After giving birth to an infant, a woman's body produces breast milk because of the release of hormones triggered by the birth.
Breast milk is the best source of nutrition for an infant. Breast milk can provide all or nearly all the nutrition an infant needs during the first 6 to 12 months of life. Both the American Dietetic Association and the American Academy of Pediatrics support breastfeeding for the first 4 to 6 months of life. They also support breastfeeding supplemented by solid foods for at least 12 months.
During the first few days after birth, a woman's body produces a watery fluid called colostrum. Colostrum is high in protein, zinc, and other minerals. It contains less fat, carbohydrates, and calories than breast milk.
Between the third and sixth day after birth, colostrum changes to a transitional form of breast milk. During this time, the amount of protein and immune factors in the milk gradually decrease, while fat, lactose, and calories in the milk increase. By the tenth day after birth, the mother produces mature breast milk.
Colostrum and human milk are both rich in antibodies and have anti-infective factors. These help protect the newborn infant from viruses and bacteria that the infant was exposed to in the birth canal. They also help to protect the infant's immature gut from infection.
Breast milk also promotes the growth of bacteria in the digestive tract. These bacteria are helpful rather than harmful and assists with the digestion of food. In addition, breast milk contains immune factors that help an infant fight infection. These immune factors also help prevent the infant from developing possible food allergies.
Most women have similar nutrients in their breast milk, but these may vary slightly because of what a woman eats and how her body produces the breast milk. If the mother does not eat a healthy diet, she may produce less milk containing smaller quantities of nutrients.
A woman who is breastfeeding should eat 500 calories per day more than her usual healthy intake. This helps her make sure she provides the infant with the quality and quantity of milk needed. Milk content can also change from one time of the day to another, and from beginning to end of a breast-feeding session.
The nutrients in breast milk also change from the early months of infancy to the later months of infancy. These changes match the changing nutritional needs of the growing infant. The proteins in human breast milk are mostly whey and casein. Cow's milk contains more casein, and human breast milk contains more whey. Whey is more easily tolerated by an infant's digestive system.
The fat in human breast milk is easily absorbed by an infant's digestive system. An enzyme called lipoprotein lipase helps an infant absorb the fat in breast milk. A mother's breast milk contains essential fats. It also contains cholesterol. Both are needed by infants to make tissues in the nervous system. The amount of fat in breast milk rises greatly at the end of a breastfeeding session. This may be nature's way of making an infant feel full and stop feeding.
Breast milk contains large amounts of lactose, which is a carbohydrate. Lactose is used in tissues of the brain and spinal cord, and it provides the infant with energy. Bacteria in the infant's intestines feed on lactose and produce B vitamins. Lactose may also help the infant absorb essential nutrients such as calcium, phosphorus, and magnesium.
Human breast milk contains only a small amount of iron, but the iron in breast milk is easily absorbed. Fifty percent of the iron in human breast milk is absorbed compared with only 4 to 10 percent of the iron in cow's milk or commercial infant formulas.
Breast milk contains all the vitamins an infant needs for good health. The actual amount of each vitamin can vary, depending on a woman's diet and genetic makeup.
Mahan, K, MS, RD, CDE&Escott-Stump, S., MA, RD, LDN. (2000).
Krause's Food, Nutrition,&Diet Therapy (10th ed.). Pennsylvania: W.B. Saunders Company.