Human vs Animal Milk

Comparison of Human Milk and Cow's Milk

Human milk is "species-specific," uniquely adapted to meet the nutritional, immunological, and developmental needs of the human infant. Cow's milk, designed for the rapid somatic growth of a calf, differs significantly in macronutrient quality, micronutrient bioavailability, and renal solute load.

1. Protein Composition

• Quantity: Cow's milk contains approximately three times more protein (3.3 g/100 ml) than human milk (1.1–1.2 g/100 ml). This high protein load in cow's milk contributes to a high renal solute load, stressing the infant's immature kidneys.
• Quality (Casein:Whey Ratio):
  • Human Milk: Whey-predominant. The ratio is typically 60:40 (Whey:Casein) in mature milk, reaching 80:20 in early lactation. Whey proteins form a soft, flocculent curd in the stomach, facilitating easy digestion and rapid gastric emptying.
  • Cow's Milk: Casein-predominant. The ratio is 20:80 (Whey:Casein). Casein forms large, tough, hard-to-digest curds, leading to delayed gastric emptying.
• Specific Proteins:
  • Beta-lactoglobulin: The major whey protein in cow's milk. It is absent in human milk and is a potent allergen, often responsible for Cow's Milk Protein Allergy (CMPA).
  • Alpha-lactalbumin & Lactoferrin: Major whey proteins in human milk. Lactoferrin has significant anti-infective properties.
• Amino Acid Profile:
  • Taurine & Cysteine: Human milk is rich in taurine (neurotransmitter/neuromodulator for brain/retina) and cysteine (essential for preterms due to enzyme immaturity). Cow's milk is deficient in these.
  • Aromatic Amino Acids: Cow's milk contains high levels of phenylalanine and tyrosine. Infants have limited capacity to metabolize these, potentially leading to hyperaminoacidemia, metabolic acidosis, and azotemia.

2. Lipid Composition

• Quantity: Both milks contain similar total fat (~3.5–3.8 g/100 ml), providing about 50% of total energy.
• Quality:
  • Essential Fatty Acids (EFA): Human milk is rich in Linoleic acid (LA) and Alpha-linolenic acid (ALA). It has a Polyunsaturated/Saturated (P/S) ratio of 1.2:1, whereas cow's milk has a ratio of 1:2 (mostly saturated fats).
  • LCPs (DHA & ARA): Human milk contains Long-Chain Polyunsaturated Fatty Acids (Docosahexaenoic acid and Arachidonic acid), critical for brain myelination and retinal development. These are virtually absent in cow's milk.
  • Cholesterol: Higher in human milk, which may program better cholesterol metabolism in adulthood.
• Digestion: Human milk contains Bile Salt Stimulated Lipase (BSSL), which activates in the infant's intestine to digest fats. This compensates for the infant's immature pancreatic lipase. Cow's milk lacks this enzyme.

3. Carbohydrate Composition

• Quantity: Human milk is sweeter, containing 7 g/100 ml of lactose compared to 4.5–4.8 g/100 ml in cow's milk.
• Role: Lactose promotes calcium absorption and the growth of Lactobacillus bifidus.
• Oligosaccharides: Human milk contains significant amounts of galacto-oligosaccharides (GOS) and mucins, which act as prebiotics and "decoy receptors" to prevent pathogen attachment. These are negligible in cow's milk.

4. Minerals and Renal Solute Load

• Renal Solute Load (RSL): Cow's milk has a high RSL (~221 mOsm/L) due to high protein and mineral content. Human milk has a low RSL (~79 mOsm/L). High RSL puts the infant at risk of hypernatremic dehydration during illness or hot weather.
• Calcium and Phosphorus:
  • Cow's milk contains much higher absolute amounts of calcium and phosphorus but in a non-physiological ratio (Ca:P < 2:1). High phosphate interferes with calcium absorption, causing hypocalcemic tetany.
  • Human milk has less calcium but a physiological Ca:P ratio of >2:1, ensuring optimal absorption and bone mineralization.
• Iron: Both are low in iron, but absorption from human milk is ~50% (due to lactoferrin and Vitamin C) compared to ~10% from cow's milk. Cow's milk feeding is a major cause of iron deficiency anemia due to poor absorption and occult intestinal blood loss.
• Sodium: Cow's milk has nearly triple the sodium content (2.2 mEq/dL) of human milk (0.7 mEq/dL), predisposing to hypernatremia.

5. Vitamins

• Vitamin C: Adequate in human milk (if mother is nourished); poor in cow's milk (often destroyed by pasteurization/boiling).
• Vitamin D & K: Both milks are relatively deficient; supplementation is often required.

Summary Table: Human vs. Cow's Milk (per 100 ml)

Difference in Milk Composition: Preterm vs. Term

Mothers delivering prematurely produce "Preterm Milk" (PTM), which is biologically adapted to meet the rapid growth needs and immature physiology of the preterm infant. This difference typically persists for about 4 weeks.

1. Protein and Nitrogen

• Higher Concentration: PTM has significantly higher protein content (~2.3 g/100 ml in the first week) compared to term milk (1.1 g/100 ml). This supports the rapid "catch-up" somatic growth required by preterms.
• Trajectory: The protein content gradually declines, reaching term milk levels (1.3 g/100 ml) by 6 weeks postpartum.
• Amino Acids: PTM is richer in specific amino acids like cystine and taurine, which are essential for preterms due to enzymatic immaturity.

2. Electrolytes and Minerals

• Sodium: PTM has a much higher sodium concentration (1.7 mmol/100 ml vs. 0.6 mmol/100 ml in term milk) to compensate for the renal salt wasting (immature tubular reabsorption) common in preterms.
• Chloride: Similarly higher in PTM.
• Calcium & Phosphorus: Though levels are higher in PTM than term milk, they are often still insufficient to match the intrauterine accretion rates required for bone mineralization in VLBW infants, necessitating Human Milk Fortifiers (HMF).

3. Fats and Energy

• Energy: While caloric density is generally comparable (~67 kcal/100 ml), PTM is richer in Long Chain Polyunsaturated Fatty Acids (LCPs) and Medium Chain Triglycerides (MCTs).
• Digestion: The presence of Bile Salt Stimulated Lipase (BSSL) is crucial in PTM to aid fat absorption in the face of pancreatic insufficiency.

4. Immunological Components

• Enhanced Protection: PTM contains higher concentrations of Secretory IgA (sIgA), lysozyme, and lactoferrin compared to term milk. This provides enhanced protection against Necrotizing Enterocolitis (NEC) and sepsis, to which preterms are highly susceptible.

Immunological Factors in Human Milk

Human milk acts as a "living fluid," providing passive immunity and modulating the infant's active immune system. These factors are absent in formula.

1. Humoral (Soluble) Factors

• Secretory IgA (sIgA):
  • The dominant immunoglobulin in breast milk.
  • Enteromammary Axis: Mothers produce antibodies to pathogens in their gut/environment (GALT); these specific plasma cells migrate to the breast and secrete specific sIgA into the milk. This protects the infant against pathogens present in their immediate environment.
  • Mechanism: It resists proteolytic digestion in the gut and "paints" the mucosal surfaces, preventing pathogen adherence and invasion (surface immunity).
• Lactoferrin:
  • An iron-binding whey protein.
  • Bacteriostatic: It binds free iron avidly, starving iron-dependent bacteria like E. coli and Staphylococcus.
  • Also has direct bactericidal and antiviral properties.
• Lysozyme:
  • An enzyme that destroys bacterial cell walls (lysis). Found in high concentrations in colostrum.
• Bifidus Factor:
  • A nitrogen-containing carbohydrate that promotes the growth of Lactobacillus bifidus.
  • Competitive Inhibition: This beneficial flora produces acetic and lactic acids, lowering gut pH and inhibiting the growth of pathogenic coliforms like E. coli and Shigella.
• Oligosaccharides & Mucins:
  • Act as "decoy receptors" or analogues for microbial attachment sites. Pathogens bind to these sugars instead of the infant's intestinal mucosa and are excreted.
• Cytokines and Growth Factors:
  • Contains IL-10 (anti-inflammatory), Epidermal Growth Factor (EGF), and Nerve Growth Factor (NGF). These promote gut maturation, reduce inflammation (protecting against NEC), and aid neural development.
• Other Factors:
  • Fibronectin: Enhances macrophage activity.
  • PABA Deficiency: Milk is deficient in Para Amino Benzoic Acid, which suppresses malaria parasites that require it for growth.
  • BSSL: Hydrolyzes bacterial lipids and kills protozoa like Giardia and Amoeba.

2. Cellular Components

• Macrophages: The most abundant cells (esp. in colostrum). They engulf bacteria (phagocytosis) and synthesize complement and lysozyme.
• Lymphocytes:
  • T-cells: Transfer immunological memory to the infant.
  • B-cells: Produce antibodies.
• Neutrophils: Present in lower numbers; aid in bacterial killing via peroxidase activity.

These factors collectively reduce the risk of diarrhea (14x lower mortality), respiratory infections (4x lower), and NEC in breastfed infants compared to those artificially fed.