Intestinal Protective Factors in Colostrum and Milk
Several factors found in milk may function in the neonate's digestive tract to minimize the potential for enteric disease. These include:

Immunoglobulins - Even after closure the immunoglobulins in milk may protect the intestinal lumen. Immunoglobulins are relatively resistant to digestion. IgA is of particular interest in the human infant because it is the major immunoglobulin in human milk.

Lactoferrin - The iron-binding capacity of lactoferrin gives it bacteriostatic and bactericidal properties. Lactoferrin is high in human milk, low in cow milk.

Lysozyme - May degrade the cell wall of some bacteria and allow them to be lysed. Lysozyme is high in human milk, but there is essentially none in cow milk. Lysozyme can act in concert with IgA, lactoperoxidase and ascorbate to lyse bacteria.

Lactoperoxidase - Uses hydrogen peroxide and halogens (I, Cl) to halogenate proteins and make them inactive. Also causes peroxidation of substances. The lactoperoxidase system also includes an interaction of the enzyme with thiocyanate. Lactoperoxidase activity is ~20 fold lower in human milk than cow milk, but the human infant also secretes considerable lactoperoxidase in the saliva.

Milk cells - Generally the leukocytes in normal milk (in the absence of mastitis) are macrophages. These cells probably retain some of their phagocytic abilities when ingested into the neonate. However, a role for these cells in the neonate has not been completely described.

Gut Flora - One of the best mechanisms for protecting against digestive tract infections is the establishment of the proper intestinal flora. In human milk there is a carbohydrate growth factor (called the Bifidus Factor, probably an oligosaccharide) which stimulates the growth of Lactobacillus bifidus. The high lactose concentration, low protein content, low bulk and low buffering capacity of human milk also encourages L. bifidus growth. The high lactose content means that lactose is still available for bacterial fermentation in the intestine, resulting in an acidic environment which reduces viability of many potentially pathogenic bacteria. Although similar factors to the Bifidus factor have not been identified in milk of other species, there may be other milk factors that contribute specifically to the establishment of the optimal microbial flora in the digestive tract.
Breast milk vs. formulas. Infant formulas initially were based on cow milk composition, but have evolved somewhat to reflect human milk composition. This is still an area of concern. Infant formulas generally are made from cow milk or soybean ingredients. The casein : whey protein ratio for cow milk is ~80 : 20 compared to human milk with a 40 :60 ratio. Human milk does not establish as hard a curd in the stomach of the infant as cow milk casein will. The presence of ß-lactoglobulin (not present in human milk) or soy proteins in formulas can lead to a dietary protein allergy.

Several amino acid differences exist between human and cow milk that can present problems in feeding cow milk-based formulas to certain infants. Human milk has a high cysteine : methionine ratio and some taurine. Cow milk has a lower cys : met ratio and essentially no taurine. The human infant's liver and brain have only low levels of cystathionase, the enzyme that converts methionine to cysteine (the fetus and pre-term infant are completely lacking this enzyme). Cysteine is important for central nervous system development. Taurine is made from cysteine (the enzyme is cysteinesulfonic acid decarboxylase), and taurine is needed in the infant for brain development and function, retinal development and function, and conjugation of bile salts. Cow milk-based formulas may not contain optimal levels of cysteine or taurine. Another amino acid problem in human milk vs. cow milk-based formulas is the concentration of phenylalanine and tyrosine. Human milk is low in Phe and Tyr (particularly milk from mothers of pre-term infants). Infants have limited ability to metabolize these amino acids, which can build up and cause Phenylalanine Ketone Urea (PKU babies).

Cow milk has lower lactose than human milk. Lactose may be particularly important as a glucose (energy) source for the rapidly developing brain of the human infant. Generally, cholesterol is very low in formulas (1-3 mg/dl) compared to human milk (7-47 mg/dl) or cow milk (10-35 mg/dl). Cholesterol is needed by the infant in challenging the development of cholesterol metabolizing enzymes and it contributes to synthesis of nerve tissue and bile salts.

The Ca : P ratio is 2.29 for human milk vs. 1.26 for cow milk. Formulas low in cow milk can cause hypocalcemia and tetany. High P in formulas may lead to hyperphosphatemia and low serum Ca. Iron is low in human and cow milk, and most formulas are fortified with iron. Both iron and zinc are more efficiently absorbed from human milk than from cow milk.

Human Milk and Lactation
The Neonate and Colostrum