Home > Breastmilk composition

15/08/2024

Table of contents

A bio-dynamic liquid

Breast milk (BM) is a physiological product adapted to a baby’s needs. It is not only a resource of nutrients, it also contains valuable bioactive compounds (6 times more elements than other mammalian milks).

There are numerous biological effects that interweave and synergise in a complex multifactorial system, involving hormones and enzymes from the mother’s plasma and those from the baby’s digestive tract.

It is the best food for infants. It satisfies all an infant’s nutritional needs up to the age of six months, 1/2 of needs between 6 and 12 months, and more than 1/3 between 12 and 24 months.

‘Breast milk is the ideal food for infants. It is safe, clean and contains antibodies that protect them from many common childhood illnesses.
Breast milk provides all the energy and nutrients that infants need for the first six months of life, and continues to cover half or more of their nutritional needs during the second half of life and up to a third during the second year. WHO https://www.who.int/health-topics/breastfeeding

The benefits of breastfeeding are dose-dependent. The longer the child is breastfed, the greater the health benefits for both mother and child.

Prebiotic and probiotic functions

Considered a prebiotic and probiotic, breast milk helps babies adapt to their environment by colonising their digestive tract with bacteria.

Through a mechanism that is still poorly understood, the mother transmits her microbiota via the mammary glands.
Her diet, the drugs she takes (antibiotics in particular) and her environment play a major role in the composition of her own microbiota.
The variability in the bacterial composition of the intestinal microbiota of a breastfed baby can be explained by the concentration of a polypeptide cytokine or TGT (transforming growth factor).

Babies born by caesarean section do not benefit from colonisation by the maternal flora as they pass through the genital tract. Breastfeeding allows them to build up part of their flora through the microbiome of their mother’s milk and the microbiome of her skin through skin-to-skin contact.

For neonatologists, LM is a medicine that promotes the harmonious growth of premature babies and prevents certain complications linked to their immaturity, in particular necrotising enterocolitis (protective effect of the protein neuregulin-4 on the intestines).

Preterm milk is richer in total protein, IgA and lysozyme during the first 2 weeks postpartum.
During the entire breastfeeding period, the composition of LM varies and adapts to the infant’s needs.

Components

Components of breast milk

Water

Water makes up 87% of breast milk. This quantity of water ensures optimum hydration for the baby. Additional intake is not necessary, even in hot weather or fever, as long as the baby can suckle without restriction.

Carbohydrates

(80 g/L, 4 Kcal/g) of which 85% lactose Lactose is an important source of energy. It promotes calcium absorption, which is thought to help prevent rickets. It promotes a specific intestinal flora of bifidobacteria.
It is split by the Lactase enzyme into glucose (for physical activity) and galactose (for brain development).

Oligosaccharides (Human Milk Oligosaccharides) are complex sugars specific to human milk.

They act like probiotics, encouraging the establishment and proliferation of specific intestinal flora and inhibiting the development of pathogenic germs.

Lipids

Lipids account for 38 g/L (9 Kcal/g on average). They are the main source of energy (around 50% of total calories).

The baby’s gestational age influences the type and quantity of fat in breast milk.
The fatty acid content depends on the quality of the fats in the mother’s diet.

The better digestibility of fats in breast milk is due to the presence of a bile acid-dependent lipase in the newborn, which compensates for the inadequacy of pancreatic lipases in the duodenum.

Essential fatty acids are necessary for the cerebral and neurosensory development of the baby.
They have the function of converting into longer-chain polyunsaturated fatty acids: omega 6, omega 3 and DHA (docosahexaenoic acid), components which have a proven role in the cerebral and retinal maturation processes.

Cholesterol: Mother’s milk contains a lot of cholesterol (2.6 to 3.9 mM/L). It is essential for cell growth, replication and maintenance.

Breast-fed children have a high intake of cholesterol compared with adults and a higher level of plasma cholesterol than children receiving infant formula.

This high cholesterol intake could have a lasting effect on the child’s ability to metabolise cholesterol, and be protective in the long term against adult cardiovascular disease.

Protein

The low protein content of breast milk (around 8 to 12g/L, 4 Kcal/g) is adapted to the baby’s immature kidneys and liver (4 times less protein than in cow’s milk).
These proteins break down more completely and are better absorbed by the infant’s digestive system than proteins from PPN.

The proteases in breast milk actively break down the proteins in the mammary gland, initiating the release of functional peptides – over 1,100 unique peptides derived from this hydrolysis.
The child thus receives partially predigested proteins and a large number of potential bioactive peptides.

Caseins (40% of proteins)
Caseins make up only 40% of the proteins in breast milk, compared with 80% for cow’s milk, and form much smaller micelles than those in cow’s milk.
When casein Beta is broken down, it releases peptides with biological activity (opioid or anti-infectious activity).

Caseins promote an optimal calcium/phosphorus ratio for their absorption.
The high percentage of soluble proteins and the small casein micelles promote faster gastric emptying than with formula.

This fine coagulation allows gastric emptying to take place in 60 to 90 minutes, unlike the large-block coagulation of PPN, which allows gastric emptying to take place in 3 hours.

Whey proteins (60% of proteins)

  • alpha-lactalbumin (3.5 g/L), which produces the lactose needed to build the human brain
  • lactotransferrin or lactoferrin (1.5 g/L) appropriates the serum iron required for the development of certain bacteria. It is thought to be able to target pathogenic bacteria by forming capsules that encircle the bacteria and pierce their cell membrane to engulf and kill them.
  • Lysosyme is a glycoprotein whose essential role is bacterial lysis. Its lactate level increases over time.
  • Immunoglobulins, in particular secretory-type IgA (IgAs, 0.5 to 1 g/L)
  • Growth factors such as Insulin-Like Growth Factor (IGF1), Transorming Growth Factor (TGF), leukocyte growth factors (g-CSF) and Epidermal Growth Factor (EGF), which has a trophic action on the gastric and intestinal mucosa.
  • immunocompetent cells: Macrophages, polymorphs, lymphocytes
  • Immunoglobulins: The baby receives Immunoglobulin A directed against pathogenic bacteria in the mother’s environment.
  • Neuregulin-4 (NRG4), a protein found only in breast milk, provides protection against the intestinal destruction involved in ulcerative colitis in premature babies.
  • Essential amino acids: Mother’s milk is rich in taurine and cystine. It is 5 to 10 times richer in free amino acids than formula, particularly glutamate/glutamine, which are thought to play a trophic role in the intestine.

Enzymes

Proteases, lipases and amylases help digest the proteins, fats and starches ingested by the child.

Mineral salts

The mineral salt content (2.50 g/L) of breast milk limits the renal osmolar load to fairly low values (93 mOsm/L).
This low renal osmolar load provides safety in the event of excessive water loss through perspiration or diarrhoea, by helping to maintain the hydromineral balance.
The various trace elements, such as iron and zinc, are more bioavailable due to the ligands present in breast milk, which facilitate their absorption.

  • Sodium protects against dehydration.
  • Calcium: breast milk contains 4x less calcium than cow’s milk, but it is better absorbed thanks to the calcium-phosphorus ratio and the high lactose content.
  • Iron, bound to lactoferrin, is absorbed by the high lactose and vitamin C content. If breast-feeding is exclusive, there is no need for iron supplements during the first 6 months of the child’s life.
  • Iodine and fluoride: Unless the mother is deficient, supplementation is not recommended for breastfed infants. In Switzerland, dietary salt is fortified with iodine (25mg/kg of salt) and fluoride (250 mg/kg of salt).

Vitamins

The level of vitamins in breast milk is influenced by the mother’s diet

  • ADEK fat-soluble vitamins

Vitamin A is present in sufficient quantities except in undernourished mothers.

The level of vitamin D in breast milk is affected by the mother’s status for this vitamin and an increase in intake increases this level: exposure to sunlight, adequate supplementation (recommendation of 400 IU/d).

The level of vitamin E (an antioxidant) in mother’s milk is highest in colostrum and then decreases as lacation progresses.

Breast milk contains little vitamin K.
The Swiss Society of Neonatology recommends the administration of 3 doses of vitamin K (Konakion MM 2 mg for example) 4 hours after birth, on the 4th day of life and at 4 weeks of age.

  • Water-soluble vitamins

The concentration of vitamin C in breast milk correlates positively with maternal intake of vitamin C from food, in the case of a diet without supplements.
Vitamin C from food is much better absorbed and passed into breast milk than vitamin C from supplements.

Group B vitamins should be supplemented if the mother is vegan (eats no animal products).
Hormones

The interaction between insulin-like growth factor and cyclic glycine-proline in breast milk is similar to that reported in other mammals. It may play an important role in defining the growth trajectories of infants beyond the first year of life.
Milk with higher IGF-1 levels was associated with higher weight at 13 months (p = 0.004), but lower weight at 3 years (p = 0.011) and 5 years (p = 0.049).

Breastmilk contains a hormone, cholecystokinin, which promotes satiety and helps babies fall asleep at the end of a feed.

Recent data

Stem cells

Mother’s milk is a rich source of multipotent mesenchymal stem cells.
Breast milk stem cells survive in the newborn’s intestine and pass through the intestinal wall into the baby’s bloodstream.
They act on tissue repair and regeneration by various mechanisms ranging from cell replacement to modulation of immune and inflammatory responses.

HAMLET

Alphalactalbumin contained in breast milk could be an anti-cancer substance. HAMLET (Human Alphalactalbumin Made Lethal to Tumor cells) is formed when casein precipitates due to the low PH of the stomach. This leads to a change in the spatial structure of Alphalactalbumin and its binding to a fatty acid. Its efficacy has been demonstrated both in vitro and in animal models, and on various types of cancer. What’s more, it has no impact on healthy cells and no toxicity. (Ho et al. 2017 https://www.sciencedirect.com/science/article/abs/pii/S0006291X1631779X)

TRAIL

(Tumor Related Apotosis Inducing Lingand) is a cytokine that induces apoptosis in tumour cells.
Present in colostrum at a concentration 400 times higher than in human blood, and in mature milk at a concentration 100 times higher than in blood, it could play a significant role in mediating the anti-cancer activity of breast milk.


MicroRNA

Mother’s milk is rich in microRNAs, which exert specific immune protection and development functions in infant tissues.
The composition of microRNA is modified at the end of breastfeeding. This may reflect the remodelling of the mammary gland in response to infant feeding patterns, which generally change at the time of dietary diversification, suggesting an adaptation to the child’s needs.

See the page devoted to Epigenetics

Myo-inositol

According to a study of 2023, the effects of micronutrients on brain connectivity are incompletely understood. Analyzing human milk samples across global populations, we identified the carbocyclic sugar myo-inositol as a component that promotes brain development. We determined that it is most abundant in human milk during early lactation when neuronal connections rapidly form in the infant brain. Myo-inositol promoted synapse abundance in human excitatory neurons as well as cultured rat neurons and acted in a dose-dependent manner. Mechanistically, myo-inositol enhanced the ability of neurons to respond to transsynaptic interactions that induce synapses. Effects of myo-inositol in the developing brain were tested in mice, and its dietary supplementation enlarged excitatory postsynaptic sites in the maturing cortex. Utilizing an organotypic slice culture system, we additionally determined that myo-inositol is bioactive in mature brain tissue, and treatment of organotypic slices with this carbocyclic sugar increased the number and size of postsynaptic specializations and excitatory synapse density. This study advances our understanding of the impact of human milk on the infant brain and identifies myo-inositol as a breast milk component that promotes the formation of neuronal connections. Paquette AF et al. The human milk component myo-inositol promotes neuronal connectivity. PNAS 2023 ; 120(30) : e2221413120. Mots-clés : lait humain, myo-inositol, développement cérébral, formation des synapses. https://www.pnas.org/doi/10.1073/pnas.2221413120

Variations in the composition and appearance of breast milk

Over the entire breastfeeding period, the composition of breast milk varies.

Colostrum


Colostrum, the first milk at birth, is a veritable nutritional concentrate, particularly well-suited to the needs of newborn babies.

Colostrum is yellow-orange in color and thicker than mature milk.
It is particularly rich in protein (42 g/L) and antibodies.
It also contains minerals, vitamins and enzymes.

Colostrum’s small volumes enable the baby to learn to suck and swallow at the breast with progressive quantities, gently kick-starting his digestive and enzymatic systems and sparing his still immature liver and kidneys.
The salt concentration allows the baby to adapt his water percentage:
the fetus is made up of 75% water; through breathing, the newborn adjusts within a few days to its new aerial environment and stabilizes at 65% water.

Colostrum has a laxative function: it encourages the evacuation of meconium, the first stool contained in the newborn’s intestines.

The best immunity observed in breast-fed infants is thus attributed to colostrum.
The types of antibodies found in colostrum depend on the infectious agents to which the mother has been exposed during her life, either naturally or through vaccination.
They will help the newborn to fight off any infections and build a strong immune system.
In the first days of a baby’s life, the permeability of its intestinal cells still allows these large molecules to pass through; later, these antibodies will be destroyed during the digestion process.

When the baby is unable to breastfeed at birth, the mother can collect Colostrum and give it to her baby.

From Colostrum to mature milk

  • In the first few weeks, the volume of milk increases, as does the lactose content, while the oligosaccharide content decreases.
  • The milk lightens during the transition phase and becomes whiter as it matures.
  • Mineral salts are halved.
  • The proportion of proteins is divided by ten. The free amino acid content falls and then rises slightly.
  • At the same time, the proportion of lipids doubles.
  • Protein: 4-5 times more in colostrum
  • Lipids: 3 times less in colostrum
  • Lactosis: 1,5 times more
  • Other sugars: 2 times more
  • Antibodies: 4-5 times more
  • Enzymes: 2-4 times more
  • Mineral salts: 1,5 times more
  • The concentration of fats is the most important variable in breast milk, and fats contribute just over half the energy in mature breastmilk.
  • The concentration of fats may increase as the feed progresses, in proportion to the emptying of the breast but this needs some more explanation about foremilk and hindmilk:

Foremilk – hindmilk, what is it about?

You may have read that the concentration of fats increases as the feed progresses, in proportion to the emptying of the breast. This needs to be nuanced. The La Leche League International website states:

“Foremilk is the milk available when your baby starts feeding, hindmilk is the milk your baby gets at the end of a feed. Foremilk is not necessarily low in fat: fat content of the milk that is removed varies according to how long the milk has been collecting in the ducts and how much of your breast is drained at the time.

As milk is made, fat sticks to the sides of the milk-making cells and the watery part of the milk moves down the ducts toward your nipple, where it mixes with any milk left there from the last feed. The longer the time between feeds, the more diluted the leftover milk becomes. This ‘watery’ milk has a higher lactose content and less fat than the milk stored in the milk-making cells higher up in your breast.

You can’t tell how much fat your baby has received from the length of a feed. Some babies take a full feed in five minutes while others take 40 minutes to get the same amount. As long as your baby is breastfeeding effectively, you can let them decide how long to feed for and they will get all the fat they need.” https://llli.org/breastfeeding-info/foremilk-and-hindmilk/

The storage capacity of the breast influences the extent of variations in fat concentration. In a mother with a large storage capacity, the start or end of a feed does not necessarily correspond to the start or end of drainage from the breast. It is preferable to let the child regulate its needs according to its appetite.

Milk from mothers of children over one year old

The results of a study by Sinkiewicz-Darol E et al (2021) suggest a role for human milk in adapting to the nutritional needs of newborns and older children. This could encourage the promotion of long-term breastfeeding, including tandem breastfeeding. Tandem Breastfeeding: A Descriptive Analysis of the Nutritional Value of Milk When Feeding a Younger and Older Child https://pubmed.ncbi.nlm.nih.gov/33478010/

Human milk after the second year of lactation contains significantly higher concentrations of protein, SIgA and IgG. The high concentration of immunoglobulins and proteins during prolonged lactation is a further argument in favour of breastfeeding, even after the introduction of solid foods, and should be one of the main objectives in protecting children’s health. It is therefore important to consider, when drawing up recommendations, that it is not just the number of feeds per day, but breastfeeding in general, that should be continued for as long as mother and baby wish, as a complement and support to the child’s maturing immune system. Czosnykowska-Łukacka MO et al.(2020) https://doi.org/10.3389/fped.2020.00428

Lactoferrin in breast milk. The concentration of Lf in human milk is related to the stage of lactation; colostrum contains over 5 g/L, which then decreases significantly to 2 to 3 g/L in mature milk. The milk of mothers who breastfeed for more than a year has a standard value, containing macronutrients in a composition similar to that of human milk at later stages. The aim of this study was to evaluate lactoferrin concentration during prolonged breastfeeding, from the first to the 48th month post-partum. The mean value of lactoferrin concentration was lowest in the 1-12 months lactation group (3.39 ± 1.43 g/L), increasing significantly in the 13-18 months group (5.55 ± 4.00 g/L; p < 0.006), and remaining at a comparable level in the 19-24 months and over 24 months groups (5.02 ± 2.97 and 4.90 ± 3.18 g/L, respectively). Czosnykowska-Łukacka MO et al.(2019) https://pubmed.ncbi.nlm.nih.gov/31581741/

The protein content of human milk is negatively associated with milk production volume, so that a decrease in milk volume predicts an increase in protein content. Although the nutritional content of human milk is variable, on average, the protein concentration of milk during the later stages of lactation is sensitive to decreases in milk production. Studies dating back several decades have shown an increase in the proportion of immunoglobulins, lactoferrin and serum albumin during weaning or prolonged breastfeeding. Verd S et al (2018) https://www.mdpi.com/2072-6643/10/8/1124

The aim of another study was to describe the longitudinal changes in macronutrient concentrations in human milk during prolonged breastfeeding by healthy mothers from the 1st to the 48th month. With regard to the macronutrient content of the milk of mothers who breastfed for more than 18 months, fats and proteins increased and carbohydrates decreased significantly, compared with the milk expressed by women who breastfed for up to 12 months. In addition, the concentration of fats, proteins and carbohydrates in breast milk over two years old, from the 24th to the 48th month, remained stable. Czosnykowska-Łukacka MO et al.(2018) https://pubmed.ncbi.nlm.nih.gov/30513944/

Milk banks have also taken an interest in this issue and studies have shown that breast milk retains all its properties after the first year of lactation. If the child suckles sufficiently, the quantity of milk remains constant. During the 2nd year, the average consumption of breastfed children is estimated at 448 ml.

More readings about long term breastfeeding

When the breastmilk is pumped

you can see that

  • The milk looks different at the beginning and end of the feed.
  • The milk may have different smells, depending on the mother’s diet.
  • Its colour may vary:
    – Some medicines and certain pigments (green) or food colourings can colour the milk (yellow, green, red)
    – Lesions on the nipple area, such as cracks, may turn the milk pink.
    – Rusty pipe syndrome’, the presence of blood in colostrum at the end of pregnancy or at the start of lactation, gives the milk a rusty or red colour. It is linked to activation of the mammary gland in preparation for lactation. This painless phenomenon usually disappears spontaneously 5 to 10 days after birth, and the milk poses no danger to the breastfed infant. associated with the rupture of the blood capillaries of the mammary gland, can colour the milk black-brown. Blood-stained colostrum : a rare phenomenon at an early lactation stage. Weszolek K et al. Children 2022 https://www.mdpi.com/2227-9067/9/2/213
  • After refrigeration or freezing, the constituents of mother’s milk separate, the cream floats to the top and the rest of the milk looks like water. Simply shake the container gently before giving the milk to the baby.

The difference between breastfeeding and ‘transferring breastmilk’

A distinction must therefore be made between

  • breast-feeding with skin-to-skin contact, interaction between mother and baby and the transfer of the mother’s antibodies and anti-infective agents through breastfeeding, and
  • feeding with breastmilk expressed via a device and then stored in one way or another (refrigerated, frozen, pasteurised, freeze-dried).

That said, breastmilk, in whatever form it takes (expressed raw, expressed pasteurised, individual donor milk or pooled milk from different donors, etc.), will always be superior to artificial milk (industrial infant formula or Commercial Milk Formula CMF), which runs the risk of being contaminated as soon as it is produced (see dedicated page) or by the preparation of formula and feeding bottles (follow the WHO Guidelines for Safe preparation, storage and handling).

Weaning

During weaning, the amount of sodium and protein increases, while the concentration of potassium and lactose gradually decreases.

Comparison of different milks

mature breastmilkformulacow’s milk
Protides: CaséineLactoserum1g 0.85g 0.7g1,5-1,7g  3,7g 2.8g 0.7g
Lipides : Acide linoléiquecholestérol3,5 g 350 mg 20 mg2,6-3,9 g 350-740 mg3,5 g 90 mg 13 mg  
Glucides totaux : LactoseOligosaccharides7,5 g 6.5 g 1,0 g6,7-9,5 g  4,5 g 90 mg traces
Minéraux totaux : SodiumFer210 mg 10 mg 0.1 mg250-500 mg900 mg 50 mg 0.03 mg

Comparison table between breastmilk and infant formula

The list of immunocompetent factors transmitted from mother to child via the act of breastfeeding is considerable in itself, but reveals only part of how breastfeeding works: it is not a simple inert assembly of ingredients, but a dynamic biological liquid resulting from a continuous and adaptive production during breastfeeding with mother-infant skin-to-skin contact.

In other words, the finely nuanced adaptation of breast milk is the result of an ongoing dialogue between the mother’s microbial environment and that of her baby. In addition to the many non-specific immunological factors transmitted, the mother provides targeted anti-infectious agents and immunological factors for her child.

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