The Protein in Breast Milk

Breast milk

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Protein is an essential nutrient that supports the structure and function of the human body. It builds, strengthens, and repairs your organs, muscles, bones, and blood. Protein is also necessary to make hormones, enzymes, and antibodies.

The Proteins Found in Breast Milk

The protein in your breast milk is important for your baby's growth and development, but it also helps to protect your baby from illness.

In the first few days after your baby is born, your child will get colostrum. There may only be a tiny amount of this first breast milk, but it's packed with nutrition including easy to digest protein. As your breast milk comes in and the colostrum changes over to transitional milk and eventually mature milk, the amount of protein goes down.

Amount of Protein Over Time

Early days: 1.4-1.6 g/100mL

After 3-4 months: 0.8-1.0 g/mL

After 6 months: 0.7-0.8 g/mL

Protein concentration is higher in parents with a higher body weight for height and lower in parents who produce a higher volume of milk. Protein is not affected by what a lactating parent eats.

Casein and Whey

There are two types of protein in human breast milk: whey and casein (curds).

Whey proteins are liquid and very easy to digest. Whey also contains antibodies, lactoferrin, and lysozyme which help your baby fight off infections and disease.

Casein proteins are larger, more complex protein molecules that are harder to digest.

The ratio of whey to casein fluctuates throughout lactation. In early lactation it is around 80% whey and 20% casein. The approximate average throughout the normal course of breastfeeding is 60% whey and 40% casein. And as the time goes on, the whey protein continues to drop until there's about the same amount of both whey and casein later in lactation.

Human milk contains significantly more whey than other mammals. The concentration of whey proteins in cow’s milk, for instance, is only around 18%.

Amino Acids

Amino acids are the building blocks of protein. When protein is digested in your baby's stomach, it breaks down into amino acids. There are over 20 different amino acids found in breast milk.

In addition to protein-bound amino acids, breast milk contains free amino acids (FAAs). FAAs account for 5-10% of the total amino acids in human milk. Research indicates these amino acids play a role in infant immune development.

Glutamate and glutamine make up around 50% of all FAAs in breast milk. In addition to providing immune development, researchers believe these FAAs support growth of the nervous tissue and intestines.

Taurine is the second most abundant FAA in human milk. Studies show that taurine has many functions, including combining with bile acids and playing an important role in brain and eye development.

Some of the other amino acids found in breast milk include cysteine, lysine, phenylalanine, tyrosine, and methionine.


Lactoferrin is an iron-binding protein that attaches to iron and transports it throughout the body. But, that's not the only function of lactoferrin. Lactoferrin stimulates the immune system and helps breastfed babies fight off the organisms that can cause bacterial, viral, and fungal infections of the gastrointestinal tract.

Protein and Premature Babies

If your child is born prematurely, your breast milk will have even more protein so that they are able to grow adequately.

The high concentration of the immune properties, including lactoferrin, in whey protein also help to protect preemies from intestinal infections.

The Protein in Formula Versus the Protein in Breast Milk

Infant formula has as much or more protein than breast milk. However, the protein in formula is not the same as the protein in breast milk.

Formula, especially cow's milk-based formula, has more casein and less whey, which makes formula more difficult to digest than breast milk. So, even though breast milk may contain less protein, the protein that it does have is more easily digested by a baby and, therefore, used more efficiently. 

7 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Donna Murray, RN, BSN
Donna Murray, RN, BSN has a Bachelor of Science in Nursing from Rutgers University and is a current member of Sigma Theta Tau, the Honor Society of Nursing.