MILK, HUMAN. Human milk is a food that evolved to ensure optimal growth, development, and survival of human infants and young children. All female mammals are uniquely equipped to provide species-specific nourishment and immunity through the provision of milk to their newborns.
Lactation refers to the physiological process of producing milk and its removal by an infant. Women produce breast milk as a response to the baby's suckling in an efficient system of supply and demand. Two hormones, prolactin and oxytocin, play important roles in this process. Prolactin is essential for both the initiation and the maintenance of milk production, while oxytocin stimulates milk ejection. Both hormones play complementary roles in breast-feeding, helping the mother relax and easing the infant into sleep. Oxytocin is particularly intriguing because it controls milk letdown, which can be affected by fear, pain, stress, and anxiety. The oxytocin reflex is more complex than the prolactin reflex. The mother's thoughts and fears may hinder the letdown reflex, and thinking about her baby may trigger the production of oxytocin and milk ejection.
Colostrum, the first milk mothers produce after giving birth, meets all the nutritional needs of the newborn. It has strong antiviral properties, strengthens the newborn's immune system, and acts as a laxative to remove meconium (first feces) from the digestive tract. It is thicker and richer in minerals and protein than mature milk. Colostrum is particularly rich in vitamins E and A. Infants usually consume only a small amount of this first milk. Within one or two days colostrum becomes transitional milk, and the supply increases greatly. The rate at which colostrum changes to mature milk varies from woman to woman, however, mature milk is present within two weeks.
Human milk is a living substance, changing constantly and adapting to meet the changing needs of the infant. For example, it changes from the beginning to the end of a feed. The fore milk has more protein, vitamins, minerals, and water and the hind milk has more fat to signal the end of the feed. Human milk has the highest fat content in the morning and the least at night. It even changes by season, age of the infant, and according to the baby's demand. Human milk reflects the environment, the diet, and the germs of the mother. Ultimately the infant determines the composition of the feed in an interactive process. Although breast pumps are available to many women in urban settings, a breast-feeding infant is the most efficient remover of human milk.
Human milk contains the right mixture of proteins, carbohydrates, fats, vitamins, and minerals to meet all the nutritional needs of infants for about the first six months of life. After the addition of other foods, breast milk continues to offer important nutritional benefits. In May 2001 the World Health Assembly confirmed by unanimous resolution that infants should be exclusively breast-fed for six months and continue to be breast-fed to age two and beyond.
One liter of human milk provides approximately 750 calories and contains 70 grams of carbohydrate, 46 grams of fat, and 13 grams of protein in addition to vitamins and minerals. Breast milk composition is remarkably stable around the world and changes only slightly with different maternal diets and under different environmental conditions. Fat is the most variable component, since maternal diet can modify the fat content of milk. Milk fat provides essential fatty acids and fat-soluble vitamins. The fats in human milk are in forms appropriate for the age of the infant and are readily bioavailable. Lactose is the primary carbohydrate in human milk. Human milk contains both casein and whey protein, but with more whey than casein, human milk is easier for human infants to digest than cow's milk.
The variety of vitamins and minerals produced in breast milk meets the needs of a full-term healthy infant. Water soluble vitamins, however, are influenced by maternal diet. Minerals in breast milk are highest in the first few days after birth. Infants build up iron reserves in utero, and the iron in breast milk is easily absorbed. As a result breast-fed babies are rarely iron deficient. Breast milk contains enough water for a baby, even in hot climates.
The amount of milk produced by a breast-feeding mother varies from around five hundred milliliters a day at day five to around eight hundred milliliters a day at six months, with a slow decline in volume as other foods are added to the diet. Women exhibit differences in the rate of milk synthesis, although the nutritional status of the mother does not significantly affect milk volume or quality. Current research suggests that differences in breast milk storage capacity among women may exist.
Knowledge about the properties of human milk is accumulating rapidly but remained incomplete at the beginning of the twenty-first century. Debates about how human milk is affected by drugs and chemical contaminants center on health consequences for infants and on the ethics of raising concerns when evidence is inconclusive and new mothers are most vulnerable to negative suggestions about the quality of their milk.
Mother's milk has also been recognized as a medium for early flavor experiences, since it is flavored by the mother's ingestion of foods such as garlic, mint, and vanilla. Human milk provides an opportunity for infants to become familiar with the flavors that they will encounter in the household cuisine.
Breast milk is a living substance. It contains living white blood cells that fight infection. Maternal antibodies are passed to the fetus through the placenta before birth and through breast milk after birth, providing temporary immunological protection for newborns. Milk proteins, such as lactoferrin, play an important immunological role, as do enzymes, immunoglobulins, and leukocytes. Human milk is clean and free of bacteria. Unlike artificial milk substitutes, human milk contains nonnutrient substances with the capacity to enhance immunity and destroy pathogens. Human milk has antibacterial, antifungal, and anti-infective properties that have been recognized for centuries. For example, expressed human milk has been used as a folk remedy for conjunctivitis. The protective effect of human milk is strongest for gastroenteritis and respiratory infections. However, the beneficial and protective effects of human milk include lowering the risk of allergies, multiple sclerosis, Crohn's disease, and sudden infant death syndrome (SIDS).
Human milk is seldom considered as a food resource or recorded in food composition tables. It has been suggested that it should be included in the calculations of a country's food supply and food balance sheets. Norway calculated the national production of breast milk to be 8.2 million kilograms in 1992, valued at U.S. $410 million (at U.S. $50 per liter). Norway has subsequently included human milk in calculating national food balance sheets.
It is impossible to put a precise economic value on human milk because it is seldom sold in the marketplace. Attempts to calculate its value include estimating the costs of breast milk substitutes or replacements or more rarely from the price charged for donated breast milk in milk banks. As a unique, incomparable product, its value to human survival is beyond calculation.
See also Baby Food; Lactation; Nutrient Bioavailability.
Riordan, Jan, and Kathleen G. Auerbach, eds. Breastfeeding and Human Lactation. Boston: Jones and Bartlett, 1993.
Stuart-Macadam, Patricia, and Katherine A. Dettwyler, eds. Breastfeeding: Biocultural Perspectives. New York: Aldine de Gruyter, 1995.
Penny Van Esterik