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The aqueous phase of milk is predominantly a lactose solution. When progesterone is withdrawn, postpartum lactose is secreted into the acinar lumen and osmotically incorporates water, resulting in a solution that is isosmotic with plasma. Therefore, lactose is the primary controlling influence on milk volume. The electrolyte content of aqueous milk is like that of intracellular fluid (i. An adequate supply of glucose to the alveolar cells is also essential for continued milk production.

It contains proteins, minerals and vitamins, lipids, carbohydrates, and both chemical and cellular immunologic factors of great importance to the survival of the newborn, even though transplacental passage of immunoglobulins confers passive immunity on the fetus while in utero and for the first few weeks after birth.

In many species, such as ruminants, there is no transplacental transfer of immunoglobulins and the newborn is entirely dependent on the immunoglobulins present in colostrum. Although the human infant is less vulnerable initially, postpartum colostrum provides extremely important immunoglobulins and other antimicrobial substances that act locally within the intestinal tract against potential pathogens. It is high in vitamin A, the pigment of which gives it its yellow color.

This immunoglobulin is a unique double molecule linked with disulfide bridges that greatly diminish its hydrolysis and metabolism in the intestinal tract. Colostrum contains immunologically active lymphocytes and monocytes, interferon, a factor that facilitates the removal of intestinal meconium, and a factor for stimulation of the beneficial Lactobacillus bifidus microorganisms in the intestinal tract.

Production of transitional milk begins after the first week of lactation and continues through the third week postpartum. The immunoglobulin and total protein content of this milk is less than that of colostrum, whereas the content of lactose, lipid, water-soluble vitamins, and calories is greater. In many respects, mature human milk differs from cow's milk and cow's milk-based milk replacers. Human milk contains 1.

The major difference is in the content of casein, which is substantially less in human milk. The relatively low casein content facilitates the formation of curd in the infant's intestinal tract. This increases the digestibility of milk and keeps the gastric acidity high, resulting in an increased antimicrobial action. Because the content of essential amino acids in human milk is essentially the same as that found in human plasma, the transfer from intestinal tract to plasma is quite efficient.

In addition, human milk is high in the amino acid, taurine, which may be of great importance in neural development, especially in premature infants. Finally, human milk contains several specific growth factors that are missing from bovine milk, as discussed later.

It promotes the growth of L. Hydrolysis of lactose results in the formation of galactose, which is necessary for the production of cerebrosides utilized in myelin synthesis. Lipids are present in human milk in a concentration of 3. They act as the primary energy source for the growing infant.

The emulsified lipid in human milk is much finer than that of cow's milk, and the lipase activity is much greater as well. Both these factors make human milk easier for the infant to digest. The primary difference is in the concentration of sodium. Vitamin A may be insufficient in human milk if the mother is poorly nourished. Because cow's milk contains half as much vitamin A, vitamin D, and iron as human milk, there is an earlier tendency toward growth deficiencies and anemia when an infant is maintained on cow's milk.

Vitamin C in cow's milk is lower than in human milk initially, and the heating process further reduces its concentration. Human colostrum and mature milk contain several growth factors.