Not all of breast milk’s properties are understood, but its nutrient content is relatively consistent. Breast milk is made from nutrients in the mother’s bloodstream and bodily stores. Breast milk has an optimal balance of fat, sugar, water, and protein that is needed for a baby’s growth and development.  Breastfeeding triggers biochemical reactions which allows for the enzymes, hormones, growth factors and immunologic substances to effectively defend against infectious diseases for the infant. The breastmilk also has long-chain polyunsaturated fatty acids which help with normal retinal and neural development.  Because breastfeeding requires an average of 500 calories a day, it helps the mother lose weight after giving birth.

The composition of breast milk changes depending on how long the baby nurses at each session, as well as on the child’s age.  The first type, produced during the first days after childbirth, is called colostrum. Colostrum is easy to digest although it is more concentrated than mature milk. It has a laxative effect that helps the infant to pass early stools, aiding in the excretion of excess bilirubin, which helps to prevent jaundice. It also helps to seal the infants gastrointestional tract from foreign substances, which may sensitize the baby to foods that the mother has eaten. Although the baby has received some antibodies through the placenta, colostrum contains a substance which is new to the newborn, secretory immunoglobulin A (IgA). IgA works to attack germs in the mucous membranes of the throat, lungs, and intestines, which are most likely to come under attack from germs.

Breasts begin producing mature milk around the third or fourth day after birth. Early in a nursing session, the breasts produce foremilk, a thinner milk containing many proteins and vitamins. If the baby keeps nursing, then hindmilk is produced. Hindmilk has a creamier color and texture because it contains more fat.

Milk quality may be compromised by smoking, caffeinated drinks, marijuana, methamphetamine, heroin and methadone. However, the American Academy of Pediatrics (AAP) states that “tobacco smoking by mothers is not a contraindication to breastfeeding.” In addition, AAP states that while breastfeeding mothers “should avoid the use of alcoholic beverages”, an “occasional celebratory single, small alcoholic drink is acceptable, but breastfeeding should be avoided for 2 hours after the drink.” A 2014 review found that “even in a theoretical case of binge drinking, the children would not be subjected to clinically relevant amounts of alcohol”, and would have no adverse effects on children as long as drinking is “occasional”.

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