Where Does Breast Milk Come From?

Ever look at a fancy bottle of non-cow milk and think, “Wait, how do you milk an almond? Or a cashew? A hemp seed?” No shade if you've also wondered this about breast milk, because it's actually quite complex. Luckily, the term "breast milk" itself is pretty much exactly what it sounds like, says Lauren Levine, MD, assistant professor of pediatrics at Columbia University Medical Center: “It’s milk produced by specialized cells in a woman’s breast.” Here’s how it all goes down: When someone gets pregnant, hormones tell special glandular cells in an area of the breast called the alveoli that it’s time to make milk, and move it into milk ducts that lead out of the nipple, Dr. Levine says. Then, that person's body gathers "ingredients" to make the milk from their blood, and synthesizes them (sometimes pulling in water) in the mammary gland, says Katie Hinde, PhD, associate professor at the School of Human Evolution and Social Change and the Center for Evolution and Medicine at Arizona State University. Add a baby to latch on and suck, and bon appetit, you’ve got breast milk. Milk ducts and glandular cells are all part of normal breast anatomy, and women are born with them. The amount people have increases during puberty (which causes a person's breasts to grow), then again during pregnancy and in the early stages of lactation (which, again, causes their breasts to grow). You know the scene in Meet the Parents in which Robert De Niro asks, “I have nipples, Greg, could you milk me?” The answer is: only if the person is pregnant. Even though, anatomically, women's breasts have all the parts for milk-making, they can’t actually produce breast milk until the end of pregnancy and after delivery. Let's say someone just gave birth: When the infant is placed on the breast, the nerve endings send a signal to the brain to release hormones (prolactin and oxytocin) that tell the body, "More milk, pronto!" That's why doctors try to get babies to start nursing as soon as possible. "Just the action of sucking starts the chain of events in a woman's body," Dr. Levine says. The actual milk that comes out is rich in nutrients and other building blocks a newborn needs in the first months of life. Think: fats, proteins, carbohydrates, water, fatty acids, amino acids, minerals, vitamins, and trace elements. Experts aren't really sure exactly how the different components of breast milk work, but they do know they work together and on their own to make essential things for infants. "Scientists still have not exhaustively documented what all is in milk, how it all gets there, and what they all do for the infant," Dr. Hinde says. We do know that breast milk goes through three different stages, and it changes appearance and makeup as pregnancy ends and the baby grows. "Colostrum" is the first stage, produced late in pregnancy and right after delivery — this is packed with immune molecules and sugars for the baby's gut. "Transitional milk" has a creamier appearance than colostrum and more fat. A person's body will make a lot of it two to five days after delivery, when the baby is better able to digest components of milk. Finally, "mature milk" production really bulks up when someone is 10 days out from delivery, and lasts until the tail end of weaning. Unlike cow’s milk, breast milk is made up of mostly whey protein and a little casein — cow’s milk has a little whey protein and a lot of casein. And lots of research suggests that drinking breast milk helps a baby’s immune system. When someone who's breast-feeding eats or breathes in an infectious agent, their body makes antibodies to fight them, which then get passed along to the baby through breast milk, Dr. Levine says. There are also other enzymes in the milk that aid in digestion and have anti-inflammatory benefits, plus help grow gut bacteria that impacts whether or not the baby will develop allergies. Perhaps the coolest thing about breast milk is that it adapts constantly, based on the person's body. "As mom's circulating hormones vary across the day, so do those hormones in her milk," Dr. Hinde says. There's even some evidence that the immune molecules in breast milk increase when the baby gets sick, she adds. Imagine having a milk dispenser that magically adjusts to your needs... That's just the miracle of life, y'all.

More from Body

R29 Original Series