Human Breast Milk - Composition

Composition

The exact integrated properties of breast milk are not entirely understood, but the nutrient content after this period is relatively consistent and draws its ingredients from the mother's food supply. If that supply is found lacking, content is obtained from the mother's bodily stores. The exact composition of breast milk varies from day to day, depending on food consumption and environment, meaning that the ratio of water to fat fluctuates.

During the first few days after delivery, the breasts produce colostrum. This is a thin yellowish fluid that is the same fluid that sometimes leaks from the breasts during pregnancy. It is rich in protein and antibodies that provide passive immunity to the baby (the baby's immune system is not fully developed at birth). Colostrum also helps the newborn's digestive system to grow and function properly.

After 3 to 4 days, breasts will begin producing milk that is thin, watery, and sweet. This quenches the baby's thirst and provides the proteins, sugar, and minerals that the baby needs. Over time, the milk changes and becomes thick and creamy. This satisfies the baby's hunger.

Foremilk, the milk released at the beginning of a feed, is watery, low in fat, and high in carbohydrates relative to the creamier hindmilk, which is released as the feed progresses. The breast can never be truly "emptied," since milk production is a continuous biological process.

The level of Immunoglobulin A (IgA) in breast milk remains high from day 10 until at least 7.5 months post-partum.

Human milk contains 0.8% to 0.9% protein, 4.5% fat, 7.1% carbohydrates, and 0.2% ash (minerals). Carbohydrates are mainly lactose; several lactose-based oligosaccharides have been identified as minor components. The fat fraction contains specific triglycerides of palmitic and oleic acid (O-P-O triglycerides), and also quite a large quantity of lipids with trans bonds (see: trans fat) that are considered to have a health benefit. They are vaccenic acid, and Conjugated linoleic acid (CLA) accounting for up to 6% of the human milk fat.

The principal proteins are casein (homologous to bovine beta-casein), alpha-lactalbumin, lactoferrin (apo-lactoferrin), IgA, lysozyme, and serum albumin. In an acidic environment such as the stomach, alpha-lactalbumin unfolds into a different form and binds oleic acid to form a complex called HAMLET that kills tumor cells. This is thought to contribute to the protection of breastfed babies against cancer.

Non-protein nitrogen-containing compounds, making up 25% of the milk's nitrogen, include urea, uric acid, creatine, creatinine, amino acids, and nucleotides. Breast milk has circadian variations; some of the nucleotides are more commonly produced during the night, others during the day.

Mother's milk has been shown to supply a type of endocannabinoid (the natural neurotransmitters that marijuana simulates), 2-Arachidonoyl glycerol.

The breastmilk of diabetic mothers has been shown to have a different composition from that of non-diabetic mothers. It may contain elevated levels of glucose and insulin and decreased polyunsaturated fatty acids. A dose-dependent effect of diabetic breastmilk on increasing language delays in infants has also been noted, although doctors recommend that diabetic mothers breastfeed despite this potential risk.

Though it now is almost universally prescribed, in some countries in the 1950s the practice of breastfeeding went through a period where it was out of vogue and the use of infant formula was considered superior to breast milk. However, it is now universally recognized that there is no commercial formula that can equal breast milk. In addition to the appropriate amounts of carbohydrate, protein, and fat, breast milk provides vitamins, minerals, digestive enzymes, and hormones. Breast milk also contains antibodies and lymphocytes from the mother that help the baby resist infections. The immune function of breast milk is individualized, as the mother, through her touching and taking care of the baby, comes into contact with pathogens that colonize the baby, and, as a consequence, her body makes the appropriate antibodies and immune cells. This is the reason why breast milk contains very little iron, as iron is an essential precursor to the activation of micro-bacteria and gut flora; providing iron in the milk would lead to infection. However, the internal iron supplies of the infant, held in the hepatic cells of the liver, are exhausted at 4–6 months, hence this is the time that complementary feeding is introduced, as to prevent anemia (recommended by WHO).

Women breastfeeding should consult with their physician regarding substances that can be unwittingly passed to the infant via breast milk, such as alcohol, viruses (HIV or HTLV-1) or medications.

Most women that do not breastfeed use infant formula, but breast milk donated by volunteers to human milk banks can be obtained by prescription in some countries.