactocytes, which are the cells that make the milk. So these lactocytes are taking various components from the blood and converting them into specific milk components.”

Some of these components, such as vitamins and minerals, as well as certain infection-fighting immune factors, can be extracted basically intact from the mother’s blood. Other components, however, such as casein proteins or the carbohydrate lactose, must undergo assembly in the lactocytes.

“These components,” says Donovan, “are then secreted into the inside of the balloon, the alveolus.” The mixture of all these components is what constitutes milk. “As milk synthesis increases and the balloon gets bigger, the signal is sent back, ‘OK, there’s enough milk here, stop making milk.’” The alveoli are surrounded by specialized bands of cells (a type of smooth muscle) that are responsible for squeezing milk from the alveoli into the ducts.

“The milk stays in the alveoli until the body secretes oxytocin,” explains Donovan, “which signals that the milk be released into the mammary ducts.” During lactation, milk synthesis constantly occurs at low levels, but is most active when the infant’s sucking stimulates nerves in the nipple. These nerves carry a message to the brain, which then releases the hormones that regulate milk production.

Breastfeeding Science Through the Ages

Given lactation’s universal importance, it’s no surprise that human cultures throughout history have tried to understand the process of milk production as well as devise solutions to breastfeeding problems. In virtually all cultures, this has included special diets or herbal preparations. The earliest medical writings on breastfeeding go back to ancient Egypt. The Greeks also weighed in, including the physician Hippocrates nearly 2,500 years ago, who believed that there was a link between human milk and menstrual blood. Aristotle, a scientist as well as a philosopher, also considered the biological origins of breast milk.

By far the most detailed and sophisticated ancient account of breastfeeding, however, was written by Soranus, a Greek physician who practiced and wrote about gynecology. Soranus lived in Alexandria and Rome in the first and second century AD. His perspectives on women’s health live on because of the major work he wrote on the subject, titled The Gynecology. His treatise included sections on wellness and pathology, including abnormal pregnancies; the care of newborns, and a long discussion on breastfeeding, problems of nursing, and weaning. Remarkably ahead of his time, Soranus held sway over such matters in Europe until the Renaissance.

It was the Industrial Revolution, however, that really provided the impetus to understand human milk and infant nutrition as economic and social upheaval resulted in large numbers of women not breastfeeding. The pace of scientific knowledge rapidly advanced in the 20th century with the development of modern nutrition research. The result has been a deeper understanding and appreciation of the marvel of mother’s milk.

At first the grainy, black-and-white images could be mistaken for a time-lapse video of the erosion of a riverbed. But when Donna Geddes, professor of biomedical sciences at the University of Western Australia (UWA), explains what the moving images are, it becomes clear just how remarkable this video footage is. Using ultrasound imaging techniques, Geddes and her colleagues at UWA have captured in real time the actual flow of milk through the ducts in a woman’s breast. Look closely, says Geddes, and you’ll notice “small white flecks (milk-fat globules) are seen moving within the duct towards the nipple. Also, sometimes surrounding ducts are seen to dilate...and some small ducts that are not obvious prior to milk ejection become visible.”

It is indeed an amazing, and rare, view of lactation in motion. The scientific data on milk ejection has been thin. As Geddes writes in a recent paper, “Although milk ejection is integral to lactation and thus the survival of the species, the lack of knowledge regarding milk ejection in women, in comparison to other lactating and dairy animals, is surprising.” Milk ejection, or the let-down reflex, is a complex neuroendocrine process. The baby sucks on the mother’s nipple, which stimulates nerve endings in the nipple and areola, which send signals along nerve pathways to the spinal cord and brain.

“It signals to the brain,” explains Sharon Donovan, “‘OK, I need the milk down here now.’” The hypothalamus and pituitary gland release the hormone oxytocin, which then travels through the bloodstream back to the mammary gland and stimulates the contraction of specialized cells surrounding the alveoli. Milk is squeezed out, ducts widen, and breast milk begins to flow from the branching network to the nipples. And while the physical stimulation of the infant’s sucking most directly triggers this chain, many mothers find the let-down reflex also can be induced by sensory or emotional stimuli as well, hearing a baby cry, for example, even if it is someone else’s baby. On the other hand, let-down is inhibited by stress, fear, and high alcohol consumption.

Anthropologist Sarah Blaffer Hrdy notes in her book, Mother Nature, that “as I became more conditioned to nursing, just the thought of her, or her cry, was enough to trigger this ‘let-down reflex’ and cause a patch of wetness on my blouse.” It becomes automatic and, as Geddes points out, “while the first milk ejection is sensed by most mothers, subsequent milk ejections that occur during either breastfeeding or breast expression are rarely sensed.”

Hormone Star

A suite of hormones are involved in lactation. Some of the hormones play roles early in pregnancy, some in late pregnancy, and others after birth. Though different hormones play different roles at different times, none of them work in isolation; some work in tandem with other hormones, and some actually inhibit the action of other hormones. The hormones oxytocin and prolactin are critical for lactation. Prolactin is the hormone that supports milk production and oxytocin is the hormone that promotes milk let-down. The two work hand-in-glove to regulate the daily rhythm of breastfeeding. It is a supply-and-demand system, or as Blackburn puts it: “Without stimulation by suckling and removal of milk, secretion of prolactin decreases and milk production ceases…. Milk production rate depends upon the milk removal rate.”

In addition to its role in lactation, oxytocin is also important in causing uterine contractions and for that reason it is sometimes given to women in order to induce labor. During nursing, the hormone continues to help contract the uterus, which in 9 months goes from the size of a fist to the size of a basketball. But in recent years, oxytocin has also gotten attention for its role in contentment, trust, and security when it comes to human bonding. Press stories dubbed oxytocin “the cuddle hormone.” According to researchers, oxytocin is a key player in the biological basis for human attachment and bonding and helps people maintain emotional closeness in interpersonal relationships.

Fetal Feat of Coordination

The sucking stimulation provided by the infant is also automatic, of course, but even here there is a biological learning curve. In fact, the fetus spends months rehearsing the skill of sucking and swallowing. And like virtually all other motor behaviors, it is “simple” only in the sense that virtually all infants come to perform it perfectly. In fact, sucking is a complex behavior dependent on a range of neuromuscular factors. As Susan Tucker Blackburn explains in Maternal, Fetal, & Neonatal Physiology, the swallowing reflex begins at 10-14 weeks, and by 16 weeks the fetus swallows a tiny amount of amniotic fluid each day (at term, the fetus is swallowing about 2 cups of amniotic fluid daily). The sucking reflex starts to kick in at 20 weeks gestation.

But that’s not the end of the training. The challenge for the fetus is to then coordinate sucking and swallowing. According to Blackburn, “Although all components of sucking and swallowing are present by 28 weeks, the fetus is often unable to coordinate these activities. Some suck-swallow synchrony is seen at 32-34 weeks; synchrony is complete by 36-38 weeks.” And finally, once the baby arrives, she must coordinate sucking and swallowing with breathing. Practice makes perfect.

Mother's milk is a marvel. It is uniquely adapted to the needs of the newborn and serves as the bridge between fetal nutrition and development and the biological independence of a growing baby.

A nursing mother is the very image of the bond between mother and child. Nursing is also a remarkable example of nutrition coming full circle. A mother transforms her own nutrients into those in breast milk, which provides her newborn with the nutrients for his or her growth and development. The building blocks of nutrition that were broken down and re-assembled for delivery in breast milk are broken down and re-assembled again in the baby's growing body.

The complex supply line that links maternal diet and physiology to infant digestion and development doesn't begin at birth. In fetal development, the placenta is essential for the transfer of nutrients (including oxygen) to the baby and the removal of wastes (including carbon dioxide) from the developing fetus. The placenta also provides key hormones as well as growth and immune factors. The umbilical cord is the lifeline between placenta and fetus.

The rapidly growing fetus is sustained by the maternal heart/lung/immune/nutrition support system. And then, at birth, suddenly and dramatically, all of the developing systems, from digestive and respiratory to circulatory and thermoregulatory, must perform self-sufficiently. The newborn, who has grown and developed in the womb, is now thrust into physiological independence. Well, not quite.

"Breast milk has been called a transitional fluid," explains Tom Brenna, professor of human nutrition at Cornell University. "It transitions us nutritionally from being a fetus and being nourished from placental transfer, to solid foods." Human breast milk has to meet extraordinary biological demands from a developing baby. Dietary nutrients must supply the building blocks for growth and development as well as provide energy for cell division, tissue growth, and motor and neural activities. Breast milk builds babies.

Precisely what breast milk is made of is a puzzle that researchers have worked for more than a century to solve. Milk is an extraordinarily complex fluid, rich in nutrients and a wide range of minerals, vitamins, hormones, and growth factors, as well as enzymes with specific functions in the infant's digestive system. In a very real sense, breast milk is a living fluid because it also contains immune cells from the mother.

How Breast Milk Changes

The baby's nutritional transition, however, doesn't begin with what we normally think of as milk. "It starts the first week of life as not really being 'milk' at all," explains clinical nutritionist and registered dietitian Julie Balay. "It's called colostrum and it's a clear, yellow fluid and mothers often get nervous during that time. 'Why isn't the milk coming and why isn't there more of it?' The baby usually loses weight and moms get worried that it isn't working. But colostrum is exactly what the baby needs."

Compared to mature breast milk, colostrum is higher in protein, because it includes lots of protein-based immune factors, but lower in carbohydrates, fat and calories. There is also much less of it, in terms of volume. It contains higher amounts of white blood cells and antibodies than mature milk, and is especially high in immunoglobulin A, which coats the lining of the baby's immature intestines and helps to prevent germs from invading the baby's system. Over the first 10 days after birth, colostrum production gradually gives way to transition and then mature breast milk.

Even when more mature milk comes in, it is far from the standardized cow's milk version we are so familiar with in our grocery stores. The composition of breast milk varies according to maternal diet and whether the mother has been breastfeeding her child for one month or one year. There may also be some variation according to the age of the mother or the number of children she has had.

"Breast milk also changes during the course of a feeding," explains Balay. "At the beginning of the feeding, what we call foremilk is basically water. This makes sense because what we need most urgently is hydration. So the foremilk is very watery and hydrates the child. And then, as the feeding progresses, the baby gets the hindmilk, where nutrients are more concentrated."

The average volume of milk for babies breastfed exclusively increases from around 500 ml/day (about 2 cups) to 800 ml/day (3-1/3 cups) by 6 months, according to Susan Tucker Blackburn, in Maternal, Fetal, & Neonatal Physiology. Milk volume is higher in women with multiple infants. It can also be higher in women with very low body fat (which can result in less fat and fewer calories in their milk); in both cases, increased sucking stimulation leads to higher milk production.

The fat component in breast milk also varies according to the stage of lactation. "In the first 6 months, breast milk is extremely high in fat, about 50% in terms of calories," says Balay. "The fat is needed to provide energy to support growth, including brain development. But when brain growth starts to level off by 1 year, the fat level also starts to come down a bit." However, a cause and effect relationship between rate of brain growth and human milk fat composition has not been established.

Unravelling the Mystery

While there are substances in breast milk whose functions and benefits are still not perfectly understood, researchers have known the basic constituents for decades. Researchers worked to unravel the mysteries of breast milk in order to devise formula for babies who could not breastfeed as well as to understand a wide range of nutritional deficiencies that affected both term and preterm babies. In fact, the history of the research efforts to deconstruct (and then reconstruct) the components of breast milk often overlaps the history of nutrition science in general.

"A century ago," explains Brenna, "what one could measure in a food was very elementary compared to what we can measure now. In those days you could measure nitrogen. And nitrogen translates into protein. So people knew that protein was very important. Why did they know that protein was important? Because they could measure it. If you can't measure it, we have no way of understanding its importance. So at that point in history, researchers could show that animals grew better on diets that had more protein compared to diets with less protein."

There was another learning curve when researchers discovered the presence and function of vitamins. Today, scientists are focusing on components such as immune and growth factors, as well as key fatty acids that play important roles in neurodevelopment. As a result, the story of infant nutrition is now much more complicated. This is great news for infants as well as parents. A deeper and broader understanding of nutrition and the role it plays in growth and development helps all babies be as healthy as they can be.

Energy Demands of Breastfeeding

Infant nutrition begins with the food a mother ingests. A healthy diet is important for expecting mothers as the fetus grows and develops. And while women are advised that they are not, in fact, “eating for two,” doctors and dietitians do encourage expectant moms to make sure their vitamin, mineral, protein, and calorie needs are being met.

The importance of good nutrition for new mothers continues after the baby’s birth, as the mother’s body transforms nutrients from her diet into breast milk. Although 9 months of pregnancy can require between 80,000 and 120,000 additional calories, breastfeeding for the same amount of time can burn up twice that. According to Susan Tucker Blackburn in Maternal, Fetal, & Neonatal Physiology, “For the healthy, well-nourished lactating woman, an additional 500 [calories per day] is recommended to meet the energy requirements for milk production during the first 6 months of lactation.” The good news is that “approximately 170 kcal of the increased requirements are provided by maternal fat stores from pregnancy.”

It may seem astonishing that lactation is more demanding in terms of energy than pregnancy, but whereas for much of gestation the fetus is tiny, weighing just ounces, a lactating mother can be producing more than 3 cups of milk daily to fuel the growth and development of an infant whose birth weight of 7 lbs or so can double in just 4 months.

Meals to Milk

The first step in the remarkable transformation of raw ingredients, whether they are bananas, burgers, or broccoli, into mother’s milk takes place in the mouth. Food is chewed and enzymes in saliva begin to break down the components. The stomach continues the physical and chemical disassembly of food by churning it and mixing it with powerful digestive juices that turn the meal into a milkshake-like slurry of tiny food particles and gastric liquid called chyme.

From the stomach, chyme passes to the small intestine. With the help of bile, hormones, and additional enzymes from the pancreas and intestinal epithelium, it is ready to be absorbed. The small intestine is lined with folds covered with tiny fingerlike projections called villi. The villi are covered with even smaller projections called microvilli. These structures produce a vast surface area through which nutrients are absorbed. Specialized cells in the microvilli allow nutrients to cross the membrane and enter the bloodstream, where they will be carried to the breasts to be repackaged into milk.

Through Thick or Thin

We know that not all diets are created equal. That goes for kids, adults, and also lactating women. And of course healthier maternal diets will translate into healthier mothers and healthier infants, just as healthier diets translate into healthier children and healthier adults. In evolutionary terms, however, lactation is so critically important that nature has made it a particularly robust and resilient biological process. How else would humankind have survived through lean times?

Physiologically, milk production is a compromise between the needs of the mother and needs of the infant. In times of scarcity, a mother’s body will tap her owns stores of fat and nutrients to ensure the survival of her infant. The baby’s nutritional needs have priority. It’s a fine balance, of course, since the well-being of the infant is also dependent on the well-being of the mother.

This doesn’t mean lactating women today should be indifferent to diet. But what the resiliency of lactation means in the real world, says Bo Lönnerdal, is that a woman living in poverty and on a restricted diet in Africa can still produce breast milk comparable to that produced by an affluent and well-fed woman in Sweden. “Very, very nice studies have looked at Gambian women in rural villages with very restricted food intake and the quality of their breast milk is excellent.”

This is true with many important ingredients, says Lonnerdal. For example, “a woman with malnutrition can produce breast milk with adequate and normal concentrations of protein.” Severe deficiencies, though, particularly with some vitamins, can result in breast milk that is also deficient in those vitamins. Calcium, too, can vary with diet. It all depends on the specific nutrient or vitamin. Diet can indeed affect breast milk, but not always in a completely clearcut manner. There is also natural variation in breast milk.

“Some of the minerals found in breast milk are very tightly controlled,” says Sharon Donovan, “such as iron and zinc, and these are very hard to change in milk. Because their levels are very well regulated in the mammary gland, these components are very stable.”

Iron is a good example of how the mother’s body regulates the concentration of key ingredients, says Lonnerdal. “An anemic woman in India or Ethiopia would have exactly the same breast milk iron levels as a woman in Sweden or Finland consuming 60-100 mg of iron every day. Exactly the same. No difference. Because the mammary gland tightly regulates this and makes sure that breast milk contains precisely the amount of iron that infants need.”

And that’s good for an infant of a woman in India as well as an infant in Finland. “If iron levels rose with diet,” explains Lönnerdal, “it could be detrimental to the infant because iron is not an innocent nutrient; iron in excess can have negative consequences. In addition, there are few or no menstrual losses of iron during lactation. But if iron in breast milk fell in response to diet, the infant would be iron deficient and anemic, so that wouldn’t be good. The mammary gland, therefore, is set to this very, very narrow window in order to meet the needs of the breastfed infant.” By 6 months, however, the baby does need additional iron, which is supplied in solid foods.

Lönnerdal points out that “a severely anemic woman would be tapping stores in her own body, so her situation worsens. On the other hand, very little iron is required in breast milk, so the additional loss in that case may not be dramatic. But it’s very obvious that the priority is definitely on making breast milk with adequate iron rather than covering the mother’s own dire needs at this time.” So while infants can be thankful that evolution has made lactation as bulletproof as possible, a healthy diet should still clearly be a priority for all pregnant and lactating women.

Curry, Kimchi, and Kung Pao Chicken

Another fascinating aspect of maternal diet has to do with flavor preference. “There’s been a lot of research at the Monell Institute in Philadelphia on how flavors are passed from the mother to the infant through breast milk and even to the fetus via amniotic fluid,” says Donovan. “When you think about it, if mom goes out and eats kung pao chicken, those spices pack a lot of flavor.” And when it comes to taste, familiarity usually results in acceptance.

Julie Mennella is one of the Monell scientists who have been doing this research. One European study had shown that if mother rabbits were fed juniper berries, their bunnies also loved this meal. Mennella repeated the rabbit experiment with human mothers, substituting carrot juice for juniper berries and found the same result. Similar research has been done with garlic, onions, vanilla, and, in France, with anise flavors.

Some of this research also suggests that breastfed infants are more apt to appreciate novelty in flavors, which might be, says Donovan, “because during the course of lactation, the infant experiences changes in the texture and composition of the breast milk and so could be more accepting later in terms of new foods and new tastes.” Since toddlers can be picky eaters, moms can perhaps give their kids a head start by eating a large variety of healthy foods themselves.

When your newborn is getting all of his nutrients from you, ensuring that your dietary choices are healthful and balanced is more important than ever. A new mother's body assembles the building blocks of total nutrition, as well as important immune factors and other beneficial elements, and delivers them to baby in an efficient, perfect form—mother's milk.

While it is always important for a woman to eat balanced healthful meals, during nursing the benefits take on new significance. Your body is very efficient at concocting milk that's perfect for your baby. In fact, the lactation process puts baby's health ahead of yours. If there are not enough of certain nutrients (calcium, vitamin D and other vitamins) available from your diet, your owns stores of these nutrients will be depleted to ensure that your milk is packed with what your baby needs.

Feeding Baby Takes Energy

As you might expect, when your body is constantly manufacturing high-fat and nutrient-dense mother's milk, extra calories are expended. Although the total varies depending on a mom's weight and activity level, doctors estimate that breastfeeding burns about 500 extra calories per day. That is good news for moms who have some leftover pregnancy weight to lose.

You should make up the difference by eating a little more of food rich in vitamins, minerals, protein and fiber. An additional snack, or a couple during the day, with vegetables, protein and extra calcium would be ideal. A 500-calorie splurge on a candy bar or French fries may seem like a fun treat. But for the important work your body is doing for your baby, you should make every bite count.

Smartest Food Choices

Avoiding certain foods and beverages because they contain something that would harm your baby is just as important as maintaining nutritional balance. Know what you are putting in your mouth, and whether it's safe for your baby.

Limit or Avoid

Alcohol—The official word is to avoid alcohol altogether while nursing. Alcohol in the blood stream does make its way into mother's milk. However, once your baby is on a schedule that includes sleeping for several hours during the night, the alcohol in an evening glass of wine or cocktail should be cleared from your system by the next morning.

Caffeine—You don't have to give up coffee or caffeinated tea, but try to limit it to two or three daily cups. Newborns' bodies cannot break down caffeine efficiently, and it can build up in their bodies. Babies aren't all the same. If it seems your baby is out of sorts or has an upset stomach after nursing, you can try cutting out the coffee to see if it helps.

Some fishes—Fish is packed with nutrients babies need, especially omega-3 fatty acids essential to brain development. However, eating fish high in mercury can introduce this toxin into your breast milk. Mercury can harm the developing nervous system and brain. The FDA recommends that nursing women avoid shark, swordfish, king mackerel and tilefish because of the mercury threat. Many other fish contain varying levels of mercury. Try to eat 2 servings (12 oz.) of safe fish per week. These include herring, anchovies, Atlantic mackerel, rainbow trout, salmon, sardines and whitefish.

Pesticides—A sad fact of modern life is that it is nearly impossible to avoid pesticides. Commonly used pesticides make their way into breast milk. To keep intake of pesticides as low as possible, buy organic produce whenever possible. Remember that pesticides also accumulate in commercially produced beef, chicken, pork and lamb. Organically farmed animals may have lower levels of some pesticides, depending upon where they are raised and what they are fed.

Dig In!

Water—The first dietary need you'll sense when you start nursing is the need to hydrate. Breastfeeding is thirsty work! Drink plenty of water throughout the day, especially just before and during every nursing session.

Lean protein—Low-fat meats and (safe) fish, as well as egg, nuts, and vegetable-based protein sources are essential for your own health and your baby's. Get at least 6 ounces of protein every day.

Fruits & vegetables—The garden boasts the best sources of key vitamins and dietary fiber. Go for plenty of leafy greens, brightly colored vegetables, berries, mushrooms, and more. Aim for at least 3 cups of vegetables and 2 servings of fruit every day.

Whole grains—Unprocessed or lightly processed grains will give you healthy carbohydrates for energy as well as fiber, vitamins, and minerals. Include 7 or 8 ounces per day, a little less if you are trying to lose post-pregnancy pounds.

Dairy—Three cups of low-fat milk, yogurt, or cheese is recommended for nursing moms. If you use non-dairy substitutes, be sure to ask your doctor whether you should supplement your calcium and vitamin D intake.

Pathways and Packaging

The building blocks of milk arrive in the breast via two main pathways. In addition to normal digestion (discussed previously), which results in nutrients carried through the bloodstream to the breasts, other special components in milk, such as immune and growth factors, arrive through the lymphatic system. Immune tissue in the gut, for example, is the source of specialized cells that are then transported to breast tissue via this route. Once the components have arrived in the breasts, many undergo further processing and packaging. And in fact, some of breast milk’s most unique qualities are the result of this ingenious packaging.

The components leave the blood or lymphatic fluid and are taken up by the epithelial cells called lactocytes. Once inside these cells, there are several different routes by which they enter milk, though a few components are transported directly into the alveoli through the tiny spaces between the epithelial cells.

Different nutrients undergo different processing and are delivered into the alveoli via different specific mechanisms. The names of the sites in the cells and the pathways the nutrients follow sound very technical (and they are!), but think of them as various stops along a factory assembly line: here’s where they attach the doors to the car, or here’s where they wrap the package of cookies. Some nutrients arrive basically intact, while others are assembled pretty much from scratch.

Here is a quick rundown of the pathways:

• Exocytosis (“exo” meaning “exit”) is the name for the pathway by which milk components such as proteins and lactose, which are made and assembled within the lactocyte, are then transported out of the cell and into the alveoli. Amino acids are absorbed into the lactocytes from the bloodstream. Once inside, they are synthesized into proteins by ribosomes located along another special structure in the cell called the rough endoplasmic reticulum (RER) to form various milk proteins, such as casein. From the RER, the proteins are transferred to another structure, called the Golgi apparatus, where they are processed for delivery into the empty cavity, or lumen, of the alveoli. This processing can involve further packaging. Casein, for example, is attached to calcium and phosphorous to form a tiny unit called a micelle, which forms soft curds in the stomach. The simple sugar glucose is also absorbed into the lactocytes, where it is transformed by the Golgi apparatus into milk sugar, or lactose, the primary carbohydrate in breast milk.
• Lipid synthesis and secretion is the pathway by which milk fat-globules are assembled and transported. Milk fats also undergo transformation in the lactocytes. Building-block molecules, as well as some premade nutrients such as fatty acids, are taken up by the lactocytes. These components are then synthesized within the cells into fats called triglycerides. Next, tiny fat droplets fuse together into a larger drop of milk fat. The fat drop is forced out of the cell surrounded by a membrane, which is actually a tiny piece of the lactocyte cell membrane. It is now called a milk-fat globule. The packaging of fats in this way is critical because without the membrane surrounding the globules, fat droplets would coalesce into ever larger blobs of fat. This would make milk secretion very difficult.
• Transmembrane secretion. This is the pathway by which ions and water are transported directly across the lactocyte membrane into the alveolus. The small molecules transported by this route include sodium, potassium, chloride, some monosaccharides, and water.
• Transcytosis (“trans” meaning “across”) is the pathway by which intact proteins move through the interior of a lactocyte. The cell creates a tiny temporary bubble, called a vesicle, that engulfs the milk component on one side of the cell and then carries it and releases it on the other side of the cell into the empty cavity of the alveoli. This pathway is used for premade immunoglobulins, hormones, and growth factors. These components do not undergo transformation or packaging in the lactocyte.
• The paracellular pathway involves transport of substances through the tiny spaces between adjacent lactocytes. For the most part, these tight junctions are intended to prevent substances from leaking either into the milk or out of the milk back to the mother. At certain times, however, this route is important for the movement of specific substances, particularly those involved in fighting infections. In cases when the breast tissue becomes infected, anti-inflammatory substances and infectious waste products move into and out of the mammary tissue via this route.

Hormonal Preparation for Lactation

A cascade of hormones that begins in the second month of pregnancy triggers a series of changes, preparing the mother’s body to produce milk:

The hormones involved in lactation are produced in several places in the body, including the pituitary gland, ovaries, breast tissue, and placenta. Among the hormones that play roles in preparing for milk production are human placental lactogen (HPL), which stimulates breast, nipple, and areola growth before birth; estrogen, which stimulates the milk duct system to grow; progesterone, which influences the growth of alveoli and lobes; prolactin, which also contributes to the growth of the alveoli; and oxytocin, which contracts the smooth muscle of the uterus during labor. Prolactin and oxytocin work together to control milk production after birth.

By the fifth or sixth month of pregnancy, the breasts are basically ready to produce milk and are waiting for the hormonal changes triggered by birth. The initiation of milk production is called lactogenesis. It can be divided into three stages, each marked by changes in milk composition. Lactogenesis I is the initial stage, which begins in midpregnancy and ends 2-4 days after birth. Lactogenesis II can last for 2 or more weeks and is followed by lactogenesis III, in which mature milk is established and milk supply is maintained through supply-and-demand in response to the infant’s sucking.

If a woman does not breastfeed her infant, levels of prolactin and oxytocin fall and milk production ceases. Mammary tissue gradually returns to a resting state, but because new alveoli formed during pregnancy never totally disappear, the breasts do not completely return to their prepregnant state, although studies have found that the changes in the breast tissue that occur during lactation seem to protect the mother against breast cancer.

Predictable Variation

One of the fascinating things about breast milk is that it isn’t uniform. And that’s a very good thing, even though that may seem counterintuitive. The composition of breast milk changes over time, for example, in generally predictable ways, and in this process also helps meet the changing nutritional and developmental needs of the infant.

In the first few days after her baby is born, a mother produces a transparent yellow liquid called colostrum. Compared to mature milk, colostrum is lower in fat, carbohydrates, and calories, but has double the protein because of all the infection-fighting antibodies and white blood cells it contains. In terms of volume, the mother produces very little colostrum, literally only tablespoons, and yet it plays a key role in boosting the baby’s immune and digestive systems.

“Colostrum seems to have evolved primarily to provide passive immunity for the infant,” say Sharon Donovan. “There’s not a lot of nutrition here, but boy, are there a lot of immune elements. Though colostrum is perfectly designed for an infant’s first few days, it looks so unlike mature milk that in many cultures they discard it, and the baby doesn’t receive it.” The practice of discarding this critical initial milk was practiced in ancient India and is also mentioned in the Old Testament.

Within 3 or 4 days, colostrum gives way to transition, or early, milk, which is lower in proteins, but higher in lactose, water-soluble vitamins, fat, and total calories. Moms produce a cup of transition milk a day, which usually increases to 2 cups by the end of the first week. Women vary in how long they produce transition milk, but by 2-3 weeks, mature milk is being produced, 3 cups or more daily, and is characterized by more fat and less overall protein than transition milk.

Another significant difference between transition and mature milk is the nature of the milk proteins. There are two main categories of proteins in milk: whey proteins and casein proteins (casein can be thought of as the curd produced in cheese making, while whey is the leftover liquid). The average ratio of whey to casein in early milk is 80:20, while the ratio in mature milk is 60:40 and eventually close to 50:50.

Both proteins are essential. Whey proteins include lactoferrin and immunoglobulins especially important to a newborn. And while whey is more rapidly digested, this doesn’t mean casein is any less essential to an infant. On the contrary. Because casein stays in the gut longer, it may promote the growth of beneficial bacteria in the infant’s gastrointestinal tract. Casein is also needed to enhance the absorption of minerals by keeping them in solution in the gut. Even though breast milk is initially mostly whey, this doesn’t necessarily mean that whey is what newborns need most, notes Sharon Donovan. “It may also be related to the maturation of the mammary gland and the enzymes that make casein.” In any case, says Donovan, “casein is a unique protein; it’s not present in any other place in the body.”

There is also a predictable type of variation within the course of any given feeding. At the initiation of a feeding, the breast produces what is called “foremilk,” which is thin and watery like skim milk. Later in the feeding comes the “hindmilk,” which is thick and rich like cream and helps the infant feel satisfied. There are various physiological reasons breast milk is delivered in this sequence. But the result for the infant is that she obtains a higher percentage of lactose in the early part of the feeding and a higher percentage of calories and fat in the latter part of the feeding. For this reason, it is important that the mother finishes feeding from one breast before offering the other so that the baby gets the complete and balanced benefits of breast milk.

Tracking Down the Components

Is there anything in breast milk that researchers haven’t detected yet, some vital ingredient that has eluded attempts to find it? “Probably not,” says Bo Lönnerdal. “I think we are reaching the limit; the sensitivity of our analytical methods now is so high that the likelihood that something would be there, have a function, and we couldn’t detect it, has kind of disappeared. But I think a decade ago that was not the case.” Knowing everything that’s in breast milk, however, is different from knowing exactly what role or function each component plays.

Among the last components to be detected, says Lönnerdal, were proteins in very low concentrations. “In the mid-1990s we could detect in breast milk around 30 proteins with some certainty. With the tools we are using right now, we’re talking about 150 to 200.” If there is anything that hasn’t yet been identified, it is probably something from the blood. “There’s not an absolute barrier between breast milk and the blood stream,” explains Lönnerdal, “so that means that virtually all the proteins that you see in the bloodstream are also found in breast milk at fairly low concentrations. It’s tough to know if they ‘do’ something in breast milk.”

But, again, just because the long list of ingredients in breast milk is established doesn’t mean that milk chemists like Lönnerdal are satisfied that they know the exact role and function of these components. Increasingly, says Lönnerdal, the real frontier is to understand how the packaging of milk ingredients facilitates their physiological role in the infant and how various milk ingredients interact and otherwise regulate the action of other milk components.

“Some of these factors in milk might actually enhance or potentiate the actions of others,” explains Lönnerdal, “but some may inhibit others. So when you start studying the components in isolation, if you see a function, that’s fine, but if you don’t see it, well, maybe you needed the matrix of breast milk or the collaboration of other components to see the function.”

Sometimes proteins need other proteins, for instance, says Lönnerdal. “A beautiful example of this is the interaction of two proteins called lactoferrin and lysozyme. Lactoferrin in itself can do a lot of things in the body. Lysozyme can do a lot of things in the body. For instance, lysozyme can destroy certain bacteria by rupturing their cell walls. It’s part of our defense against infection. But other types of bacteria are harder for lysozyme to kill. So what happens is that lactoferrin binds to a component of the outer membrane of these types of bacteria and takes a specific piece out of it. Once lactoferrin pokes holes into the membrane, lysozyme can kill it. Lysozyme can’t do it alone. Lactoferrin can’t do it alone. But lactoferrin together with lysozome can do it.”

Indigestible Ingredients

Researchers are also investigating the “functional” roles played by certain proteins and carbohydrates, including breast milk components that are not digested or digested only partially.

Oligosaccharides are carbohydrates that don’t directly provide fuel for the infant; instead, they serve as food for beneficial bacteria that are an important part of a baby’s immune system. One way to think of these important, but indigestible, carbohydrates, says Donovan, is to “think of them as the ‘fiber’ in breast milk.” Oligosaccharides help the lining of the baby’s gastrointestinal tract mature, and they also serve as a type of decoy for roaming pathogens.

Breast milk also contains proteins that, like oligosaccharides, resist digestion in order to play functional roles in supporting the immune system and defending the infant against infection. Further evidence, says Lönnerdal, of the amazing spectrum of roles performed by the ingredients in mother’s milk.

Sources of Variation

That breast milk composition is not uniform was brought home very clearly to Sharon Donovan when she was still in school. “When I was a grad student, we’d collect breast milk samples and bring them into the lab for analysis. When you spin breast milk in a centrifuge in the cold, all of the fat rises to the top. In samples from some women, the fat layer was thick; it looked like the lard on the outside of a steak. In other women you could almost just pour it off, like corn oil.”

The underlying reasons for that sort of variation were reviewed in a United Nations report. “There are several factors that are known to influence the concentration of breast milk constituents in predictable ways,” explains the report. “These include stage of lactation; breastfeeding routine; parity, age, and other maternal characteristics; regional differences; and, in some situations, season of the year and maternal diet.” The reason it is important to better understand this variation, of course, is because “differences in breast milk composition affect the daily intakes of milk components by the breastfed child.”

One factor that influences milk composition in some parts of the world is season. “In sub-Sahelian Africa,” according to the UN report, “where food availability, infection rates, farm work, and childcare patterns vary between seasons, variations in the concentrations of some constituents, such as fat, immunoproteins, and water-soluble vitamins, have been observed.” An example of a seasonal effect in this region can be found in the relative concentration of vitamin C in breast milk: levels are high during the season when mangoes are plentiful but low for the rest of the year.

An important point made by the UN report is that, overall, “the similarities between regions are more striking than the differences, particularly with respect to the major nutrients.” This doesn’t mean all the differences are negligible, but rather, that the amount of variation within any given population is likely to be larger than the amount of difference between one region and another region. Significant variation among individuals, rather than populations, seems to be the rule.

There are still many reasons, however, to study both the similarities and differences in breast milk around the globe. “I’ve been involved in studies where they’ve picked nine different regions in the world and looked at the composition of breast milk,” says Bo Lönnerdal, “and it turns out to be very similar when it comes to most of the components.” But looking at what components do vary, and why, can be a rich source of information on the genetic, cultural, and environmental factors that influence breast milk. Tracking mothers’ dietary intake and infant health also provides information that can be used to improve the well-being of both mothers and babies worldwide.

Role of Genetics

Researchers are coming to believe that genetics could be playing a significant role in breast milk variation. But, as Lönnerdal notes, “I think we need to be clear about the distinction between genetics and ethnicity.” And that can be tricky. “It’s very difficult to disassociate them because different ethnic groups have different dietary habits, but I haven’t seen anything that would convince me that breast milk from, say, Hmong women in California would be different from Caucasian women in California if they had identical dietary habits.”

Lönnerdal, who has also studied zinc deficiencies in breastfed infants, believes that new (and increasingly inexpensive) genetic screening tools will help researchers better understand the origin of much breast milk variation. “We have found that some women produce breast milk that is extremely low in zinc. It’s the only thing that is abnormal; everything else is fine as far as we can judge. And we know it is a genetic anomaly. I think this is uncommon, but how uncommon is it? We don’t know.”

Genetics can influence the composition of breast milk through an uncommon mutation, but genetics may sometimes play a broader role. An example of this sort of genetic variation in breast milk involves the oligosaccharides that help support infants’ immune systems. There are about 200 different known oligosaccharides in human milk, and one individual’s milk can contain anywhere from a few dozen to more than 100. It turns out that different oligosaccharides protect the infant from different pathogens. So an infant who receives one type of oligosaccharide in breast milk may be protected against one type of pathogen, but not another.

While oligosaccharide profiles don’t strictly reflect geography (they are actually closely tied to blood group types), recent studies, says Donovan, have shown some interesting regional variation. “If you go to Mexico, for example, you see that the women there are creating oligosaccharides that are a little different than women elsewhere. So it does seem that women in different areas of the world have different oligosaccharide patterns.” But, again, there is also substantial variation within a given population in Mexico.

A type of variation in breast milk composition that is strongly geographical is the mother’s production of antibodies, immune factors that help fight pathogens. All infants receive antibodies in breast milk, but it’s particularly interesting that each infant receives antibodies to the organisms the mom actually encounters, and these can be very local. As a result, the antibodies a woman produces in Sri Lanka will be different than those a mother produces in Boston or San Francisco. In this way, breast milk “knows” which germs are likely to threaten the infant and arms that baby with the right immune defenses. Pretty smart.

Add to this mix the fact that a breastfed infant is exposed to the flavors and spices of the local cuisine the mom is eating, and what you have, in physiological terms, are babies who “know” where they are living long before they can ever send a postcard.

It seems timing is everything when it comes to breast milk. The cascade of hormones, the stepwise preparation of the mammary gland for milk production, the sequence of nerve stimulation and response, and the supply-and-demand rhythm of breastfeeding all proceed in sequence. Not surprisingly, even the changing composition of breast milk is in synch with the growth and development needs of the infant.

When newborns most need immune protection, they receive more defenses from their mothers’ colostrum. When a week-old baby needs readily digestible protein, breast milk contains a high concentration of soluble whey proteins. When the baby’s brain is growing fastest, breast milk is full of key fatty acids such as DHA. The list goes on and on. Sometimes the timeframe is days, other times it is weeks or months, but breast milk delivers vital nutrients when the infant needs them most.

The equation also holds true in reverse. When a baby’s immune system is fully developed, the concentrations of antibodies and immune factors in breast milk level off. When a baby’s gastrointestinal tract is mature and flourishing with beneficial microflora, the level of oligosaccharides in breast milk subsides. When a baby’s pancreas starts making digestive enzymes, the temporary digestive enzymes supplied by breast milk taper off. The resources of breast milk reflect the developmental needs of the growing infant.

Tracking Growth

Overall growth is one obvious way in which parents and physicians track the health and development of a baby. An infant’s growth is fueled by the nutritional energy as well as the building blocks that are delivered in breast milk. The development of the baby’s digestive and immune systems, bones, muscles, and circulatory and respiratory systems may be less visible, but here, too, breast milk delivers nutrients to babies when they need them most.

A baby’s birth weight typically doubles by 4 months of age and triples by 1 year. A baby will increase in length by roughly 50% by his or her first birthday. Another key measure for infants is head circumference. At birth the average is just over 14 inches (36 cm), roughly two thirds the circumference of an adult’s head. At 2 months the infant’s head circumference is approximately 15-3/4 inches (40 cm), at 4 months about 17-3/8 inches (44 cm), and at 1 year a little over 18 inches (46 cm). All of these figures are averages, of course. It’s important to remember that healthy babies come in all shapes and sizes. In any population, there will always be individuals, whether babies or adults, who are shorter than average, taller than average, heavier or lighter than average. An average is just a number, and there is a wide range of normal variation within any population.

At the same time there is variation in weight and height, there is also variation in stages in the development, such as when a baby will hold up his head, utter her first word, or take her first step. Developmental milestones are averages.

Evolution and the Recipe for Human Milk

“When we look at human milk compared to other species,” says Donovan, “it seems kind of dilute; its protein content in particular is quite low compared to other species. But it turns out that you can almost draw a direct line correlation between the richness of the milk and the growth rate of the infant.” Another way to look at it is the thinner the milk, the longer lactation lasts.

We are used to thinking of babies as growing fast, and by some measures they do. But compared to other mammals, human infants are real slowpokes. Hooded seals, for example, according to Sarah Blaffer Hrdy, “have one of the shortest periods of lactation known, as brief as a week. Mothers stockpile blubber in advance, then give birth atop ice floes to pups who imbibe pure cream, a high-protein formula containing 60% fat, approximately 1,400 calories in an 8-oz glass.” The seal pup can gain 50 lbs in a matter of days.

Growing slowly has its advantages, particularly for highly social, language-using primates like ourselves. Growing slowly also represents, in evolutionary terms, an enormous reproductive investment on the part of the mother. “Our reproductive rate is slow compared to other species,” explains Donovan, “and we typically have one infant per birth, and so we put a lot of resources into raising that infant compared to species that can have several litters in a year.”

It might strike parents as odd or even off-putting to talk about babies in terms of resources and investment, but examples of a mother’s biological “investment” can be clearly seen in the composition of breast milk. It takes energy for the mother to assemble, transport, and manufacture many of the special components of breast milk, particularly those components that protect the infant from pathogens and disease. “So while human milk doesn’t need to provide lots of protein for quick growth,” says Donovan, “it has proteins and other components that pack a lot of bioactivity and functionality. Much of that functionality is related to helping defend the infant against infections. That seems to be really a hallmark of many of the proteins, such as lactoferrin, lysozyme, and the immunoglobulins. And even when it comes to carbohydrates, we also have all of these oligosaccharides which again have a role in defending the infant against infection.” Breast milk protects our precious investments.

During fetal development, all nutrients for growth are supplied by the placenta, which also removes wastes from the amniotic sac. At birth that changes in one fell swoop, when the newborn digestive and excretory systems must take over.

The infant's gastrointestinal tract is also one of the key "surfaces" that are newly exposed to the environment. We are not used to thinking of it in this way, but the lining of a baby's digestive system, in a fundamental way, is actually on the "outside" of the baby. It is the boundary and interface between food taken in from the environment, whether that is breast milk or formula, and the organ systems this food will nourish. The gastrointestinal (GI) tract must therefore serve as a protective boundary against a wide variety of environmental microbes and other contaminants. It is a very dynamic but vulnerable surface.

And yet, the baby's stomach, and digestive system in general, is still immature. One significant difference is size, of course. A baby's stomach capacity is extremely small, which is why infants require frequent feeding to supply adequate amounts of nutrients for growth and development.

"It's an important point for new parents to keep in mind," says registered dietitian Julie Balay. "The big question that moms always ask is, 'Is my baby eating enough?' That's certainly the concern with newborns. And maybe even more so with breastfeeding, since there is no 'last drop' in a breast to gauge when a mom feels her infant has had enough. But it also extends to the second 6 months, when some solid foods and finger foods are being introduced."

The small size of the stomach and digestive system is another reason there is so much fat in breast milk. A gram of fat contains more than twice the caloric energy of a gram of either carbohydrate or protein. And that means the most efficient fuel for a small digestive system will include a lot of fat. "Most of what we eat is burned as fuel," says Professor Tom Brenna of Cornell University. "We use it for energy. We could track the components of a sandwich and after a week, two thirds of it would be gone, burned as energy. And even the components that are not burned for fuel are turned over as cells grow, die, and are replaced. After a month, there would be very little of that sandwich still in the body."

According to Susan Tucker Blackburn, in her book, Maternal, Fetal, & Neonatal Physiology, researchers have even calculated the overall energy bill for fetal development. In the second trimester of pregnancy, a woman needs an additional 300-340 kcal per day and up to 450 additional kcal per day in the third trimester to meet the energy and growth demands of the fetus. The total minimum energy cost of reproduction is 80,000 kcal and can be as high as 120,000 kcal for some women.

How much energy is that? In fast food terms, it's roughly equivalent to 100 double-cheeseburgers, 200 small orders of fries, and 90 hot fudge sundaes for good measure. And that's just the energy bill at birth. The energy needs of a 2-month-old, 11-lb baby girl is more than 500 kcal a day, or roughly the equivalent of a Big Mac.

The Immature Digestive System

An infant's digestive system, besides having a very limited capacity, is also immature. The infant's pancreas is not yet up to speed, which means that certain digestive enzymes are at much lower levels than they are in older children or adults. The infant compensates by relying on nonpancreatic enzymes such as those found in breast milk as well as in the baby's own salivary secretions. Another difference is that the esophageal valve, which is the opening at the top of the stomach, is not fully developed. That's why it is common for babies to have problems with spitting up.

Although infants can digest proteins, fats, and carbohydrates at birth, their ability to do so improves as they develop and the GI tract lining matures while levels of digestive enzymes and stomach acids increase. Meanwhile, the limitations of their immature digestive and renal systems can put them at risk for dehydration, malabsorption of nutrients, and electrolyte imbalance.

The infant's protective mucosal barrier is immature at birth. As a consequence, pathogens and other macromolecules can potentially penetrate the intestinal epithelium and enter the infant's circulatory system. This can increase the infant's risk of infection as well as the development of allergies. "There are antibodies and other large immune molecules present in breast milk," says Brenna, "that can help support the baby's immune system." The main antibody providing protection in breast milk is called secretory IgA. As the infant's digestive system continues to develop, it will become much less permeable to large molecules. This coincides, however, with the baby's increasing ability to make his or her own antibodies.

The infant's gastrointestinal system matures with the help of many factors, including vitamins and minerals, hormones, and other components of the diet. Breast milk is rich in intestinal growth factors and other components that help establish the colonization of the GI tract by beneficial bacteria, which in turn help the intestinal lining mature as well as prevent the spread of pathogenic microbes.

It is no surprise that an infant's digestive system encounters a learning curve at birth. What is remarkable is the extent to which the fetus has been rehearsing for this debut. Even sucking and swallowing, the behavioral and motor skills most fundamental to the success of the baby's nutritional changeover, have been practiced while the infant was still developing in the womb.

The term metabolism comes from a Greek word meaning "to change." It is often used in a narrow sense, as a synonym for burning calories, especially in diet books that promise ways to speed up a person's metabolism. But metabolism actually refers to the entire range of biochemical processes involved in assembly and disassembly, transport and transformation, that are constantly taking place within any living organism.

The metabolism of a developing fetus is fueled in large part by glucose, though amino acids and lipids are also required for protein synthesis and tissue growth. At birth, this constant supply of glucose comes to an abrupt end. The newborn must now rely on breast milk or formula, which in turn triggers a series of changes in the infant's body. Fat becomes a critical energy source. The infant's body must balance its simultaneous needs for the biological building blocks necessary for tissue growth and the energy required to fuel this growth.

The small intestine is the site of most of the infant's absorption of nutrients. A newborn has lower levels of digestive enzyme activity than does an older child or adult, but the newborn is able to compensate for this shortfall by using other enzymatic pathways. This ensures that he or she is able to obtain all of the nutritional components necessary for healthy growth and development.

Protein

The recommended protein intake for infants from birth to 6 months is about 9.1 g per day (1.52 g per kg of body weight per day). From the age of 7 months to 12 months, it is about 11.0 g per day (1.2 g per kg of body weight per day). [Institute of Medicine, Dietary Reference Intakes.]

There are thousands of different proteins in the human body, but they are all combinations of fewer than 2 dozen common amino acids. Some of these amino acid building blocks can be made by tissues in the body from even smaller components. Nine of the amino acids, however, cannot be synthesized in the body and therefore must be obtained through the diet. These are called "essential amino acids." Developing fetuses, as well as preterm infants, require at least 2 other amino acids in their diet because their capacity to manufacture them is still undeveloped.

Proteins are required for the building, maintenance, and repair of tissues throughout the infant's body. They are also necessary as a source of building blocks for the synthesis of digestive enzymes, regulatory hormones, growth factors, and important components of the immune system such as antibodies. "Most people think of protein as just being muscle," says registered dietitian Julie Balay. "But protein is really the building block for all tissues. It's important for muscles, yes, but there are proteins circulating throughout our body, proteins that are part of our immune system. Our enzymes are actually little proteins that help us digest our food. Protein is really a critical component throughout our whole body."

Infants are able to digest proteins, though the range of dietary protein they encounter as infants is very limited. The process begins in the stomach, where the enzyme pepsin, with the help of acid, hydrolyzes, or breaks apart, large protein molecules. In the small intestine, enzymes such as trypsin and chymotrypsin enable these components of protein, small peptide fragments and amino acids, to be absorbed by the intestinal lining.

The predominant protein fractions in human breast milk are referred to as whey and casein and are present in mature milk at a ratio of about 60% whey to 40% casein. Both whey and casein proteins are important components of human milk and provide complementary amino acid profiles to meet the requirements of developing infants. In addition to providing building blocks, casein aids in the absorption of minerals in the infant's gut, and casein may also help promote the growth of beneficial bacteria and discourage the growth of pathogens. The protein content of colostrum is twice the level of protein in mature milk, but much of the additional protein in colostrum is in the form of immune factors such as antibodies and immunoglobulins, which help the infant resist infection.

A newborn's initial gastric pH is less acidic than that of an older child or adult and this reduces enzyme activity and the breakdown of proteins. The secretion of gastric acid in the infant's stomach begins to increase at birth and doubles by 2 months. Levels of many enzymes in the infant's digestive system are also lower, between 10% and 60% of adult levels, according to Susan Tucker Blackburn, in Maternal, Fetal, & Neonatal Physiology. Other enzymes, such as trypsin, are closer to adult levels, but their activity is initially reduced.

Carbohydrates

An infant's intake of carbohydrates from birth to 6 months is about 60 g per day. From the age of 7 months to 12 months this increases to about 95 g per day. Carbohydrates are important because they supply food energy for growth, body functions, and activity, as well as facilitate the efficient use of protein and fat in the diet. Carbohydrates are also building blocks for some essential body compounds. Nucleotides, for example, are compounds made up of a sugar (a simple carbohydrate) plus a nitrogen base (a component of protein) and phosphate group; nucleotides, in turn, are the building blocks of DNA and RNA, the genetic instructions that guide the cellular development, division and function of all living organisms. Free nucleotides are present in breast milk and they also play important roles in a baby's metabolism, including cellular signaling and important enzymatic reactions.

While the developing fetus depends on the blood glucose from the mother to supply its carbohydrate needs, a newborn relies primarily on lactose from breast milk. There are small amounts of other carbohydrates in breast milk, including indigestible components that promote the healthy development of beneficial intestinal bacteria that inhibit pathogens. Called human milk oligosaccharides (HMOs) or prebiotics (in formula versions), "a good way to picture the function of these compounds," says Professor Tom Brenna of Cornell University, "is to consider them as digestive fiber for the baby."

Adults digest carbohydrates with the help of salivary and pancreatic amylase, as well as carbohydrate-digesting enzymes secreted by the lining of the small intestine. At birth, however, infants have only a third of the amylase activity of adults, according to Blackburn, and levels of pancreatic amylase are less than 5% of adult levels. Salivary amylase, as well as mammary amylase from breast milk, help the infant compensate for the reduced activity of pancreatic enzymes. An infant is able to digest carbohydrates fairly easily, with the exception of complex carbohydrates a baby would have no reason to encounter until the introduction of cereals at about 6 months. And at that point, the baby has developed the digestive capacity to break down this nutrient.

Fat

An infant's intake of fat, or lipids, is approximately 31 g per day from birth to 6 months. From the age of 7 months to 12 months, with the introduction of complementary foods such as cereal, intake decreases to 30 g per day. Fatty acids are building blocks for lipids and can be used as a source of energy. There are two fatty acids in particular that are called "essential" because they cannot be manufactured in the body and therefore must be obtained through the diet.

About 50% of the calories in breast milk come from fat. The function of lipids is to provide energy; serve as insulation to reduce body heat loss and protect the infant from injury; facilitate the absorption of the fat-soluble vitamins A, D, E, and K; and provide fatty acids that are critical for normal brain and eye development, healthy skin and hair, and resistance to infection and disease.

Lipids are also critical components, both structurally and functionally, of cells in the brain. They are the main components of the myelin sheath, which facilitates the efficient transmission of impulses along nerve cells.

And although we generally associate the image of plump newborns with health, that chubbiness develops gradually. According to Blackburn, fetal fat content, as a percentage of body weight, increases from a miniscule 0.5% in early gestation to approximately 3.5% by 28 weeks and then to a pudgy 16% of body weight by term birth. The gain in body fat is a result of both the transfer of dietary fat components from the mother as well as the production of fat in the fetal liver and other tissues.

Fat digestion in adults relies on an enzyme also produced in the pancreas called lipase, and again, at birth an infant's lipase production is only a fraction of that in older children and adults. The activity of another key component of fat digestion, bile acid, is also considerably lower. Lipase found in breast milk helps close this gap, however, and enzyme activity quickly accelerates in healthy term infants. This allows babies to absorb up to 90% or more of fat, though infants born prematurely usually absorb only 70% or less.

We know that complete nutrition is necessary for healthy growth and development. But is it possible to identify single nutrients that are responsible for the health and function of a specific organ, a vitamin that determines good hearing, healthy joints, or efficient liver function? Not exactly. While researchers have indeed identified many different nutrients that play key roles in specific systems and physiological pathways, in nearly all cases, it is the interaction of many different nutrients that determine healthy growth and development.

Much of what we know about the impact of single nutrients has been learned through the study of deficiencies. The classic investigation of a deficiency, and one of the first scientific nutrition experiments, was conducted in 1747 by a physician in the British Navy, who believed that citrus could prevent scurvy, a disease of malnutrition that was a terrible scourge for sailors who spent long periods of time at sea without fresh fruit or vegetables. It would take decades before the significance of the experiment was widely appreciated, but eventually, the consumption of citrus on ships to prevent scurvy gave rise to the slang term "limey" for British sailors.

Other common deficiencies include pellagra, caused by a niacin deficiency; kwashiorkor, a form of malnutrition caused by insufficient protein; beriberi, caused by a thiamine deficiency and often a consequence of eating only polished white rice; rickets, the result of inadequate mineralization of bone caused by a vitamin D deficiency or defect in its metabolism; and iodine deficiency, which is the most common cause of preventable mental retardation and brain damage worldwide.

Anemia caused by insufficient iron, according to international health organizations, is currently the most common deficiency overall, affecting 700 million worldwide. In some developing countries the immediate cause of the deficiency may be blood loss, a consequence of hookworm infestation, for example, but in many other countries it is caused by not getting enough of this mineral in the diet.

Iron is also a good example of the balance that must be struck in a diet, particularly an infant's diet. There is very little iron in breast milk, but what is there is easily absorbed by the infant. While iron is indeed essential for the production of red blood cells as well as the development and regulation of other organ systems, iron can also fuel the growth of pathogenic bacteria in a baby's gut. Breast milk provides enough iron to meet the infant's needs, but not enough to encourage disease-causing microbes. And in fact, there is a key protein, lactoferrin, also present in breast milk, which binds to iron in order to maintain this balance.

The more researchers learn about infant nutrition, the more examples we have of the critical role played by micronutrients. Occasionally, says registered dietitian Julie Balay, this can be the source of debate. "There are two issues with breast milk that are controversial, and those are fluoride and vitamin D. Even if the mother drinks fluoridated water, it doesn't show up in breast milk. And often we find that vitamin D is at very low levels in mothers." So this raises the question: should we fortify breastfed babies with fluoride and vitamin D?

"It's controversial, because on one hand you think, well, it's breast milk. It must be perfect," says Balay. "But on the other hand, we now know a lot more about nutrition than we used to. We didn't use to live as long as we do now; maybe we need fluoride to make our teeth last 80 years." The issue with vitamin D, explains Balay, is trickier. "We know that vitamin D has hormonal effects on the body and that it is also critical for the proper absorption of calcium. The concern is whether all babies get enough in breast milk. Well, we now we have recommendations that all breastfed babies be given vitamin D."

A pregnant woman doesn't need to eat the same diet as a newborn to assure that all the right nutrients get to the baby. Some of the necessary nutrients are going to get shifted and shunted to the fetus. The same goes for breastfeeding. Take fat, for example. A nursing mother should still eat a moderate diet, not one in which 50% of the calories come from fat. However, not all nutrient gaps can be overcome through nutritional shunting. Careful consideration of dietary choices and supplementation may be necessary to optimize the health of the mother and developing infant.

Micronutrients are clearly critical. But what is not as obvious, explains Balay, is that both mothers and developing fetuses as well as infants may absorb a higher percentage of some micronutrients than would a nonpregnant woman or older child. In other cases, an infant might absorb less of a particular micronutrient. "That's the neat thing about human bodies and nutrition. We have these safety valves. When we are low in something we will absorb a lot more of it, and by and large, when we have enough, our absorption rate will go down. It's often a protective mechanism."

While it is impossible to completely isolate the impact of a single nutrient, the USDA's Nutritional Needs of Infants offers information on some of the roles and functions that specific vitamins and minerals play in infant growth and development.

• Vitamin D: essential for utilization of calcium and phosphorous, formation of bones
• Vitamin A: plays role in formation and maintenance of healthy skin, hair, and mucous membranes; proper vision; growth and development; healthy immune and reproductive systems
• Vitamin E: protects vitamin A and essential fatty acids in the body, prevents breakdown of tissues
• Vitamin K: necessary for proper blood clotting
• Vitamin C: necessary for the formation of the protein collagen found in bones, cartilage, muscle, blood vessels, and other connective tissue; helps maintain capillaries, bones, and teeth; plays role in healing of wounds and body's ability to resist infections and absorb iron
• Vitamin B12: necessary for healthy blood cells, proper functioning of the nervous system
• Folate: essential for cell division, growth and development of healthy blood cells, formation of genetic material within cells throughout body
• Vitamin B6 (pyridoxine): helps the body use protein to build tissues, aids in metabolism of fat
• Vitamin B1 (thiamine): helps body release energy from carbohydrates during metabolism, plays vital role in normal functioning of nervous system
• Vitamin B2 (riboflavin): helps body release energy in the metabolism of protein, fat, and carbohydrates
• Niacin: helps body release energy during protein, fat, and carbohydrate metabolism
• Calcium: necessary for bone and tooth development, blood clotting, and maintenance of healthy nerves and muscles
• Iron: essential for growth and formation of healthy blood cells
• Zinc: plays role in the formation of protein in the body and in wound healing, necessary for growth and maintenance of all tissues, taste perception, healthy immune system
• Sodium: essential for the maintenance of water balance, regulation of blood volume, proper functioning of cell membranes