What is the physiology of breast milk production and secretion, and what factors affect it?

Physiology of Breast Milk Production and Secretion

Breast milk production is initiated by the withdrawal of progesterone after placental delivery, maintained by prolactin secretion from the pituitary gland in response to infant suckling, and ejected through oxytocin-mediated contraction of myoepithelial cells. 1, 2, 3

Hormonal Control of Lactation

Initiation Phase (Lactogenesis)

• Insulin and glucocorticoids (particularly hydrocortisone) are essential for lactation initiation, working alongside prolactin as the principal lactogenic hormone 1
• Milk synthesis begins during the second half of pregnancy, supported by prolactin and cortisol, which directly act on enzyme activities and differentiation processes of alveolar cells 4
• The sudden surge in milk secretion after delivery results from the rapid decline in progesterone levels following placental expulsion 4, 2
• Mothers typically experience copious milk production (lactogenesis II) by the third to fourth day after delivery 5

Maintenance Phase (Galactopoiesis)

• Frequent and intensive suckling is the critical factor maintaining postpartum lactation 4, 3
• Suckling stimulates both oxytocin release (for milk ejection) and prolactin secretion (for continued milk production) 4, 3
• Oxytocin is released in a pulsatile pattern early postpartum (5 pulses per 10 minutes), which coalesces into a more sustained rise as lactation progresses 3
• The number of oxytocin pulses during early breastfeeding correlates with greater milk yield and longer lactation duration 3

Milk Ejection Reflex

• Suckling triggers a neuroendocrine reflex that releases oxytocin from the posterior pituitary 4
• Oxytocin induces contraction of myoepithelial cells surrounding the alveoli, increasing intramammary pressure and expelling milk 4, 6
• This oxytocin release is immediate and short-lasting (approximately 20 minutes) 3

Stages of Milk Composition

Colostrum (Days 1-6)

• Contains high concentrations of immunoglobulins (particularly secretory IgA at 88-90% of total immunoglobulins) and proteins that establish the infant's immune system 7, 1
• Secretory IgA is highest in colostrum and decreases in mature milk 7
• Lactoferrin, comprising approximately 25% of total protein content, is most concentrated in colostrum 7

Transitional Milk (Days 7-14)

• Immunoglobulin and protein concentrations decrease as colostrum transitions to mature milk 7, 1

Mature Milk (After 2-3 Weeks)

• Exhibits less compositional variability but undergoes subtle biologically relevant changes throughout lactation 7, 1
• Average energy content is 0.62 kcal/g (2.6 kJ/g), with fat being the main energy source 7, 5
• Hind milk (end of feeding) contains 2-3 times more fat than fore milk (beginning of feeding) 7, 1

Factors Affecting Milk Production and Composition

Temporal Variations

Diurnal Rhythm:

• Glucocorticoids (cortisol and cortisone) follow maternal hypothalamic-pituitary-adrenal axis activity, peaking in the morning and reaching lowest levels in the evening 7
• Melatonin concentrations are higher in milk during nighttime 7
• Day and evening milk samples are more energy-dense than night or morning samples 7, 1

Lactation Stage:

• Milk cortisol increases during the first three months postpartum 7
• Leptin declines across early lactation days 7
• Ghrelin increases during lactation 7
• Secretory IgA concentrations remain stable within individual mothers from day 6 to at least day 90 postpartum 7

Maternal Factors

Parity:

• Multiparous women have higher oxytocin levels compared to primiparous women 3

Stress:

• Maternal stress reduces the number of oxytocin pulses during breastfeeding 3
• Breastfeeding-induced oxytocin release is associated with lowered ACTH and cortisol (stress hormones), reduced anxiety, and enhanced sociability 3

Medical Interventions:

• Emergency cesarean section reduces oxytocin and prolactin release in response to breastfeeding and impairs maternal psychological adaptations 3
• Epidural analgesia reduces prolactin release and maternal mental adaptation 3
• Synthetic oxytocin infusions can restore negative effects induced by cesarean section and epidural analgesia 3

Nutritional Status:

• A breastfeeding woman requires approximately 500 more kcal/day compared to a non-pregnant woman 7
• Total milk volume produced averages 780 mL/day (range 450-1200 mL/day) 7

Infant Factors

Suckling Frequency:

• Feeding 8-12 times per 24 hours (approximately every 2-3 hours) is optimal for maintaining lactation 5
• Frequent feeding (9-10 times/day) is associated with lower bilirubin concentrations and decreased newborn weight loss 5
• Feeding fewer than 7 times per day is associated with inadequate milk transfer 5

Mechanical Stimulation:

• Mechanical breast pumping (but not bottle-feeding) is associated with oxytocin and prolactin release and decreased maternal stress levels 3

Transport Mechanisms

• Five general pathways exist for transport of proteins, lipids, ions, nutrients, and water into milk 6
• Four pathways are transcellular (crossing at least two membrane barriers); one is paracellular (allowing direct exchange between interstitial fluid and milk) 6
• These transport processes are regulated by hormonal, developmental, and physiological factors 6

Common Pitfalls

• Avoid unnecessary supplementation: Breast milk supplements should only be used after optimizing breastfeeding technique and frequency, or when medically necessary 5
• Recognize stress impact: Maternal stress can significantly impair oxytocin release and milk production; addressing psychological factors is essential 3
• Monitor birth interventions: Be aware that cesarean section and epidural analgesia can negatively affect lactation initiation and may require additional support 3
• Understand individual variability: There is substantial individual variability in milk hormone concentrations across lactation, requiring individualized assessment 7