Breast milk is a thing of beauty. Generally, babies are breastfed exclusively for the first 6 months of life, and there’s a reason why it is considered the best source of nutrition for your infant.

Breast milk is still a mystery!

Years of research have been dedicated to understanding everything we can about breastmilk.

There’s still a lot we don’t know…. hey – it’s complicated!

Breast milk is more than nutrition!

But based on what we DO know, breast milk contains many different bioactive components that help in the optimal development and function of your baby’s gastrointestinal tract, immune system, and brain. Studies have even shown that breast milk helps reduce the risk of your child developing metabolic diseases like obesity and type 2 diabetes later in life.

What is remarkable is that the composition of breast milk changes as baby develops to continually ensure optimal nutrition is provided. Yep, we always say it…Mama’s got the magic!

Perspective: aside from the nutritional benefits, breast milk is cheap (free) and convenient, and offers a bonding opportunity for mama and baby.

What’s in breast milk?

The approximate composition of breast milk is 87% water, 3.8% fat, 1.0% protein, and 7% lactose. It’s truly the fat and lactose (which is a type of carbohydrate, or sugar) that provide most of the energy in breast milk. And that is exactly what baby needs!

In addition to this, a ton of other molecules make breast milk more than just about nutrition. For example:

  • Boosting immunity: Immune cells, stem cells, antibodies
  • Establishing the microbiome: probiotics, good bacteria, oligosaccharides
  • Supporting the growth: nucleotides, proteins, growth factors
  • Providing adequate nutrition: vitamins, minerals
  • Helping baby adjust to life outside the womb: hormones, enzymes
  • Exposing to new flavours: molecules from mom’s diet with their own taste and flavour


There are probably even more molecules present that we have not yet discovered!


What are the main nutrients in breast milk?


  • Fats

Fats are a very important component of breast milk, that not only supply energy, but also aid in the central nervous system’s development. Plus, fat provides aroma and taste for baby.

Breast milk fat content ranges from 3.5-4.5%. A molecule of fat is made of a combination of various fatty acids. Over 200+ types of fatty acids can be found in breast milk! Sure, many of these fatty acids are found in very low concentrations, but it does highlight how complex breast milk is!

Why so many different types of fat? Well, each type contributes a unique function to normal health and development. Short chain fats are a source of quick energy, but also help in developing the gastrointestinal tract. While certain medium chain fats contribute to immune function, such as the suppression/inactivation of pathogens like Group B streptococcus. Also, a group of fats called “sphingolipids” found in breast milk are especially important in neuronal development, participating in a process called myelination.

A lot of different fat molecules are present in breast milk, each having a specific function that goes beyond simply nourishing your child!


  • Carbohydrates

The carbohydrate (aka carbs, aka sugars) profile of breast milk is diverse and complex. The predominant carb in breast milk is lactose. Unlike other nutrients, lactose concentration remains relatively constant around 3-weeks postpartum onwards. The high concentration not only helps meet the high energy demand of the developing brain, but it also helps maintain consistency, and helps in absorption of minerals including calcium.

Breast milk also contains a significant amount of another group of carbohydrates called “human milk oligosaccharides” (HMOs). It is a fairly new discovery, and over 200 different HMOs have been identified to date. HMO content is almost twice a high in colostrum than in mature milk, and the immunity boost of the colostrum milk would be due to these HMO. These HMOs are indigestible for baby, and function as prebiotics. They promote the growth of beneficial gut bacteria strains including Bifidobacterium infantis, and help protect baby from harmful bacteria. Ultimately, this helps prevent neonatal diarrhea, respiratory infections, and help the establishment of the healthy gut microbiota.

Carbs provide more than energy: they also help the establishment of the gut microbiome.


  • Proteins

Breast milk contains more than 400 different proteins in two overarching classes called casein and whey. They provide nutrition but also help absorb other nutrients and possess antimicrobial and immune-supporting functions.

Just like fat is made of smaller molecules of fatty acid, proteins are made of smaller molecules called amino acids. Whey and casein have their own specific amino acid profiles. The content of breast milk and the ratio of whey to casein change from colostrum to mature milk. Whey generally is the predominant protein in breast milk, ranging from 50-80% of total protein content depending on the stage of milk.

Nucleotides, the building blocks of our DNA (our genes) are also found in breast milk. Nucleotides are conditionally essential nutrients during the early stages of life. In infants, breast milk nucleotides help the immune system, and in the development and repair of the gastrointestinal tract.

Whey, casein and nucleotides are all proteins found in breast milk and promoting growth and development in your child.


Does the composition of breast milk ever change?


Breast milk composition changes during one single feed

Breast milk composition changes during each nursing session. The first milk that is expressed is called the foremilk, and it has a thinner consistency along with a higher content of lactose to satisfy baby’s thirst.

This is followed by the creamier hindmilk which is higher in fat content. Pretty neat right? It means the duration of each nursing session will affect the nutrients your child receives.

The action of your child sucking on the breast trigger the change in composition. The milk fat content is relative to how “full” or “drained” your breast is. If your breast is fuller (assuming there is more milk stored), the fat content is diluted in more breast milk volume, resulting in a milk with a lower fat percentage. On the opposite, if you breast is more drained, the same amount of fat will be concentrated in a smaller volume of milk, making it a milk with a higher fat percentage.

Yet, despite all these changes, it has been observed that babies consume a similar amount of fat at the end of each day!

Breast milk composition changes during a feed, and fat concentration increases as the nursing session goes.


Breast milk composition changes as baby grows in age

Breast milk also changes composition to match the age and other nutritional requirements of your child. Scientists believe that the composition of breast milk is UNIQUELY TAILORED by each mom to meet the SPECIFIC needs of her baby. Talk about personalized nutrition!

Think about breast milk as a gradual change rather than truly distinct types.

  • Colostrum: the first few days

Thick and sticky yellow milk. It is not abundant in quantity, but it is a packed with quality!

Colostrum’s role would be more immunologic than nutritious, providing antibodies, bioactive cells, good bacteria, human oligosaccharides and other protective cells.

  • Transitional milk: the first few weeks

The transition from colostrum to transitional milk is often refereed to as the time when “your milk is coming in”. Your milk increases in quantity (that explains the rather drastic change in breast size!), and becomes creamier as the fat content increases, as well as lactose and calories. That is perfect for baby who is drinking more and growing fast.

  • Mature milk: after the first month

Once you reach 4 weeks, your milk is mature and the composition will remain fairly stable: high in proteins, lactose, and other minerals and vitamins. Although it has reached a “mature” and stable stage, your breast milk will still vary feed to feed, and in time as baby grows.

The nutrition content of the breast milk changes as baby grows to ensure all the nutrients needs are met!


What factors influence changes in breast milk composition?

Several factors like stage of nursing or time of day influence breast milk composition, and in general fat seems to change the most. In addition, other factors also play a role:

  • Age of your baby: Colostrum, transitional milk, and mature milk have different nutrient profile.
  • Time since last feeding: Milk fat content is highly affected by the length of time between the last feed and volume consumed. That is, the greater the time interval, the lower the fat content because it is diluted in more volume of milk.
  • Stage of nursing: Prominent changes in fat and lactose content changes between foremilk and hindmilk.
  • Time of day: Fat content in breast milk peaks mid-morning, and is at its lowest overnight.
  • Age of mama: Generally, the mother’s age doesn’t play a huge factor in breast milk composition, but protein content is highest in mothers aged 20-30 years old.
  • Mama’s diet: Breast milk is highly susceptible to changes in dietary fat. In fact, breast milk fat content changes within 2-3 days to mimic changes in dietary fat. The best way to ensure adequate nutrition to breast fed baby is to make sure mama herself is eating well!
  • Baby’s birth weight: fat content has been reported in some studies to be higher in babies with low birth weight as well as in infant with high birth weight. Carb and protein content seem to remain unaffected


A lot can affect breast milk composition, but not much in our control. Aiming for a healthy diet is likely the best bet to ensure baby received everything needed!


What doesn’t affect milk composition?


  • Ethnicity: Based on an analysis of women from seven different countries, ethnicity does not significantly influence breast milk composition
  • Gestational weight gain: The jury is still out on this one, but for now there does not seem to be a relationship between gestational weight gain and breast milk fat content.
  • Breast size: the size of the breast is mostly determined by the amount of fatty tissue it is made of. This has nothing to do with the production of breast milk, so breast size is irrelevant when it comes to breast feeding.


Breast size, ethnicity and the amount of weight mom gained during pregnancy doesn’t affect the composition and volume of breast milk produced.


Does frozen breast milk lose its nutritional value?

Yes…but it is still very nourishing!

Fresh breast milk is preferable to refrigerate breast milk, and refrigerate breast milk is better than frozen breast milk. That is because all the bioactive molecules don’t all resist to cold temperature. Yet, all these options are still very nourishing for baby! So go ahead and pump and freeze breast milk if that option fits best your lifestyle and choices.

Always follow the recommendations in term of storage and use of expressed/pumped breast milk.

Expressed and stored breast milk tend to separate, where the fat rising at the top. Rather than mixing vigorously, it is suggested to gently mix the milk to preserve all the components.

If you notice the pumped breast milk you thawed is different than your freshly expressed breast milk, that can be due to the enzyme lipase which continues to break down fat into fatty acids, changing the look and taste of it! Some babies don’t mind while others do. Make sure your baby drink pumped breast milk before you put in the energy to make yourself a stock of pump breast milk! Test whether baby will drink refrigerated and frozen-thawed pumped breast milk.


Pumping and freezing breast milk is a adequate alternative to feeding at the breast, but some bioactive molecules do not resist to cold temperature. Make sure baby likes pumped breast milk before you go on and stash your freezer!


Can I feed my child frozen breast milk from several months ago?

It is likely that you will be able to make reserve of expressed breast milk when your mature milk is well established. Because mature milk nutrient’s content is more stable than the ones from colostrum and transitional milk, it is appropriate to feed to a child of a different age than when it was pumped. It may not been exactly the same nutritional content as milk produce at that moment but it is still adequate for baby.

In a perfect world, you would be able to feed baby breast milk that has been expressed a couple of days or weeks ago, not months (or year!) ago. But some circumstances can make this impossible to achieve, and breast milk has many virtues that are beneficial for babies, so don’t let that stop you.

Feeding expressed mature breast milk to older babies is adequate for their nutritional needs.


Is formula comparable to breast milk?

Formula isn’t identical to breast milk, but a lot of work has been put into replicating the nutritional profile of breast milk. Formula uses soy or cow’s milk as a base, and both macronutrients (carbs, fats, proteins) and micronutrients (vitamins and minerals) are added to achieve the ideal composition.

Formula is generally sold by age groups:

  • Newborn formula – Birth
  • Infant formula – 6 months+
  • Toddler formula – 12 months+

The only time formula composition is likely to vary is when you change age groups or formula brands, and can be further stratified by factors like allergies/sensitivities and special nutritional requirements. It is an adequate milk for your child, but it is unlikely that we will ever be able to market a formula that has the same bioactive molecules than breast milk.

Formulas are adequate milk for infants but do not contain all the bioactive molecules that breast milk does.


Martin C, Ling P-R, Blackburn G. Review of Infant Feeding: Key Features of Breast Milk and Infant Formula. Nutrients 2016;8(5):279.

Andreas NJ, Kampmann B, Mehring Le-Doare K. Human breast milk: A review on its composition and bioactivity. Early Hum Dev. Elsevier Ireland Ltd; 2015;91(11):629–35.

Lawrence RA, Lawrence RM. Breastfeeding: a guide for the medical professional. Elsevier Health Sciences; 2010 Sep 30.

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