Colostrum is the special milk that is secreted in the first 2–3 days after delivery. It is produced in small amounts, about 40–50 ml on the first day (12), but is all that an infant normally needs at this time. Colostrum is rich in white cells and antibodies, especially sIgA, and it contains a larger percentage of protein, minerals and fat-soluble vitamins (A, E and K) than later milk (2). Vitamin A is important for protection of the eye and for the integrity of epithelial surfaces, and often makes the colostrum yellowish in colour. Colostrum provides important immune protection to an infant when he or she is first exposed to the micro-organisms in the environment, and epidermal growth factor helps to prepare the lining of the gut to receive the nutrients in milk. It is important that infants receive colostrum, and not other feeds, at this time. Other feeds given before breastfeeding is established are called prelacteal feeds. Milk starts to be produced in larger amounts between 2 and 4 days after delivery, making the breasts feel full; the milk is then said to have “come in”. On the third day, an infant is normally taking about 300–400 ml per 24 hours, and on the fifth day 500–800 ml (12). From day 7 to 14, the milk is called transitional, and after 2 weeks it is called mature milk. 2.3 Animal milks and infant formula Animal milks are very different from breast milk in both the quantities of the various nutrients, and in their quality. For infants under 6 months of age, animal milks can be home-modified by the addition of water, sugar and micronutrients to make them usable as short-term replacements for breast milk in exceptionally difficult situations, but they can never be equivalent or have the same anti-infective properties as breast milk (13). After 6 months, infants can receive boiled full cream milk (14). Infant formula is usually made from industriallymodified cow milk or soy products. During the manufacturing process the quantities of nutrients are adjusted to make them more comparable to breast milk. However, the qualitative differences in the fat and protein cannot be altered, and the absence of anti-infective and bio-active factors remain. Powdered infant formula is not a sterile product, and may be unsafe in other ways. Life threatening infections in newborns have been traced to contamination with pathogenic bacteria, such as Enterobacter sakazakii, found in powdered formula (15). Soy formula contains phyto-oestrogens, with activity similar to the human hormone oestrogen, which could potentially reduce fertility in boys and bring early puberty in girls (16). 2.4 Anatomy of the breast The breast structure (Figure 3) includes the nipple and areola, mammary tissue, supporting connective tissue and fat, blood and lymphatic vessels, and nerves (17,18). The mammary tissue – This tissue includes the alveoli, which are small sacs made of milk-secreting cells, and the ducts that carry the milk to the outside. Between feeds, milk collects in the lumen of the alveoli and ducts. The alveoli are surrounded by a basket of myoepithelial, or muscle cells, which contract and make the milk flow along the ducts. Nipple and areola – The nipple has an average of nine milk ducts passing to the outside, and also muscle fibres and nerves. The nipple is surrounded by the circular pigmented areola, in which are located Montgomery’s glands. These glands secrete an oily fluid that protects the skin of the nipple and areola during lactation, and produce the mother’s individual scent that attracts her baby to the breast. The ducts beneath the areola fill with milk and become wider during a feed, when the oxytocin reflex is active. 2. The physiological basis of breastfeeding 11 2.5 Hormonal control of milk production There are two hormones that directly affect breastfeeding: prolactin and oxytocin. A number of other hormones, such as oestrogen, are involved indirectly in lactation (2). When a baby suckles at the breast, sensory impulses pass from the nipple to the brain. In response, the anterior lobe of the pituitary gland secretes prolactin and the posterior lobe secretes oxytocin. Prolactin Prolactin is necessary for the secretion of milk by the cells of the alveoli. The level of prolactin in the blood increases markedly during pregnancy, and stimulates the growth and development of the mammary tissue, in preparation for the production of milk (19). However, milk is not secreted then, because progesterone and oestrogen, the hormones of pregnancy, block this action of prolactin. After delivery, levels of progesterone and oestrogen fall rapidly, prolactin is no longer blocked, and milk secretion begins. When a baby suckles, the level of prolactin in the blood increases, and stimulates production of milk by the alveoli (Figure 4). The prolactin level is highest about 30 minutes after the beginning of the feed, so its most important effect is to make milk for the next feed (20). During the first few weeks, the more a baby suckles and stimulates the nipple, the more prolactin is produced, and the more milk is produced. This effect is particularly important at the time when lactation is becoming established. Although prolactin is still necessary for milk production, after a few weeks there is not a close relationship between the amount of prolactin and the amount of milk produced. However, if the mother stops breastfeeding, milk secretion may stop too – then the milk will dry up. More prolactin is produced at night, so breastfeeding at night is especially helpful for keeping up the milk supply. Prolactin seems to make a mother feel relaxed and sleepy, so she usually rests well even if she breastfeeds at night. Suckling affects the release of other pituitary hormones, including gonadotrophin releasing hormone (GnRH), follicle stimulating hormone, and luteinising hormone, which results in suppression of ovulation and menstruation. Therefore, frequent breastfeeding can help to delay a new pregnancy (see Session 8 on Mother’s Health). Breastfeeding at night is important to ensure this effect. Oxytocin Oxytocin makes the myoepithelial cells around the alveoli contract. This makes the milk, which has collected in the alveoli, flow along and fill the ducts (21) (see Figure 5). Sometimes the milk is ejected in fine streams.