Colostrum is present at birth. For 1 to 5 days postpartum, the breasts secrete this thick, yellow fluid. Colostrum is higher in protein and lower in fat and carbohydrates than are transitional milk and mature milk. It contains high levels of immunoglobulins, especially immunoglobulin A (IgA), which “is the most important immunoglobulin in milk, not only in concentration but in biologic activity” (Lawrence & Lawrence, 1999). A “mucosa-protecting non-inflammatogenic secretory immunoglobulin,” IgA protects against Vibrio cholerae, enterotoxogenic E. coli, Campylobacter, Shigella, and Giardia liamblia, to name a few pathogens (Hanson, 1998). Maternal malnutrition affects immunoglobulin concentrations.

Colostrum also contains high levels of the iron-binding protein lactoferrin, which is known to have a bacteriostatic effect on many organisms. Colostrum helps with the passage of meconium and with the establishment of Lactobacillus bifidus flora, which inhibits certain pathogenic bacteria in the gut. Vitamin E, an antioxidant that helps protect lung tissue and the eyes, is two to three times higher in colostrum than in mature breast milk (Lawrence & Lawrence, 1999). Sodium is higher in colostrum than in mature milk. Sodium that remains high in mature milk may indicate lactation problems. Colostrum has a mean energy of 67 kcal/dL, whereas mature milk has 75 kcal/dL (Lawrence & Lawrence, 1999).

Transitional milk is produced between the colostrum and mature milk stages. It gradually changes to mature milk from 7 days to 2 weeks postpartum. Immunoglobulin levels, total protein, and fat-soluble vitamins decrease. Lactose, fats, and water-soluble vitamins increase. Milk prolactin is high in transitional milk and then declines steadily throughout the breast-feeding duration.

Mature milk is made up of mostly water and about 10% solids. The water is important for the infant’s temperature regulation. Breast-feeding infants in hot climates do not require supplemental water, because breast milk meets their water requirement needs.

Lipids provide a good source of energy. They make up about 50% of the calories in mature human breast milk and about 3% to 5% of total milk composition (Lawrence & Lawrence, 1999). The total fat content usually increases during a feeding and with the stage of lactation. The baby’s gestational age at birth and the mother’s diet and diurnal rhythm impact the lipid content. Ninety-eight to 99% of the fat in milk is made up of triglycerides that are broken down into fatty acids, which the intestines absorb easily. Unsaturated fatty acids and monoglycerides inactivate lipid-enveloped viruses, such as herpes simplex, Semliki Forest, and influenza. Breast milk is an important source of long chain polyunsaturated fatty acids, which are considered very important for brain development. DHA, one essential fatty acid found in breast milk, is essential to visual development
and may be especially important to preterm infants and the development of visual acuity and retinal responses to light (Carlson, 1999).

Adrenocorticotropic hormone (ACTH), a peptide hormone that can cross the intestinal barrier, is found in breast milk. ACTH influences the production of cortisol, a hormone that induces change in the microvillus membrane in the intestine, which in turn favors the colonization of nonpathogenic microbes in the gut. Cortisol also helps the intestinal barrier mature, which is important because the primary cause of necrotizing enterocolitis is an immature intestinal barrier (Bernt & Walker, 1999).

Growth hormone (GH), found in milk, may help GI tract function. GH also may play a role in preventing or delaying GI allergies. Nucleotides and their derivatives, which appear in higher concentrations in human milk than in cow’s milk-based formula, appear to have significant effects on the immune and GI systems (Carver, 1999).

Lactose, the most abundant carbohydrate found in human milk (about 7 gm/dL), is synthesized in the mammary gland. Lactose easily breaks down into simple sugars, essential for providing accessible energy for the infant. Other carbohydrates found in small amounts include peptides, monosaccharides, and more than 80 neutral and acid oligosaccharides. The functions of the oligosaccharides are not understood fully, but they do play an important role in enhancing defenses against viruses, bacteria, and their toxins (Coppa et al., 1999). They also have a role in promoting the growth of bifidogen flora and indirectly providing protection against GI infections in newborns.