**2. A historical perspective**

#### **2.1. Body research methods**

A prospective study investigated the frequency of use of breast milk for the delivery of preterm infants of a high-risk neonatal unit. These are 244 preterm infants and were performed at a Hospital Friends of Children. The observed frequency of breast milk use at discharge from the neonatal unit was 94.6%. The finding that preterm infants were receiving breast milk reflects a well-structured program to promote breastfeeding of preterm infants at this institution, which is based on an interdisciplinary team to provide adequate support to mothers [6].

#### **3. Physiology**

With regard to the physiology of the creation of breast milk should be taken into account the stage of embryology. From that period on, the basic components of the genesis of the organs are developed.

In the intrauterine period the amniotic fluid contains amino acids, proteins, vitamins, minerals, hormones and growth factors. Although the concentration of these nutrients is much lower than that found in human milk, the large volumes of amniotic fluid ingested (up to one liter per day in the last stage of gestation, considerably more than the newborn after birth). Have a significant impact on growth and maturation of both the fetus and the fetal intestine. This makes it beneficial for his extrauterine life especially in the period of prematurity. Animal studies and limited human observations suggest that ingested amniotic fluid accounts for approximately 15% of fetal growth. The stage of prematurity, less than 37 weeks gestational age, affects the milk of mothers who give birth prematurely as it differs from women who give birth at term. The milk of the premature newborn, according to studies, is initially in concentration higher in proteins, fats, free amino acids and sodium, but during the first weeks after the birth these levels decrease. Keep in mind that the mineral content of preterm milk is similar to that of full-term milk, with the following exceptions: calcium is significantly lower in preterm milk than term milk and apparently does not seem to increase in over time.

With respect to carbohydrates in breast milk, lactose is the main carbohydrate. This disaccharide is an important source of energy, according to studies, it is relatively low in colostrum (first secretion of the mammary gland) and increases over time with more dramatic increases in preterm milk. Complex oligosaccharides are the second most abundant carbohydrate in breast milk. These oligosaccharides of human milk (HMO) are not digestible by host glycosidases and, however, are produced in large quantities with very variable structures by the mother. HMOs have three important functions: prebiotic (stimulation of commensal bacteria containing glycosylated bacteria to deconstruct and consume HMOs), strategy (structural similarity with glucans in enterocytes allows HMOs to competitively bind to pathogens) and the supply of fucose and sialic acid that appears to be important in host defense and neurodevelopment, respectively. In addition, preterm milk is relatively more variable in the HMO content. The differences between mothers are due to genetic diversity. Glycosaminoglycans (GAGs) also appear to act as decoys that provide binding sites for pathogenic bacteria to avoid adherence to the enterocyte. Premature milk is richer in GAG than full-term milk. We have to know the components of this stage of prematurity to contain and guide the mother in its extraction and access of premature to breast milk.

In turn, biological molecules actively in human milk are important components of the innate immune system. The differences in cytokines, growth factors and lactoferrin between premature milk and full-term milk are more important in colostrum and early milk and, in most cases, resolve after 4 weeks after delivery. Leptin is produced by the mammary glands, secreted in human milk, and may be important in postnatal growth. The leptin in human milk does not seem to differ between the milk to the preterm newborn and the milk of the term newborn. The activity of the lipase stimulated by the bile salt is similar in the term and in the preterm milk while the activity of the lipoprotein lipase is higher in the milk at term. The physiological basis of the composition of breast milk in its states of prematurity in the newborn provides us with guiding elements in the importance of the promotion of breast milk and the constant support that should be provided to the mother in neonatal units.
