Composition of Human Milk

Nutritional Composition of Human Milk

Human milk is a species-specific, biological fluid described as "white blood" because of its complex composition of living cells, nutrients, and bioactive factors. It is the gold standard for infant nutrition, providing optimum nutrients for growth and development while offering immunological protection.

General Characteristics

• Energy Density: Human milk provides approximately 65–67 kcal/100 mL,,.
• Osmolality: It has a low osmolality of approximately 7.9 mOsm/L (renal solute load ~79 mOsm/L), which ensures a gentle load on the infant's immature kidneys,,.
• Water Content: It provides adequate water for the infant; no additional water is required even in hot climates for exclusively breastfed infants,.

Proteins and Nitrogenous Compounds

Human milk protein content is lower than that of animal milks but is biochemically superior and easily digestible.

• Total Protein: The protein content is approximately 1.1–1.2 g/100 mL in mature milk,,.
• Whey:Casein Ratio:
  • Breast milk is whey-predominant. The ratio is typically 60:40 (Whey:Casein) in mature milk.
  • In early lactation, the ratio can be as high as 80:20.
  • Whey proteins form a soft, flocculent curd in the stomach, facilitating easy digestion and rapid gastric emptying.
• Specific Protein Components:
  • Alpha-lactalbumin: The major whey protein (about 40% of whey), rich in tryptophan.
  • Lactoferrin: Constitutes about 30% of whey proteins. It binds iron (bacteriostatic effect against E. coli) and enhances the absorption of iron and zinc,.
  • Secretory IgA (sIgA): Represents 15–20% of whey proteins; resistant to proteolysis and provides surface immunity.
  • Absence of Beta-lactoglobulin: Human milk lacks beta-lactoglobulin, the major allergen found in cow's milk,.
• Amino Acid Profile:
  • Taurine: High concentration; functions as a neurotransmitter and neuromodulator essential for brain and retina development.
  • Cysteine: High cysteine:methionine ratio. Cysteine is essential for preterms due to delayed maturation of cystathionase.
  • Aromatic Amino Acids: Low levels of phenylalanine and tyrosine, preventing amino acid toxicity in infants with immature enzyme systems.
  • Tryptophan: Rich source; precursor to serotonin and niacin.
• Non-Protein Nitrogen (NPN):
  • NPN accounts for a significant portion (~0.2 g/100 mL) of total nitrogen.
  • Includes urea, nucleotides, peptides, free amino acids, carnitine, and polyamines.
  • Nucleotides: Essential for DNA synthesis, immune function, and gut flora development,.

Lipids (Fats)

Fats are the main source of energy, providing approximately 50% of the total calories,.

• Total Fat: Approximately 3.5–3.8 g/100 mL,.
• Fatty Acid Profile:
  • Unsaturated Fats: Human milk is rich in unsaturated fats. The Polyunsaturated/Saturated (P/S) ratio is 1.2:1.
  • Essential Fatty Acids (EFA): Contains Linoleic acid (n-6) and Alpha-linolenic acid (n-3).
  • Long Chain Polyunsaturated Fatty Acids (LCPs): Rich in Arachidonic Acid (ARA) and Docosahexaenoic Acid (DHA), which are critical for brain myelination, retinal development, and membrane fluidity,.
  • Trans vs. Cis forms: Amino acids and fatty acids in breast milk are in the biologically active 'trans' form (Note: Source mentions amino acids in trans form, likely referring to structural configuration relevance in context of microwaving, while lipids are generally cis in nature biologically; however, source text explicitly states amino acids are mostly in 'trans' form).
• Digestibility:
  • Bile Salt Stimulated Lipase (BSSL): An enzyme present in human milk that activates in the intestine to hydrolyze fats, compensating for the infant's low pancreatic lipase,.
• Cholesterol: Higher concentration than in cow's milk, which may program cholesterol metabolism for later life.
• Carnitine: High content facilitates the transport of long-chain fatty acids into mitochondria for oxidation,,.

Carbohydrates

Human milk is the "sweetest milk" due to its high lactose content,.

• Lactose:
  • Concentration is approximately 7.0 g/100 mL,.
  • Provides glucose (energy) and galactose (essential for brain development/myelination).
  • Enhances calcium and magnesium absorption,.
• Oligosaccharides:
  • Constitute approximately 15% of total carbohydrates.
  • Galacto-oligosaccharides (GOS): Act as prebiotics (Bifidus factor) to promote the growth of Lactobacillus bifidus,.
  • Act as decoy receptors to prevent pathogen attachment to the gut mucosa.

Micronutrients: Vitamins

The vitamin content of breast milk is generally adequate for the infant, provided the mother's status is optimal, with exceptions for Vitamin D and K.

• Fat-Soluble Vitamins:
  • Vitamin A: ~53 µg/100 mL (1250 IU/L). Rich in beta-carotene,.
  • Vitamin D: Low content (~30–40 IU/L or <10 IU/100 mL), which is insufficient to meet the RDA (400 IU), necessitating supplementation,,.
  • Vitamin E: ~0.2 mg/100 mL; sufficient to prevent anemia.
  • Vitamin K: Low concentration (15 ng/100 mL or 0.3 µg/100 mL). Supplementation is required at birth to prevent Hemorrhagic Disease of the Newborn,,.
• Water-Soluble Vitamins:
  • Vitamin C: ~4.3 mg/100 mL. Adequate to prevent scurvy,.
  • B-Complex: Generally adequate. B12 may be low in strict vegetarian mothers,.

Micronutrients: Minerals and Trace Elements

Minerals in human milk have high bioavailability despite lower absolute concentrations compared to cow's milk.

• Calcium: ~33 mg/100 mL. The Calcium:Phosphorus ratio is >2:1, ensuring optimal absorption and preventing hypocalcemic tetany,,.
• Phosphorus: ~15 mg/100 mL.
• Iron: Low absolute value (0.05–0.15 mg/100 mL) but very high absorption (~50%) due to the presence of lactoferrin and Vitamin C,,.
• Sodium: Low concentration (16 mg or 0.7 mEq/100 mL), protecting the infant from hypernatremic dehydration,.
• Zinc: 0.53 mg/100 mL. Highly bioavailable due to binding ligands.
• Copper: ~0.04 mg/100 mL.

Bioactive and Immunological Factors

Human milk is a "living fluid" containing cells and bioactive molecules that provide passive immunity and modulate development.

• Cellular Components: Macrophages (predominant), T and B lymphocytes, and neutrophils.
• Enzymes:
  • Lysozyme: Lyses bacterial cell walls.
  • Peroxidases: Lactoperoxidase and leukocyte myeloperoxidase aid in killing bacteria,.
  • PAF-Acetylhydrolase: Protects against Necrotizing Enterocolitis (NEC) by blocking platelet-activating factor,.
• Growth Factors:
  • Epidermal Growth Factor (EGF): Promotes intestinal maturation and repair,.
  • Nerve Growth Factor (NGF): Promotes neural growth.
  • Insulin-like Growth Factors (IGF): Mediate growth.
• Hormones: Contains erythropoietin, prolactin, insulin, cortisol, thyroxine, and prostaglandins,.
• Anti-parasitic Factors: PABA deficiency in milk suppresses malaria parasites. BSSL kills Giardia and Amoeba.

Variations in Composition

The composition of breast milk is dynamic and changes based on several factors.

• Colostrum (Days 1–3):
  • Small quantity (10–40 mL).
  • Rich in protein and immunoglobulins (especially sIgA).
  • Higher in sodium, chloride, and magnesium compared to mature milk.
• Transitional and Mature Milk:
  • Colostrum is replaced by transitional milk and then mature milk by ~2 weeks.
  • Protein content declines while lactose and fat content stabilize.
• Preterm Milk (PTM):
  • Produced by mothers delivering before 37 weeks.
  • Protein: Significantly higher (~2.3 g/100 mL in week 1) to support rapid catch-up growth,.
  • Electrolytes: Higher sodium (1.7 mmol/100 mL) and chloride to compensate for renal losses.
  • Energy: Slightly variable but optimized for preterm needs with higher LCPs and MCTs.
  • Immunology: Higher concentrations of IgA and lactoferrin.
• Foremilk vs. Hindmilk:
  • Foremilk: Watery, thirst-quenching, lower in fat,.
  • Hindmilk: Richer in fat and energy, provides satiety and promotes weight gain,.