Chapter 29 Infant Feeding

Anatomy and physiology of the breast

The breasts are compound secreting glands, composed mainly of glandular tissue, which is arranged in lobes.

Each lobe is divided into lobules that consist of alveoli and ducts.

The alveoli contain acini cells, which secrete the components of milk and are surrounded by myoepithelial cells, which contract and propel the milk out.

Small lactiferous ducts, carrying milk from the alveoli, unite to form larger ducts.

Myoepithelial cells are oriented longitudinally along the ducts; under the influence of oxytocin, these smooth muscle cells contract and the tubule becomes shorter and wider.

The nipple is composed of erectile tissue and plain muscle fibres, which have a sphincter-like action in controlling the flow of milk.

Surrounding the nipple is an area of pigmented skin called the areola, which contains Montgomery’s glands. These produce a sebum-like substance, which acts as a lubricant during pregnancy and throughout breastfeeding.

The breast is supplied with blood from the internal and external mammary arteries with corresponding venous drainage.

Lymph drains freely between the two breasts and into lymph nodes in the axillae and the mediastinum.

During pregnancy, oestrogens and progesterone induce alveolar and ductal growth, as well as stimulating the secretion of colostrum. When the levels of placental hormones fall, this allows the already high levels of prolactin to initiate milk secretion. Continued production of prolactin is caused by the baby feeding at the breast, with concentrations highest following night feeds.

Prolactin is particularly important in the initiation of lactation. As lactation progresses, the milk removal becomes the driving force behind milk production, due to a protein feedback inhibitor of lactation. This protein accumulates in the breast as the milk accumulates and it exerts negative feedback control on the continued production of milk. Removal of this autocrine inhibitory factor, by removing the milk, allows milk production to be stepped up again.

Milk release is under neuroendocrine control. Tactile stimulation of the breast also stimulates the oxytocin, causing contraction of the myoepithelial cells. This process is known as the ‘let-down’ or ‘milk ejection’ reflex and makes the milk available to the baby. This occurs in discrete pulses throughout the feed and may well trigger the bursts of active feeding.

In the early days of lactation this reflex is unconditioned. Later, it becomes a conditioned reflex, which can be enhanced or suppressed by environmental factors.

Properties and components of breast milk

Human milk varies in its composition. The most dramatic change in the composition of milk occurs during the course of a feed.

At the beginning of the feed the baby receives a high volume of relatively low-fat milk.

As the feed progresses, the volume of milk decreases but the proportion of fat in the milk increases, sometimes to as much as five times the initial value.

The baby’s ability to obtain this fat-rich milk is not determined by the length of time spent at the breast, but by the quality of attachment to the breast. The baby needs to be well attached so that he or she can use the tongue to maximum effect, stripping the milk from the breast, rather than relying solely on the mother’s milk ejection reflex.

Fats and fatty acids

For the human infant, with a unique and rapidly growing brain, it is the fat and not the protein in human milk that has particular significance.

Ninety-eight per cent of the lipid in human milk is in the form of triglycerides: three fatty acids linked to a single molecule of glycerol.

Over 100 fatty acids have so far been identified, about 46% being saturated fat and 54% unsaturated fat.

Fat provides the baby with more than 50% of calorific requirements.

It is utilised very rapidly because the milk itself contains the enzyme (bile salt-stimulated lipase) needed for fat digestion, but in a form that only becomes active when it reaches the infant’s intestine.

Pancreatic lipase is not plentiful in the newborn, so a baby who is not fed human milk is less able to digest fat.

Carbohydrate

The carbohydrate component of human milk is provided chiefly by lactose, which supplies the baby with about 40% of calorific requirements.

Lactose is converted into galactose and glucose by the action of the enzyme lactase and these sugars provide energy to the rapidly growing brain.

Lactose enhances the absorption of calcium and also promotes the growth of lactobacilli which increase intestinal acidity, thus reducing the growth of pathogenic organisms.

Protein

Human milk contains less protein than any other mammalian milk. Human milk is whey dominant (the whey being mainly alpha-lactalbumin) and forms soft, flocculent curds when acidified in the stomach.

Vitamins

All the vitamins required for good nutrition and health are supplied in breast milk, although the actual amounts vary from mother to mother.

Fat-soluble vitamins

Vitamin A

This is present in human milk as retinol, retinyl esters and beta-carotene. Colostrum contains twice the amount present in mature human milk, and it is this which gives colostrum its yellow colour.

Vitamin D

This is the name given to two fat-soluble compounds:

calciferol (vitamin D2)

cholecalciferol (vitamin D3).

Vitamin D3 plays an essential role in the metabolism of calcium and phosphorus in the body and prevents rickets in children. Adults can obtain these substances from dietary sources and the conversion of 7-dehydrocholesterol in the skin to vitamin D3 from exposure to sunlight.

For light-skinned babies, exposure to sunlight for 30 minutes per week wearing only a nappy, or 2 hours per week fully clothed but without a hat, will keep vitamin D requirements within the lower limits of the normal range.

The babies of dark-skinned mothers living in temperate zones and preterm babies may be at risk of vitamin D deficiency.

Vitamin E

Although vitamin E is present in human milk, its role is uncertain. It appears to prevent the oxidisation of polyunsaturated fatty acids and may prevent certain types of anaemia to which preterm infants are susceptible.

Vitamin K

This vitamin (83% of which is present as alpha-tocopherol) is essential for the synthesis of blood-clotting factors. It is present in human milk and absorbed efficiently. Because it is fat soluble, it is present in greater concentrations in colostrum and in the high-fat hind-milk, although the increased volume of milk as lactation progresses means that the infant obtains twice as much vitamin K from mature milk as from colostrum.

Water-soluble vitamins

Unless the mother’s diet is seriously deficient, breast milk will contain adequate levels of all the vitamins. An improved diet is always more beneficial than artificial supplements. With some vitamins, particularly vitamin C, a plateau may be reached where increased maternal intake has no further impact on breast milk composition.

Minerals and trace elements

Iron

Normal term babies are usually born with a high haemoglobin (Hb) level (16–22 g/dl), which decreases rapidly after birth. The iron recovered from Hb breakdown is utilised again. Babies also have ample iron stores, sufficient for at least 4–6 months. Although the amounts of iron are lower than those found in formula, the bioavailability of iron in breast milk is very much higher:

70% of the iron in breast milk is absorbed.

Only 10% is absorbed from formula.

Zinc

A deficiency of this essential trace mineral may result in failure to thrive and typical skin lesions.

Although there is more zinc present in formula than in human milk, the bio-availability is greater in human milk.

Breastfed babies maintain high plasma zinc values when compared with formula-fed infants, even when the concentration of zinc is three times that of human milk.

Calcium

Calcium is more efficiently absorbed from human milk than from formula milks because of human milk’s higher calcium:phosphorus ratio.

Infant formulas, which are based on cow’s milk, inevitably have a higher phosphorous content than human milk.

Other minerals

Human milk has significantly lower levels of calcium, phosphorus, sodium and potassium than formula. Copper, cobalt and selenium are present at higher levels. The higher bioavailability of these minerals and trace elements ensures that the infant’s needs are met while also imposing a lower solute load on the neonatal kidney than do breast milk substitutes.

Anti-infective factors

Leucocytes

During the first 10 days of life there are more white cells per ml in breast milk than there are in blood. Macrophages and neutrophils are among the most common leucocytes in human milk and they surround and destroy harmful bacteria by their phagocytic activity.

Immunoglobulins

Five types of immunoglobulin have been identified in human milk: IgA, IgG, IgE, IgM and IgD. Of these the most important is IgA, which appears to be both synthesised and stored in the breast. Although some IgA is absorbed by the infant, much of it is not. Instead it ‘paints’ the intestinal epithelium and protects the mucosal surfaces against entry of pathogenic bacteria and enteroviruses. It affords protection against:

Escherichia coli

salmonellae

shigellae

streptococci

staphylococci

pneumococci

poliovirus

the rotaviruses.

Lysozyme

This kills bacteria by disrupting their cell walls. The concentration of lysozyme increases with prolonged lactation.

Lactoferrin

This binds to enteric iron, thus preventing potentially pathogenic E. coli from obtaining the iron needed for survival. It also has antiviral activity (HIV, cytomegalovirus, herpes simplex virus), acting by interfering with virus absorption and/or penetration.

Bifidus factor

The bifidus factor in human milk promotes the growth of Gram-positive bacilli in the gut flora, particularly Lactobacillus bifidus, which discourages the multiplication of pathogens.

Hormones and growth factors

Epidermal growth factor (and insulin-like growth factor) found in breast milk and colostrum stimulate the baby’s digestive tract to mature more quickly and strengthen the barrier properties of the gastrointestinal epithelium. Once the initially leaky membrane lining the gut matures, it is less likely to allow the passage of large molecules and becomes less vulnerable to micro-organisms. The timing of the first feed also has a significant effect on gut permeability, which drops markedly if the first feed takes place soon after birth.

Management of breastfeeding

The first feed

Early feeding contributes to the success of breastfeeding. The first feed should be supervised by the midwife. It should proceed without pain and the baby allowed to terminate the feed spontaneously.

< div class='tao-gold-member'>

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. The Placenta
2. Prolonged Pregnancy and Disorders of Uterine Action
3. The Third Stage of Labour
4. Problems of Pregnancy

Stay updated, free articles. Join our Telegram channel

Join

Full access? Get Clinical Tree