Breast Milk Production and Composition

Hormonal Control and Milk Composition

The American Academy of Pediatrics recommends that breastfeeding women require approximately 500 more kcal/day compared to a non-pregnant woman, with total milk volume produced averaging 780 mL/day (range 450-1200 mL/day) 1
Colostrum contains high concentrations of immunoglobulins (particularly secretory IgA at 88-90% of total immunoglobulins) and proteins that establish the infant's immune system, with secretory IgA being highest in colostrum and decreasing in mature milk 2
Mature milk exhibits less compositional variability but undergoes subtle biologically relevant changes throughout lactation, with an average energy content of 0.62 kcal/g (2.6 kJ/g), and fat being the main energy source 1
Hind milk (end of feeding) contains 2-3 times more fat than fore milk (beginning of feeding) 1

Diurnal rhythm affects milk composition, with glucocorticoids (cortisol and cortisone) peaking in the morning and reaching lowest levels in the evening, and melatonin concentrations being higher in milk during nighttime 2, 3
Milk cortisol increases during the first three months postpartum, while leptin declines across early lactation days, and ghrelin increases during lactation 2, 3
Secretory IgA concentrations remain stable within individual mothers from day 6 to at least day 90 postpartum 2

There is substantial individual variability in milk hormone concentrations across lactation, requiring individualized assessment 2

Insulin and glucocorticoids, such as hydrocortisone, are essential for lactation to begin, in addition to prolactin, which is the principal lactogenic hormone, according to the American Academy of Nutrition and Dietetics 4
Glucocorticoids support milk synthesis and regulate stress response, energy regulation, and have anti-inflammatory effects, as stated by the Academy of Nutrition and Dietetics 4

Transitional milk follows colostrum from days 7-14 postpartum, as recommended by the American College of Nutrition 5
Mature milk is established around 2-3 weeks postpartum, with less compositional variability, but still changes throughout the day and during feedings, according to the Academy of Nutrition and Dietetics 5, 6

Colostrum contains high concentrations of immunoglobulins and proteins that help establish the infant's immune system, as stated by the American Society for Nutrition 5
Hind milk (end of feeding) contains 2-3 times more fat than fore milk (beginning of feeding), as reported by the Academy of Nutrition and Dietetics 5, 6
Milk composition varies with time of day, with day and evening samples being more energy-dense than night or morning samples, according to the American College of Nutrition 5