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.
