Breast milk changes its recipe during a single feed, and between day and night
Human milk is not one fixed fluid. Its fat content climbs steadily as a baby feeds, some of its nucleotides are made mostly at night and others by day, and its antibodies are tailored to germs the mother picks up from her own infant. No commercial formula has managed to copy it.
The first milk, colostrum, is a thin yellowish liquid produced in the days after birth, rich in protein and antibodies that give the newborn borrowed immunity while its own defences mature. It is packed with immunoglobulin A, which coats the gut, and has a mild laxative effect that helps clear the first stool and limit the build-up of bilirubin linked to jaundice. Over the next few days it turns into sweeter, thinner milk, and later into something thicker and creamier. IgA levels stay high from around day 10 until at least seven and a half months.
By one breakdown, mature human milk is roughly 0.8 to 0.9 percent protein, 4.5 percent fat and 7.1 percent carbohydrate, mostly lactose, plus sugars called human milk oligosaccharides that are tied to many of its infection-fighting effects. Lactation specialists in the 1980s and 1990s spoke of separate foremilk and hindmilk, but that split is now considered misleading: there is a single milk that grows gradually fattier through a feed. In the acidic stomach one protein, alpha-lactalbumin, can bind oleic acid into a complex called HAMLET that kills tumour cells in laboratory settings.
Production runs on the hormones prolactin and oxytocin, and the supply responds to demand: more nursing or pumping yields more milk. A genuine inability to produce enough is rare; mothers in malnourished regions still make similar amounts of comparable quality. A Cochrane review found that relaxing audio, warmth and massage were associated with larger expressed volumes. Oxytocin also helps the uterus shrink back and reduces bleeding after birth.
The World Health Organization and UNICEF recommend feeding only breast milk for the first six months, then adding solid foods, with breastfeeding continuing up to two years. The sources credit it with fewer infections such as diarrhoea and a lower risk of sudden infant death syndrome. In the 1950s some countries went through a phase of regarding formula as superior. Informally shared, unpasteurised milk from another woman carries a risk of passing on infections, including HIV.
Source: Breast milk
