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 THEY 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.