Milk is milk, right? Mammals make it (humans are mammals) and babies drink it. There’s more to the story than that. Each species of mammal makes a unique kind of milk, which meets all the nutritional requirements of its offspring at the beginning of life. Each species’ milk has specific qualities that insure the survival of the young in a particular environment. This principle is known as the biological specificity of milk. Mother seals, for example, make a high-fat milk because baby seals need lots of body fat to survive in cold water. Since brain development is crucial to the survival of humans, human milk provides nutrients for rapid brain growth.
No matter what animal it comes from, milk contains the basic nutritional elements of fats, proteins, carbohydrates, vitamins, and minerals. Let’s look at each one of these nutrients in human milk, comparing them to the same nutrients in formula or cow milk, so you can further appreciate how your milk is custom-made to meet the needs of your baby.
Unique nutrition for unique humans. As hormones levels change in the days after birth, the mother’s body starts to make more plentiful amounts of milk. Colostrum gradually changes into mature milk–the stuff babies have been thriving on for thousands of years. Milk’s basic ingredients are fat, proteins, lactose, vitamins, minerals, and water. This is true of milk from all kinds of mammals. Yet, the proportions of these ingredients differ, as do the kinds of protein and fat. This is what makes each species’ milk uniquely suited to its young. It’s also why cow’s milk and cow’s milk-based formulas are not the ideal food for human infants.
High Quality Protein
Protein is a prime example of how human milk is unique nutrition for human babies. Human milk is low in protein, at least when compared with the milk of other species, especially cow’s milk. This isn’t a nutritional deficiency; there are good reasons for this. Human infants are designed to grow slowly. While it’s important for humans to develop strong bodies, even more important is brain development and the learning of social skills. The experiences that shape the brain come from close contact between mother and baby when baby is held and carried. If human infants doubled their birthweight in less than 50 days the way baby calves do, and then continued growing, how could their mothers carry them and talk to them and keep them close? Baby cows need to learn where to find the best grass in the meadow; baby humans need to learn how to work with others so that everyone’s needs get met.
Though the protein content of human milk is generally low, the types of amino acids that make up these proteins are important. One particular amino acid, taurine, is found in large amounts in human milk. Studies show that taurine has an important role in the development of the brain and the eyes. The body can’t convert other kinds of amino acids into taurine, so its presence in human milk is significant–so significant that some formula manufacturers have begun adding it to artificial baby milks.
If you let milk stand out of the refrigerator and sour, you will see that milk proteins fall into two categories, curds and whey. (Remember Miss Muffet?) The curd portion, the casein proteins, are the white clots; the liquid is the whey. Cow’s milk is mostly casein protein, which forms a rubbery, hard-to-digest curd in babies’ tummies. Human milk has more whey than curd, and the curds that are formed are softer and more quickly digested. Breastfed babies get hungry sooner than babies who are formula-fed because human milk proteins are digested so efficiently. It doesn’t take as much energy to digest human milk as it does to digest formula. Frequent feedings also ensure that human babies get lots of attention from their mothers.
There’s another reason why babies digest human milk so quickly: the fat in human milk comes with an enzyme, lipase, that breaks the fat down into smaller globules so this important nutrient can be better absorbed into the bloodstream. Fat is a valuable source of energy for babies, so the presence of lipase makes the fat in human milk more available. This is one of the reasons human milk is so good for premature babies, who need lots of energy to grow but whose digestive systems are very immature.
A changing nutrient for changing needs. The fat content of human milk changes constantly. Typically, fat levels are low at the beginning of a feeding and high at the end. Babies nurse eagerly to get the low-fat, thirst-quenching foremilk, then slow down and linger over the high-fat dessert at the end of their meal. Babies who nurse again soon after the end of the last feeding get more high-fat milk, so babies who breastfeed more frequently during a growth spurt get more calories. Longer intervals between feedings bring down the fat content of the milk stored in the breast. This nutritional fact of human milk is one of the many reasons why the rigid 3 to 4 hour scheduled style of feeding is biologically incorrect.
Smarter fats. The special kind of fat in human milk is important to brain development. As newborn babies grow, the nerves are covered with a substance called myelin which helps the nerves transmit messages to other nerves throughout the brain and body. To develop high-quality myelin, the body needs certain types of fatty acids–linoleic and linolenic–which are found in large amounts in human milk.
Vitamins and Minerals
The vitamins and minerals listed on the formula can are no match for those in the milk made by mom, even if milligram by milligram comparisions suggest otherwise. When formula researchers want to know how much of a particular vitamin or mineral babies need each day, they look first at how much of that nutrient is present in human milk and how much milk a baby of a given age takes in a day. But just doing the math doesn’t tell the whole story. More important than the amounts of nutrients in the milk is the amount that is available for the infant to use, a nutrient principle called bioavailability. The bioavailability of a nutrient is influenced by many factors, including its chemical form and the presence of other substances.
The three important minerals calcium, phosphorus, and iron are present in breastmilk at lower levels than in formula, but in breastmilk these minerals are present in forms that have high bioavailability. For example, 50 to 75 percent of the iron in breastmilk is absorbed by the baby. With formula, as little as four percent of the iron is absorbed into baby’s bloodstream. To make up for the low bioavailability of factory-added vitamins and minerals, formula manufactures raise the concentrations. Sounds reasonable, right? If only half gets absorbed by the body, put twice as much into the can. Yet, this nutrient manipulation may have a metabolic price.
Baby’s immature intestines are required to dispose of the excess. Meanwhile, the excess unabsorbed minerals (especially iron) can upset the “ecology of the gut,” interfering with the growth of healthful bacteria and allowing harmful bacteria to flourish. This is another reason formula-fed infants have harder, more unpleasant smelling stools.
To enhance the bioavailability of nutrients, breastmilk contains facilitators – substances that enhance the absorption of other nutrients. For example, vitamin C in human milk increases the absorption of iron. Zinc absorption is also enhanced by other factors in human milk. In an interesting experiment, researchers added equal amounts of iron and zinc to samples of human milk, formula, and cow’d milk, and fed them to adult volunteers. More of the nutrients in the human-milk sample got into the bloodstream compared to the formula and cow’s milk. In essence, breastmilk puts nutrients where they belong – in baby’s blood, not in baby’s bowels.
Hormones and Enzymes
Every year medical journal articles describe more valuable substances discovered in human milk. Scientists are only beginning to write the story on other factors in human milk that may be important to baby’s growth and development. For example, other enzymes besides lipase are available to aid infant digestion. Epidermal growth factor, present in human milk in significant amounts, may promote the development of tissues in the digestive tract and elsewhere. Other hormones in milk may influence a baby’s metabolism, growth, and physiology. The effects may be subtle, but they may also have far-reaching implications. Being breastfed has advantages that reach into adulthood. Science is only beginning to learn what these benefits are.