Breast milk components

What is in my milk?

Breast milk is a complex bio-active mixture of ingredients that include water, fats, sugars, proteins, minerals, vitamins, cells, and hormones, among other components. Roughly half the calories in breast milk come from fat. Breast milk changes to meet the needs of the growing baby, moving from colostrum for the first two to four days after birth, then transitional milk, and mature milk until the start of weaning when weaning milk is made. Breast milk is specific to each mother. Some breast milk component amounts change over the course of the day and provide the baby with timed nutrition. Infant formula components are static, not bio-active, and often different from breast milk. 

A) Milk content

Human breast milk is an incredibly complex biofluid that contains a collection of components that work together to ensure the optimum nutrition and growth of human babies. It is very different from the milk of other species.

Research has identified many components in breast milk but little is known about how they interact.

Human milk contains roughly:

  • 87% Water
  • 8% Fat
  • 1% Protein
  • 7% Milk sugar (lactose)

Fat and lactose respectively provide 50% and 40% of the energy in breast milk (Martin 2016). Compared with the milk of other mammals, human milk is relatively low in fat.

Human milk is high in lactose to fuel our large brains and help babies calcium. It contains large amounts of unique sugars (human milk oligosaccharides [HMOs]).

Breast milk components are derived from three sources:

  1. The mother’s diet
  2. The mother’s body
  3. Made in the milk-producing cells (lactocyctes)

The components of human milk are not largely affected by race, age, or the number of children a mother has had (Jenness 1979) and when available in adequate amounts, nearly always supports the healthy growth of a child. However, each mother produces milk that is unique and amounts of each component often vary as the baby grows, allowing breast milk to meet the baby’s exact needs.

B) Classes of breast milk components

Breast milk has a wide range of components, which can be divided into classes. 

Table: Classes of Breast Milk Components (Ballard and Morrow 2013)

(Links to more information about the topics in the above table: cholesterol; glycanslactoferrin; vitamin B12; vitamin Dvitamin KmicroRNAbacteria; food proteins from mother’s diet; iron; zinc; iodide)

1) Fats

Fat is the most variable component in breast milk (Mitoulas 2003). Milk fat profiles change slightly with the mother’s diet, weight, genetic makeup, health, environment, and with the baby’s age at birth and current age (Amaral 2019; Aumeistere 2019; Dickton 2018; Miliku 2019).

Milk fat is organized into tiny balls (milk fat globules) that float in milk. If expressed milk is left to sit in the refrigerator, the fat will rise and form a layer on top of the milk. This is normal.

Triglycerides make up 95% of all the fat in breast milk. Triglycerides are made of a three-carbon backbone (glycerol) that is joined to chains of fatty acids. Half of the fatty acids are saturated and the other half are unsaturated. Palmitic acid, a saturated fat, is 23% of total fat and oleic acid, an unsaturated fatty acid, is 36% of the total fat (Martin 2016).

While all fatty acids provide energy, they also have different lengths and different functions. Shorter fatty acids help the gut mature and longer ones support brain development, adjust the immune system, and have anti-viral and anti-parasitic activity (Schlievert 2019).

Infant formula uses triglycerides and other oils that are mostly plant-based and have a different structural make-up (Mendonça 2017). In particular, palmitic acid is connected to a different place on the glycerol molecule and this may account for the increased risk of constipation in infant formula-fed babies.

2) Proteins

There are three classes of protein in breast milk: whey, casein, and mucins. Whey will remain liquid whereas casein will become solid in the stomach, forming curds. Mucins are present in the milk fat globules.

The whey/casein ratio in breast milk fluctuates between 90/10 soon after birth and decreases to 60/40 later in the breastfeeding period (Donovan 2019; Martin 2016).

Breast milk has higher amounts of whey and lower amounts of casein compared to the milk of other animals. For example, with a whey to casein ratio of about 20/80, cow’s milk has much more casein than breast milk. Whey is added to cow’s milk-based infant formula to make it more digestible and the ratio of whey to casein can vary between various infant formulas.

a) Whey proteins

Whey proteins include:

  • α-Lactalbumin (protects the baby from infection, helps with gut maturation, hold onto zinc and calcium)
  • Lactoferrin (stores iron and keeps bacteria from using it)
  • Secretory IgA (an antibody which prevents harmful microbes from attaching to the baby’s cells and neutralizes them)
  • Osteoponin
  • Serum albumin
  • Several classes of enzymes:
    • Lysozyme (breaks down the cell wall of harmful bacteria)
    • Lipase (breaks down fats)
    • Proteases (break down proteins)
    • Amylase (breaks down starch, has anti-bacterial actions)
  • A large number of proteins present in low amounts

The antibody secretory IgA makes up 90% of the antibodies in human milk. Instead of being absorbed or destroyed by the baby’s body, it remains active in the gut, protecting the baby from infection. It has demonstrated activity against many viruses including those that cause diarrhea and chest infections. In contrast, cow’s milk contains much lower levels of secretory IgA, which are further reduced when processed into infant formula. 

Enzymes are proteins that break down nutrients into a form that can be used by the body. Various types are present in breast milk and help the baby digest milk and food.

b) Casein

Casein in breast milk is mostly β-casein although it also has α-casein and κ-casein whereas α-casein is the main casein in cow’s milk (Liao 2017). Human casein is a source of calcium and phosphate for bone growth and amino acids for protein growth and has anti-oxidant, anti-bacterial, and anti-inflammatory activity.

3) Cells

Each day, breastfeeding babies take in around 100,000,000 (108) cells. There are five main types of cells (stem cells, macrophages, neutrophils, lymphocytes, and epithelial cells) (Valverde-Villegas 2019). A newer class of cell (innate lymphoid cells) has also been found in breast milk (Baban 2018). The exact number of each type changes as the baby grows (Li 2019).

These cells help the baby’s immune system work against harmful organisms and prevent the system from over-reacting (Baban 2018). They also help to keep breast milk from being contaminated with microorganisms.

Live cells are killed by pasteurization and freezing breast milk. There are no live cow’s milk cells in infant formula as they are killed during its processing.

4) Antioxidant activity

A variety of compounds such as vitamins, minerals, and enzymes in breast milk have antioxidant activity (Cacho 2017).

C) Changes with the baby’s age

Breast milk donations from mothers, showing a range of colour. Courtesy of the NorthernStar Mothers Milk Bank.

Breastmilk undergoes gradual changes to meet the baby’s needs (Gidrewicz 2014). These phases are:

  1. Colostrum for the first two to four days after birth
  2. Transitional milk until the end of the second week after birth
  3. Mature milk
  4. Weaning milk

The table below describes how nutrients change between these four milk types and compares them to the amounts in infant formula.

Table: Changes in Breast Milk Nutrients With Age (Czosnykowska-Łukacka et al. 2018; Garwolińska et al. 2018; Hester et al. 2012)

1) Appearance of milk changes through the entire period of lactation

Colostrum is clear and straw-coloured, transitional milk looks very rich and creamy, and mature milk will look thinner and slightly more blue over time. These are all normal changes.

2) The transition from colostrum to mature milk

a) Colostrum

The role of colostrum is to protect the newborn baby from infection and help the gut to start working.

Colostrum protects the newborn baby from infection with higher amounts of antibodies (secretory IgA), white blood cells, and lactoferrin.

Compared to mature milk, colostrum has lower energy content but higher amounts of proteins, protein building blocks, vitamins, minerals, and cholesterol and other sterols (Golinelli 2014; Hamdan 2018; Sundekilde 2016; Yang 2018). The enzyme amylase is present in higher amounts in colostrum, when the baby’s own amylase levels are low, and the amounts decrease over the next few months as the baby’s own levels rise.

The amino acid glutamine is a building block for proteins but also appears to act as a neurotransmitter in the brain and provide nutrition for cells in the baby’s gut (Agostoni 2000). The amount of glutamine is 20 times higher in mature milk than in colostrum (Zhang 2013).

b) Transitional milk

Transitional milk helps the baby return to birth weight. It is made in much larger amounts than colostrum.

3) Mature milk

Protein levels decrease during the first 6 months of lactation, and the type of milk protein changes from mostly whey after birth to equal amounts of whey and casein later in lactation. 

As babies enter the second year of life, breast milk continues to provide many nutrients and is relatively consistent in its makeup. There are small changes in some components. For example, it has fewer carbohydrates and more (Perrin et al. 2017):

  • Total protein (Verd et al. 2018)
  • Energy
  • Fat (Mandel and  Lubetzky  2005)
  • Lactoferrin (Czosnykowska-Łukacka et al. 2019)
  • Agents (lysozyme) to help good bacteria grow in the child's gut (Minami et al. 2016)
  • Human milk oligosaccharides (HMOs)  
  • Antibodies  (Czosnykowska-Łukacka et al. 2020)

4) Weaning milk compared to mature milk

As the amount of milk removed from the breast decreases during weaning, there are changes in some components of breast milk.

Weaning milk has increased amounts of (Garza 1983; Goldman 1983; Prosser 1984):

  • Protein
  • Lactoferrin
  • Lysozyme
  • Antibodies
  • Iron
  • Sodium

This can provide the baby with a boost to iron levels and immune protection before weaning. 

It has decreased amounts of (Garza 1983; Goldman 1983; Prosser 1984):

  • Potassium
  • Glucose
  • Lactose
  • Zinc

5) Milk of mothers of premature babies

The milk of mothers who have delivered a premature baby is slightly different from that of mothers of term babies. It has higher levels of white blood cells, antibodies, lactoferrin, growth factors, and HMOs (Bardanzellu 2020). It also has higher levels of fat, lactose, protein, and energy (Bauer 2011; Maly 2018; Mimouni 2017).

D) Changes with the time of day

Some breast milk components will increase and decrease at various times during the day and night. These variations likely provide nutrients when they are most needed by the baby (Hahn-Holbrook 2019; White 2017).

The building block tryptophan is highest at noon, fats and cholesterol peak at 6 pm, melatonin, the sleep hormone, at 2 am, and cortisol and iron are maximized at 6 am (Italianer 2020). 

As such, human milk can be called timed nutrition (chrononutrition), telling the baby which time of day it is, helping the baby establish body (circadian) rhythms and sleep patterns, and synchronizing the mother’s and baby’s body rhythms. Chronic disruption of these cycles can have many negative health effects (Kecklund 2016).

E) Differences between mothers

A mother’s diet can affect her breast milk and changes can be found in its:

The mother’s genetic make-up will influence the type of HMOs and the makeup of nucleic acids (microRNA) in her milk. 

The types of infections she has had will determine the immune protection that she can provide to her baby. 

A mother’s weight status, before, during and after pregnancy can cause small differences in hormone, protein, and fat levels and immunological properties (Amaral 2019; Bzikowska-Jura 2020; Erliana 2019; Leghi 2020; Sadr Dadres 2019). These differences may have an impact on the baby’s future weight and development (de la Garza Puentes 2019; Yu 2018).

Some, but not all studies have shown that mothers have slight differences in their milk based on the sex of their baby (Barreiro 2020; Bzikowska-Jura 2020; Hosseini 2020; Mangel 2020).

F) Changes during illness of the baby or the mother

The breast monitors the baby’s health. If the baby develops an infection, the milk is adjusted to increase the number of white blood cells and other disease-fighting components to minimize its severity (Breakey 2015; Riskin 2012). This response is stronger when mothers are exclusively breastfeeding (Hassiotou  2013).

Illness in the mother will also trigger an increase in the number of breast milk immune cells to protect the baby.


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