Chapter 6.docx

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10 Apr 2012
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Chapter 6: Nutrition During Lactation
Lactation of the Mammary Gland
Functional Units of the Mammary Gland
Mammary gland: the source of milk for offspring also commonly called the breast
o The presence of mammary glands is a characteristic of mammals
Alveoli: a rounded or oblong-shape cavity present in the breast
Secretory cells: cells in the acinus (milk gland) that are responsible for secreting milk components into the ducts
Myoepithial cells: specialized cells that line the alveoli and that can contract to cause milk to be secreted into the ducts
Oxytocin: a hormone produced during letdown that causes milk to be ejected into the ducts
Mammary Gland Development
The cyclic release of estrogen and progesterone governs pubertal breast development
The mammary gland develops its lobular structure under the cyclic production of progesterone and is usually complete within 12
to 18 months after menarche
As the ductal system matures, cells that can secrete milk develop, the nipple grows, and its pigmentation changes
Fibrous and fatty tissues increase around the ducts
In pregnancy, the luteal and placental hormones allow further preparation for breastfeeding
Estrogen stimulates development of the glands that will make milk
Progesterone allows the tubules to elongate and the cells that line the tubules to duplicate
Lactogenesis
Breast milk production (lactogenesis) occurs in three stages:
o Lactogenesis I. Milk begins to form, and the lactose and protein content of milk increase
Extends through the first few days of postpartum
o Lactogensis II. Begins 2-5 days postpartum and is marked by increased blood flow to the mammary gland
Considered the onset of copious milk secretion (when milk comes in)
Significant changes in both the milk composition and the quantity of milk that can be produced occur over the
first 10 days of the baby’s life
o Lactogensis III. Begins about 10 days after birth and is the stages in which the milk composition becomes stable
Hormonal Control of Lactation
Prolactin and oxytocin are necessary for establishing and maintaining a milk supply
Prolactin: hormone that stimulates milk production
Suckling is a major stimulator of prolactin secretion (levels double)
Stress, sleep, and sexual intercourse also stimulate prolactin levels
To prevent milk production in the last 3 months of pregnancy, prolactin activity is suppressed by a prolactin-inhibiting factor that
is released by the hypothalamus
o Allows the mother’s body to prepare for milk production during pregnancy
Oxytocin release is also stimulated by suckling or nipple stimulation
Main role is in letdown, or the ejection of milk from the milk gland (acinus) into the milk ducts
Women may experience tingling or sometimes sharp shooting pain and corresponds with contractions in the milk ducts
o Oxytocin also acts on the uterus, causing it to contract, seal blood vessels, and shrink in size
Secretion of Milk
Secretory cell in the breast uses five pathways for milk secretion
Some components like lactose are made in the secretory cells and secreted into ducts
Water, sodium, potassium, and chloride are able to pass through alveolar cells membranes in either direction (passive diffusion)
Milk fat comes from triglycerides from the mother’s blood and from new fatty acids produced in the breast
Fats are made soluble in ilk by addition of a proteins carrier to form milk-fat globules
Milk-fat globules are then secreted into ducts
Immunoiglobulin A and other plasma proteins are captured from the mother’s blood and taken into the alveolar cells
These proteins are then secreted into the milk ducts
Letdown Reflex
Stimulates milk release from the breast
Stimuli from the infant suckling are passed through nerves to the hypothalamus, which respond by promoting oxytocin release
from the posterior pituitary gland
Oxytocin causes contraction of the myoepithelial cells surrounding the secretory cells
Milk is released through the ducts, making it available to the infant
Other stimuli, such as hearing a baby cry, sexual arousal, and thinking about nursing, can also cause letdown, and milk will leak
from the breasts
Human Milk Composition
For approximately 6 moths, yhe composition of milk is designed not only to nurture, but also to protect infants from injectious
and certain chronic diseases
Colostrum
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The first milk, colostrum: produced in the first 2-3 days after the baby is born
o Higher in protein and lower in lactose than milk produced after a milk supply is established
o Thick, often yellow fluid produced during lactogenesis II infants may drink only 2 to 10 mL per feeding in the first 2-3
days
o Secretory immunoglobulin A and lactoferrin are the primary proteins present
o Higher concentrations of sodium, potassium, and chloride than more mature milk
Water
Largest component
Breast milk is isotonic with plasma
The biological design of milk means that babies do not need water or other fluids to maintain hydration
Water allows suspension of the milk sugars, proteins, immunoglobulin A, sodium, potassium, citrate, magnesium, calcium,
chloride, and water-soluble vitamins
Energy
Human milk provides approximately 0.65 kcal/mL
Breastfed infants consume fewer calories than infants fed HMS
Lipids
Second largest component of breast milk by concentration
Provide half the energy of human milk
Human milk is low at the beginning of a feeding in foremilk, and higher at the end in the hindmilk that follows
Effect of Maternal Diet on Fat Composition
When diets rich in polyunsaturated fats are consumed, more polyunsaturated fatty acids are present in the milk
When a mother is losing weight, the fatty acid profile of her fat stores is reflected in the milk when very low-fat diets with
adequate calories from carbohydrate and protein are fed, more medium-chain fatty acids are synthesized in the breast
DHA
Milk DHA levels are increased by maternal supplementation
DHA is essential for retinal development and accumulates during the last months of pregnancy
Trans Fatty Acids
Trans fat concentrations are similar in American and Canadian women, but lower in the milk of women from European and
African countries
Removal of trans fatty acids from many food products in Canada led to lower levels of trans fat in human milk
Cholesterol
Essential component of all cell membranes, is need for growth and replication of cells
Breastfed infants have higher intakes of cholesterol and higher levels of serum cholesterol than infants fed HMS
Early consumption of cholesterol through breast milk appears to be related to lower blood cholesterol levels later in life
Protein
The protein content of mature human milk is relatively low compared to other mammalian milks
Concentration of proteins synthesized in the breast are more affected by the age of the infant than maternal intake and maternal
serum proteins
Have important nutritive and non-nutritive value
Proteins and their digestive products, such as peptides, exhibit a variety of antiviral and antimicrobial effects
Enzymes present in human milk might also provide protection by facilitation actions that prevent inflammation
Casein
Major class of protein in mature milk from women who deliver either a term or preterm
Casein, calcium phosphate, and other ions such as magnesium and citrate appear as an aggregate and are the source of milk’s
white appearance
Keeps calcium in soluble form and facilitates its absorption
Whey Proteins
Proteins that remain soluble in water after casein precipitated from milk by acid or enzymes
The enzymes present in whey proteins aid in digestion and protection against bacteria
Nonprotein Nitrogen
Nonprotein nitrogen provides 20-25% of the nitrogen in milk
Urea accounts for 30-50% of nonprotein nitrogen, and nucleotides for 20% depending on the stage of lactation and the diet of the
mother
Some of this nitrogen is available for the infant to use for producing nonessential amino acids
Some of the nonprotein nitrogen is used to produce other proteins with biological roles such as hormones, growth factors, free
amino acids nucleic acids, nucleotides, and carnitine
Nucleotides appear to play important roles in growth and disease resistance
Milk Carbohydrates
Lactose is the dominant carbohydrate in human milk
Other carbohydrates including monosaccharides, polysaccharides, oligosaccharides, and protein-bound carbohydrates are also
present
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