What Is Breast Milk Made Of? Nutrients, Stages and Immune Factors

Breast milk is often called a “living” food, and that is a fair description. Besides nutrients, it carries antibodies, immune cells and many other active substances. It also changes: from the first days to the first months, during a single feed and even over the course of a day.

Here’s what breast milk is made of, in plain terms, based mainly on a widely cited scientific review by Ballard and Morrow (2013). For how breastfeeding works in practice, see our breastfeeding basics overview.

The three stages of breast milk

Colostrum (first few days)

Colostrum is the thick, often yellow milk your breasts make in late pregnancy and the first days after birth. It comes in small amounts, about a teaspoon per feed at first, which suits a newborn’s tiny stomach. It is especially rich in immune factors such as secretory IgA, lactoferrin, white blood cells and growth factors that help the gut mature. It has less lactose than later milk, which tells us its main job is protection and gut development rather than calories. Our guide on when breast milk comes in explains this stage in more detail.

Transitional milk (about day 5 to 2 weeks)

As your milk volume increases, usually around 2 to 4 days after birth, your milk gradually changes. This transitional milk typically lasts from about day 5 to 2 weeks after birth, with lactose and fat rising and volume increasing quickly.

Mature milk (from about 4 to 6 weeks)

By four to six weeks after birth, milk is considered fully mature. Its composition then stays relatively stable, although it still varies within each feed and between parents.

	Colostrum	Transitional milk	Mature milk
When	First few days	About day 5 to 2 weeks	Fully mature by 4–6 weeks
Look	Thick, yellow or orange	Gradually whiter and more plentiful	Thinner at the start of a feed, creamier at the end
Volume	Small, about a teaspoon per feed at first	Increases quickly	Matches your baby’s demand
Key features	Highest in antibodies, lactoferrin, white blood cells; lower in lactose	Lactose and fat rising	Main source of calories and nutrients for growth

Macronutrients in mature breast milk

These are typical values for mature milk from parents of full-term babies. Every parent’s milk is different, and individual samples can vary widely.

Nutrient (per 100 ml, about 3.4 oz)	Typical amount	What it does
Carbohydrate (mainly lactose)	6.7–7.8 g	Main carbohydrate and a major energy source
Fat	3.2–3.6 g	Provides around half the calories, plus fatty acids for brain and eye development
Protein	0.9–1.2 g	Growth, plus immune and digestive proteins
Human milk oligosaccharides (HMOs)	About 1 g	Feed healthy gut bacteria and help block germs
Energy	65–70 kcal	Closely tied to the fat content

Source: Ballard and Morrow, 2013. The fat figure is the most variable. In donated milk samples, fat ranged from under 2 g to almost 9 g per 100 ml.

A few things stand out. Protein is the smallest of the three main nutrients, and lactose varies the least between parents, while fat varies the most. And HMOs, which babies can’t digest themselves, are present in roughly the same amount as all the protein combined. Their job is to feed beneficial gut bacteria.

Foremilk and hindmilk

Breast milk changes during a feed. The milk at the start (foremilk) is thinner and more watery. As the breast empties, fat content rises, and the milk at the end of a feed (hindmilk) can have two to three times the fat of foremilk. It isn’t two different kinds of milk, just a gradual shift.

Fat also varies over the day. One study in the Ballard review found fat was lower in night and morning feeds than in afternoon and evening feeds.

What this means for you: you don’t need to time feeds or worry about the balance. Let your baby finish the first breast until they slow down or come off on their own before offering the second. If you pump, the milk may separate in the fridge, with a cream layer on top. That’s normal: swirl it gently before feeding.

Immune factors and living cells

This is what formula can’t replicate. Breast milk contains:

• Secretory IgA: the main antibody in breast milk. It coats the lining of your baby’s gut and helps block germs from attaching.
• Lactoferrin: one of the most abundant protective proteins, especially in colostrum. It binds iron that bacteria need to grow.
• HMOs: complex sugars that act as food for healthy gut bacteria and as decoys that germs stick to instead of your baby’s gut.
• Living cells: white blood cells, including macrophages and lymphocytes, plus stem cells, whose role is still being studied. In early lactation, a baby may take in billions of maternal white blood cells a day.
• Other bioactive factors: enzymes, growth factors and hormones that support digestion and gut development.

These components help explain why breastfed babies have fewer ear, chest and stomach infections. Read more in our guide to the benefits of breastfeeding.

Vitamins and minerals: two gaps to know

Breast milk provides most vitamins and minerals a baby needs, with two important exceptions.

• Vitamin D: breast milk contains only small amounts, especially when the parent gets little sun. The AAP recommends that breastfed and partially breastfed babies get 400 IU of vitamin D a day, starting in the first few days of life, usually as liquid drops. Ask your pediatrician which product and dose to use.
• Vitamin K: levels in breast milk are very low no matter what you eat, which is why the AAP recommends a vitamin K shot for all newborns at birth to prevent dangerous bleeding.

Your diet affects some parts of your milk more than others. The types of fat, especially the omega-3 fat DHA, depend on what you eat, and the Ballard review notes that DHA is low in North American milk. See our guide to foods to avoid while breastfeeding for diet advice while nursing.

Breast milk vs formula, briefly

Formula is designed to match the main nutrients in breast milk and supports normal growth. It doesn’t contain living cells, the full mix of antibodies or milk that changes to fit your baby. For a fair comparison, see breastfeeding vs formula.

When to talk to a doctor or lactation consultant

Talk to your pediatrician about vitamin D drops, and if your baby was born early, isn’t gaining weight well, or has fewer wet and dirty diapers than expected. Ask your doctor before taking supplements while breastfeeding. If you’re worried your milk is “too thin” or “not rich enough,” ask an IBCLC to assess a feed. Thin-looking milk is usually perfectly normal, and the real question is whether your baby is feeding often and gaining well.