Breast milk composition changes over time: a complex process!


What are the changes in the composition of breast milk over time?

Does breast milk composition even change?

The short answer is yes.

This is why breast milk is a thing of beauty.

Babies usually breastfeed exclusively for the first 6 months of life.

Breast milk is the gold standard of nutrition for infants.

Years of research have helped us better understand the complexities of breast milk.

We start to understand WHY it is so beneficial for infants; but there’s still a lot we don’t know… hey – it’s complicated!


Summary of breast milk composition

Image adapted

Breast milk benefits for baby: more than nutrition!


From what we DO know, breast milk has many different bioactive components.

They help with the development of your baby’s gastrointestinal tract, immune system, and brain.

Studies show breast milk helps reduce metabolic disease risk later in life. This includes obesity and type 2 diabetes.

What’s remarkable is that breast milk composition changes as your baby develops.

Its dynamic and helps provide optimal nutrition.

Yep, we always say it…Mama’s got the magic!

Perspective: Beyond nutrition, breast milk is cheap (free) and convenient. Best of all is the bonding opportunity for mama and baby.



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What is the composition of breast milk?


The approximate composition of breast milk is 87% water, 7% lactose, 4% fat, and 1% protein.

The fat and lactose (a type of carb/sugar) provide most of the energy in breast milk.

And that is exactly what baby needs!

Also, 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 baby’s growth: nucleotides, proteins, growth factors
  • Providing adequate nutrition: vitamins, minerals
  • Helping baby adjust to life outside the womb: hormones, enzymes
  • Exposure to new flavours: from mom’s diet with their own taste and flavour

There are probably even more we haven’t discovered!


What are the main nutrients in breast milk?


Breast milk contains all the nutrients babies need to growth to their full potential.

Breastmilk nutrient composition breakdown


Fats and breast milk

Fats are a very important component of breast milk.

Fat not only supplies energy, but also aids 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%.

Over 200+ types of fatty acids are in breast milk; a great example of its complexity!

Why so many different types of fat?

Each fat has a unique function for normal health and development.

Here are some examples:

Short chain fats provide quick energy. They also help in developing the gastrointestinal tract.

Some medium chain fats support immune function.

For example, they help suppress pathogens like Group B streptococcus (GBS).

Other fats called “sphingolipids” aid neuronal development in the brain.

This is why it is difficult to simplify fats as “good” or “bad” like with fad diets.

Remember, fats are diverse and play a huge part in normal growth and development.

Breast milk has diverse fat types that serve specific functions beyond nourishing your child!


Carbohydrates and breast milk

The carb (or sugar) profile of breast milk is diverse.

The predominant carb in breast milk is lactose.

Its concentration remains pretty stable around 3-weeks postpartum onward.

The high lactose content provides energy, especially for the developing brain.

Lactose also helps maintain milk consistency, and absorption of minerals like calcium


Human milk oligosaccharides in breast milk

Breast milk contains a category of carbs called “human milk oligosaccharides” (HMOs).

This is a pretty recent discovery! To date, researchers have found over 200 different HMOs.

HMOs may shed light on the immune benefits of breast milk.

For example, in comparison to mature breast milk, HMO content is almost twice as high in colostrum.

Baby’s don’t digest HMOs. Instead they function as prebiotics.

HMOs promote the growth of beneficial gut bacteria (i.e. Bifidobacterium infantis).

This helps protect baby from harmful bacterial…but how?

By preventing neonatal diarrhea, respiratory infections, and supporting healthy gut microbiota.

Carbs provide more than energy: they help establish the gut microbiome.


Protein and breast milk

Breast milk has 400+ different proteins.

These proteins fit in two categories: i) casein and ii) whey.

Protein itself is a nutrient, but also helps absorb other nutrients.

Proteins also have antimicrobial and immune-supporting functions.

Amino acids are the building blocks of proteins, and each has a unique combination.

Therefore, the amino acid profile of whey and casein proteins are distinct.

From colostrum to mature milk, the ratio and amounts of whey/casein protein changes.

Between the two, whey protein is predominant. It makes up 50-80% of protein content in breast milk.

Suffice to say that the protein composition of breast milk is dynamic!

Nucleotides are also found in breast milk. They are the building blocks of our DNA.

Nucleotides are conditionally essential nutrients during the early stages of life.

In infants, they help the immune system and the gastrointestinal tract.

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


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How does the composition of breast milk change over time?


Lets explore some of the short- and long-term reasons why breast milk composition changes.

It will certainly help you appreciate the beauty of this dynamic process.


Breast milk composition changes during one single feed


Breast milk composition changes during each nursing session.

Foremilk is the first milk that’s expressed. It has a thinner consistency and higher lactose content to quench baby’s thirst.

This is followed by the fattier hindmilk. Pretty neat right?

This means how long your baby nurses will affect the nutrients s/he receives.

The mechanical action of sucking on the breast triggers the change in composition.

The amount of milk fat depends on how “full” or “drained” your breast is.

The fat content in a a fuller breast is diluted due to milk volume. This means the milk has a lower fat percentage.

Whereas, in a drained breast, the same fat content becomes concentrated. This makes for a higher fat milk.

But, does it make a big difference in the end?

Not so much; studies suggest 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 with longer nursing sessions.


Breast milk composition changes as baby grows in age


Breast milk composition changes to match the age and various nutritional requirements of your child.

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’s low in volume, and packed with quality!

Colostrum’s role is more immune related than nutritious.

Colostrum provides antibodies, good bacteria, HMOs, and other protective cells.


Transitional milk: the first few weeks

After colostrum comes transitional milk.

This is often refereed to as the time when “your milk is coming in”.

Milk volume also increases, explaining the drastic change in breast size!

The milk becomes creamier due to higher fat and lactose content.

This is perfect for baby who is drinking more and growing fast.


Mature milk: after the first month

After approximately 4 weeks, your milk is mature.

The composition will remain fairly stable: high in proteins, lactose, and other vitamins/minerals.

Although mature milk is stable, your breast milk will still vary feed-to-feed, and as baby grows.

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


Breast milk composition: what factors influence changes?


In genreal, fat seems to change the most.

Several factors like nursing stage, or time of day influence breast milk composition.


But, 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 by your baby.

That means, the longer you wait until the next feed, the lower the fat content because it is diluted in more volume of milk.

Stage of nursing: 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: Mama’s age doesn’t play a huge factor in breast milk composition.

But! Data shows protein content is highest in mothers aged 20-30 years old.

Mama’s diet: Breast milk is highly susceptible to changes in dietary fat.

It changes within 2-3 days to mimic dietary fat in mama’s diet.

This is why mama needs to eat well too!

Baby’s birth weight: In some studies, fat content was higher in babies with low or high birth weight.

Carb and protein content seem to remain unaffected.

A lot of what affects breast milk is not in our control. A healthy diet for mom is the best way to help breastfeeding babies!


What doesn’t affect breast milk composition?

Ethnicity: Based on an analysis of women from seven different countries, ethnicity does not significantly influence breast milk composition.

Gestational weight gain: Based on current data, weight gain during pregnancy doesn’t seem to affect the fat content of breast milk.

Breast size: Breast size is linked to the amount of fat tissue.

This has nothing to do with the production of breast milk.

Basically, breast size is irrelevant when it comes to breast feeding nutrition.

Breast size, ethnicity and pregnancy weight gain don’t affect breast milk volume and composition.


Breast milk composition: tailored immunity through baby spit backwash


Breast milk is packed with antibodies and bioactive cells,  helping protect against infections.

This is also a dynamic process!

Researchers hypothesize that baby is able to signal his/her specific immune needs to the mother.

This occurrs through a phenomenon called “baby spit backwash”.

During breastfeeding, the suckling creates a vacuum and forces a mixture of breast milk and baby’s saliva to go back upstream into the mother’s nipple.

This “retrograde milk flow” from the baby’s mouth to the mother’s nipple, produces on-demand immune cells for baby!

The immune cells, called leukocytes, are then passed back to baby through breast milk, helping baby fight the infection.

Although still being studied, this idea of tailored immunity makes sense.

A key concept of immunity is the exposure to pathogens.

This triggers the immune system to respond and fight back.

Evidence shows that leukocytes levels spike in breast milk when baby is sick.

This happens in response to baby’s need to have immune protection to fight infections.

The call for leukocytes could happen through this spit-backwash action.

It happens when baby’s mouth interacts and signals to the nipple of the nursing mothers.

Evidence is not clear whether this same phenomenon happens when an infant is fed breast milk in a bottle.

This is where the direct contact from mouth-to-nipple is missing.

Baby spit backwashes into the mother’s nipple, signalling the exact type of antibodies needed to fight back the infection!


Does frozen breast milk lose its nutritional value?


Yes…but it is still very nourishing!

Fresh breast milk is preferable to refrigerated breast milk.

Refrigerated breast milk is better than frozen breast milk.

That is because all the bioactive molecules don’t always resist the cold temperature.

Yet, all these options are still very nourishing for baby!

So go ahead… pump and freeze breast milk if that option best fits your lifestyle and choices.

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

Expressed and stored breast milk tend to separate, with fat rising to the top.

Rather than mixing vigorously, mix gently the milk to preserve all the components.

Have you noticed that thawed breast milk that was pumped is different than freshly expressed milk?

This can be due to the enzyme lipase which continues to break down fat into individual fatty acids, changing the look and taste of it!

Some babies don’t mind while others do.

Make sure your baby accepts the taste of 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. Some bioactive molecules do not resist to cold temperature. Make sure baby likes pumped breast milk before you go and stock your freezer!


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


You can likely make a reserve of breast milk when “mature milk” is well established.

Recap: mature milk has stable nutrient content versus colostrum and transitional milk.

This means it’s fine to pump, freeze, and feed you infant at a later time.

The nutrient content of older breast milk may not exactly match fresh breast milk.

That’s fine, it is still adequate for your baby!

Yes, in a perfect world, your baby would get freshly expressed milk.

Or, even milk that is a few days or week-old, instead of months.

But some circumstances can make this impossible to achieve.

Breast milk has many virtues that benefit 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.

However, a lot of work has been invested in replicating the nutritional profile of breast milk.

Formula uses soy or cow’s milk as a base.

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 likely varies is when you change age groups or formula brands.

However, formula can be further stratified by factors like allergies, sensitivities, and special nutritional requirements.

Still, formula is an adequate milk for your child.

It’s just unlikely that we will ever be able to market a formula that has the same bioactive molecules as breast milk.

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




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