Breast Milk

Image Caption : Mother with Visible Cardiovascular System. When a mother's digestive system breaks down food nutrients are absorbed into her bloodstream. The nutrients travel to capillaries in the breast tissue where they are taken up by lactocytes (milk-producing cells) and repackaged into breast milk.

Breast Milk

Baby's Nutrition Begins with You

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 inMaternal, 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.

Ingredients May Vary

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.

Breastfeeding and Breast Milk: Condition Information

Breastfeeding, also called nursing, is the process of feeding human breast milk to an infant, either directly from the breast or by expressing (pumping out) the milk from the breast and bottle-feeding it to the infant. Breastfeeding and breast milk provide an infant with essential calories and nutrients.

According to the American Academy of Pediatrics (AAP) Policy Statement on Breastfeeding, women who don't have health problems should exclusively breastfeed their infants for at least the first 6 months of life.

The AAP suggests that a woman should try to breastfeed her infant for the first 12 months of life because of the benefits to both the mother and the infant.

Although breastfeeding is the recommended method for feeding infants, and breast milk provides most of the nutrients an infant needs, it does not provide infants with adequate vitamin D.4 The current AAP-recommended daily vitamin D intake is 400 IU per day for all infants and children beginning from the first few days of life.5 Human breast milk contains a vitamin D concentration of 25 IU per liter (about 4 cups) or less. Therefore, to meet the 400 IU daily requirement, supplementation is required.


Breast milk is the milk produced by the breasts (or mammary glands) of a human female to feed a child. Milk is the primary source of nutrition for newborns before they are able to eat and digest other foods; older infants and toddlers may continue to be breastfed, either exclusively or in combination with other foods from around six months of age when solid foods may be introduced.

The material on this site is for informational purposes only and is not intended as medical advice. It should not be used to diagnose or treat any medical condition. Consult a licensed medical professional for the diagnosis and treatment of all medical conditions and before starting a new diet or exercise program. If you have a medical emergency, call 911 immediately.