Components of breast milk (I)

Breastfeeding is the best way to provide the baby with all the nutrients adapted to its physiological conditions and the necessary immune protection during the first months of life; in addition, the contact with the mother transmits confidence, security and love.

Breast milk has the capacity to change both in quantity and composition according to the infant's needs. For example, from the fourth postpartum day, there is an abrupt increase in milk production known as "milk surge", varying from the initial composition of "colostrum" milk to an intermediate milk "transition milk" that becomes "mature milk" around the fifteenth postpartum day.

The mature milk also varies in composition throughout the day and during the same feeding:

• Throughout the day: during the day, especially mid-morning, the fat content of milk is higher than during the night; in addition, night feedings contain endorphins, neurotransmitter substances that help in falling asleep.
• During the same feeding: At the beginning of the feeding, the milk is lighter, helping to quench thirst and prepare the digestive system; afterwards, the milk is denser and richer in fat, making it ideal for quenching the baby's appetite. Therefore, it is recommended that the baby suckle on the first breast as much as he/she wants until he/she lets go, so he/she will drink the most nutritious milk. Then the other breast will be offered.

During the first six months, the mother produces, on average, between 700 and 900 milliliters of milk per day; and from the sixth month on, about 600 milliliters per day.

The components of breast milk can be grouped into the following four groups:

• Macronutrients: Proteins, lipids and carbohydrates.
• Micronutrients: Vitamins and trace elements.
• Bioactive components: Hormones, antimicrobials, growth factors, etcetera.
• Human milk microbiota:

Carbohydrates present in breast milk.

Carbohydrates are a source of energy for immediate use by cells. The most abundant carbohydrate in breast milk is lactose, which provides about 40% of the total calories required by the baby for normal growth and development, promotes calcium absorption and provides sufficient osmotic force for the formation of milk fluid.

Oligosaccharides are also carbohydrates contained in breast milk, constituting its third major component after lactose and fat. They are resistant to digestion and play a role similar to that of soluble fiber, preventing the development of pathogenic microorganisms.

Lipids or fats present in breast milk

Fats are the main source of energy for the baby, providing about 50% of the total calories required; they also provide the body with essential fatty acids, transport fat-soluble vitamins and facilitate their absorption.

About 98% of the fat contained in breast milk is in the form of triglycerides. Saturated fatty acids account for about 44% and unsaturated fatty acids for about 56%.

Breast milk contains long-chain polyunsaturated fatty acids - not present in cow's milk - which play an important role in the growth and maturation of the baby's nervous system and brain. Their content in breast milk depends on the mother's diet; therefore, it is recommended to consume at least two servings of fatty fish such as salmon, tuna, trout, etc., per week.

Proteins present in breast milk

There are two nitrogen fractions in breast milk: protein nitrogen and non-protein nitrogen.

Protein nitrogen: Breast milk contains about 400 different types of proteins, 70% of which are found in the serum (seroproteins), and the rest - 30% - form aggregates called casein micelles; this balance is essential to promote digestion and proper development of the baby.

Proteins concentrated in serum (seroproteins): Within this group, alpha-lactoalbumin, lactoferrin, immunoglobulins, lysozyme and serum albumin stand out. Let's see the main functions of three of them:

• Alpha-lactalbumin: It is the main serum protein. It participates in the formation of lactose and is the source of numerous amino acids.

• Lactoferrin: It plays a fundamental role in protecting the baby against gastrointestinal infections. Its concentration is ten times higher in breast milk than in cow's milk.

• Immunoglobulins (Ig): Breast milk -especially colostrum- is rich in immunoglobulins, antibodies capable of binding to viruses and bacteria, preventing them from penetrating the mucous membranes; protecting the infant while its own immune system matures.

Casein micelles: These proteins play roles related to the absorption of calcium ions, antihypertensive and antithrombotic actions.

Non-protein nitrogen: Among the components of the non-protein nitrogenous fraction are creatine, carnitine, free amino acids, nucleotides, etcetera.