Chapter 2 – The Ideal Food and Much More

We have long known, in broad terms, what human milk contains: proteins, sugars, fats, vitamins, and minerals. But only in recent years—thanks to advances in molecular analysis—has science begun to explore the diversity of its components and their importance for the child’s body.

The more time passes and the more it is studied, the more we understand how precious breast milk is for the baby’s development and future health. It is the species-specific food, ideal for the human infant, and it is produced each time tailored to the baby’s changing needs.

Between the first and third day of the baby’s life, the new mother produces just a few drops of yellow-colored liquid.

This is colostrum, also known as “liquid gold”, which contains less sugar and fat than mature milk but is extremely rich in highly digestible proteins, minerals, and vitamins, as well as phagocytes(“scavenger cells”), antibacterial, and antifungal agents. Its yellow, serum-like color is due to its high concentration of immunoglobulins (components of the immune system), nucleotides, minerals, and vitamins (especially A, D, K, and B12). Of the more than 200 different carbohydrates present in colostrum, only a small portion is digested and absorbed by the newborn (1). The rest nourishes the bacteria that have started to colonize the baby’s gut. By offering this first milk, the mother initiates the selection of her child’s intestinal flora, supported by the action of maternal antibodies passed through the milk and by proteins that slow down the growth of pathogenic microorganisms.

After a few days of colostrum, the milk “comes in”—the beginning of full breast milk production. The timing of this event can vary, but it usually occurs between the third and fourth day after birth and is marked by specific symptoms. The most common ones include breast engorgement, possible pain, warmth, or tingling inside the breast. These symptoms are usually relieved by the baby through breastfeeding.

Milk production is triggered by increased levels of prolactin, a hormone stimulated by the baby’s sucking, which allows milk to flow from the glandular tissue of the breast into the milk ducts.

From the 4th to the 10th day after birth, breast milk becomes transitional milk: abundant, slightly yellow, energy-dense, and high in fats and carbohydrates, though lower in minerals and proteins. This is followed by mature milk, rich in fats and carbohydrates and with a carefully balanced content of proteins and minerals, adjusted to meet the baby’s growing nutritional needs.

By one month after birth, the transition from colostrum to mature milk is complete—but the milk continues to adapt constantly to the baby’s needs. For instance, the milk taken at the start of a feed, when the breast is full, is more watery and sugar-rich. It quenches thirst and meets immediate energy needs. As the feeding continues, its composition changes, and the fat content increases. Interrupting the baby’s feed shortly after it begins and switching sides would prevent the baby from accessing the more nutritious portion of the milk (2). On hot days, the milk becomes more watery. In the evening and nighttime, it becomes richer in hormones like oxytocin, which promote relaxation and sleep.

Breast milk plays a critical role in supporting the baby’s immune system y providing the necessary defenses to fight off pathogens through antibodies (mainly immunoglobulin A – IgA). It also acts as an immunomodulator, enriching the baby’s gut microbiome.

The percentage and composition of fats also evolve over time. From two to six months, and even more so by one year, the baby grows, feedings become less frequent, and the energy required per meal increases. Contrary to the myth that breast milk “turns into water,” it does not lose its nutritional power—on the contrary, it becomes richer, aligning with the baby’s needs. Its composition includes a higher proportion of fatty acids, essential for the development of the central nervous system, vision, and neuro-motor functions. Among the proteins, those contributing to brain development also increase (3). Breastfeeding is a flawless system, perfectly refined through millennia of evolution.