How does breast milk form?

Energy estimates for breast milk range from 650 to 700 kcal/L. It is composed of:

Water

Water accounts for 88% of breast milk, making it particularly thirst-quenching. It is therefore not necessary to offer additional bottles of water between on-demand feeds.
 

Nutrients

0.9% to 1.2% protein

Breast milk contains free amino acids, as well as amino acids bound in the form of proteins or enzymes.

Among the proteins, on average 70% are soluble whey proteins and 30% are insoluble proteins, namely caseins. This ratio fluctuates over the course of lactation, reaching 50/50 at the end of lactation [1]. 

- Lactalbumin

The main nutritional protein in breast milk is lactalbumin (0.32%), which enables the production of lactose. 

- Immunoglobulins

Immunoglobulins (0.15%), such as immunoglobulin A, act on immune defences by inhibiting the binding of pathogens, for example, which helps protect the baby whose digestive mucosa is immature during the first 4 months.

- Free amino acids

In terms of free amino acids, which represent 5 to 10% of the total amino acid content, breast milk has high levels of glutathione (an antioxidant and selenium transporter) and taurine. Taurine is a neurotransmitter but is also present in the structure of bile acids. It is involved in brain development and bile conjugation [2].

Did you know?

Cow's milk contains more protein and has an inverted ratio of soluble to insoluble proteins. The caseins in breast milk differ from those in cow's milk; some have bifidogenic effects (protective at the intestinal level) and they are also smaller and more digestible for the infant [3].

3.2% to 3.6% fat

Lipids are the primary energy source in breast milk, providing 50% of total calories. Lipids are organised into microscopic globules, which facilitates their digestion and absorption. 
 

Breast milk is rich in polyunsaturated fatty acids, containing three times more than cow's milk, which is rich in saturated fatty acids. 

Among these fatty acids, it contains arachidonic acid (0.5%) and docosahexaenoic acid (DHA, 0.2–1.2%), which are derived from essential fatty acids: linoleic acid (10%) and alpha-linolenic acid (1%). These are major constituents of neuronal cell membranes. 

Newborns have limited capacity to synthesise DHA from alpha-linolenic acid, which is why having a dietary source is important. Arachidonic acid is essential for growth, whilst DHA is essential for the development of the central nervous system and the retina during the prenatal and early postnatal period.
 

Did you know?

Breast milk is richer in cholesterol than cow's milk. Cholesterol plays important roles in membrane structure, as a hormonal precursor and in brain development [4].

6.7% to 7.8% carbohydrates
 

- Sugar molecules

With 200 sugar molecules, breast milk is the most complex mammalian milk, given that cow's milk contains only between 30 and 50 [5]. These sugars serve a nutritional purpose (with lactose in particular) but not only that, as oligosaccharides have health effects by acting, for example, on gastrointestinal function. 

- Lactose

5.7% for lactose. It is the main sugar in breast milk, providing 40% of calories. This sugar promotes the absorption of calcium and iron and has a beneficial effect on intestinal bacteria. Breast milk contains more lactose than cow's milk (4.5%).

- Oligosaccharides

1.3% for oligosaccharides. Breast milk contains nearly 130 different ones, whereas cow's milk contains very few. They are beneficial for bacteria in the colon [6].   

0.2% mineral salts

The mineral content of breast milk is relatively low but sufficient to meet the infant's needs. It contains, for example, 330 mg/L of calcium, 150 mg/L of sodium, 550 mg/L of potassium, 430 mg/L of chlorine and 0.5 mg/L of iron [7]. 

Iron is present in a highly bioavailable form and will be absorbed five times more effectively by the infant than from cow's milk or iron-fortified infant formula [8]. The mineral content of breast milk is optimal for your baby, even if it is lower than that in cow's milk, as the minerals will be better absorbed.

Vitamins 

The vitamin composition of breast milk is strongly influenced by nutritional intake for the majority of vitamins, as we will see later!

Bioactive components
 

These components are diverse; for example, there are numerous growth factors in human milk that have very varied effects on the intestinal tract, vascularisation, the nervous system and the endocrine system.
 

The main bioactive factors in human milk are [9]: 

- Cells, such as stem cells, whose function is regeneration and repair.

- Immunoglobulins, such as IgA, which inhibit the binding of pathogens.

- Antibodies (their concentration changes over time and decreases by more than 90% after one month, which may be explained by the fact that the baby is developing its own immune system) [10]

- Cytokines, involved in inflammatory processes.

- Chemokines, which act on the immune system.

- Growth factors. For example, a growth factor involved in tissue repair

- Hormones 

- Antimicrobials, including lactoferrin, which binds iron and has antimicrobial and antioxidant properties.

- Mucins, derived from the maternal plasma membrane, which block viral and bacterial infections. 

- Oligosaccharides, the best known being Human Milk Oligosaccharides: these are prebiotics that stimulate beneficial colonisation and reduce colonisation by pathogens; they are also involved in reducing inflammation.

Where do the nutrients in breast milk come from?

The origin of nutrients in breast milk is varied.

They can come from the maternal bloodstream following the digestion of food into nutrients and be filtered at the level of the mammary gland — which is the case for water, mineral salts and vitamins, fatty acids, glycerol, and amino acids. The time it takes for molecules from food to reach the composition of breast milk varies and depends on many factors: digestion speed, foods eaten together, and so on.

Other constituents can be synthesised directly by the gland itself, such as proteins, lipids, and lactose.

The nutritional composition of your milk does not always depend on your diet — it depends on the nutrients. Whatever the case, an postpartum nutrition optimal is essential to meet your needs and help you recover from pregnancy and childbirth.

The stages of breast milk synthesis

The synthesis of breast milk is a complex mechanism that takes place in several stages [11].

Towards the end of pregnancy, the cells in the mammary gland differentiate to become secretory cells in order to enable milk production. The secretory products accumulate in the secretory glands (known as acini). The cells that produce milk are grouped into alveoli, and milk accumulates in the centre (what is known as the "lumen"). 

Colostrum: the golden milk of the early days

The first fluid produced by mothers after birth is colostrum. It is a thick, salty, yellow-coloured liquid with a strong smell and a bitter taste. 
It is rich in immunological components such as immunoglobulin A, lactoferrin and leucocytes, as well as developmental factors.

Colostrum also contains relatively low concentrations of lactose, but is rich in proteins, fats and vitamin A.

Important: Colostrum quantities are always sufficient and its composition meets the needs of the newborn at birth. The stomach of newborns is the size of a hazelnut at birth (20–25 ml) and the small quantities of colostrum are perfectly suited to your baby's needs in the first few days.

Transitional milk

Transitional milk marks the onset of secretory activation. It shares some characteristics with colostrum but is more concentrated in lactose and better suited to the nutritional needs of the rapidly growing infant. This transition varies from one woman to another, but generally occurs between five days and two weeks after birth, after which the milk is considered "mature".

Mature milk

Between four and six weeks after birth, human milk is considered fully mature. Its composition remains relatively consistent, although subtle changes occur throughout lactation [14]. 

The nutrient composition differs between preterm milk and term milk.

The protein content of milk from mothers who give birth prematurely is significantly higher than that of mothers who give birth at term, most likely to meet the needs of the premature baby. Protein levels in human milk decline during the first 4 to 6 weeks of life or beyond, regardless of the timing of delivery [15]. 

Preterm milk also tends to be higher in fat [16].

Preterm milk also contains significantly higher concentrations of sodium and chloride, and lower concentrations of lactose [17]. Calcium is significantly lower in preterm milk than in term milk and does not appear to increase over time, whereas copper and zinc levels are both higher in preterm milk than in term milk and decrease with the duration of lactation [18].

Hindmilk, defined as the last milk of a feed, can contain two to three times the fat concentration of the milk found in foremilk, defined as the first milk of a feed [19]. During each breastfeeding session, the milk expressed first is thinner and contains more lactose, which satisfies the baby's thirst, while the hindmilk is creamier and contains more fat to meet the baby's needs [20].

Furthermore, one study found that the fat content of breast milk was significantly lower during night and morning feeds compared to afternoon or evening feeds [21]. 

Important: these variations during breastfeeding or throughout the day should not concern you. Your baby will always receive the nutrients they need each day.

Breast milk contains all the nutrients your baby needs, as well as bioactive components including growth factors and antibodies. The composition of breast milk varies according to the stage of the post-partum period, beginning with colostrum, and also changes throughout the day and in line with your diet. 

No need to worry about the composition of your milk — it will always be tailored to your baby's needs! To top up on nutrients, our post-partum food supplement, 100% compatible with breastfeeding, will help prevent deficiencies and boost your energy levels.

And don't forget, mama: our bodies are magical! Your body knows how to produce enough milk for your baby — don't go searching for the best breastfeeding food supplement, your body knows what to do.