Composition of Human Milk
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Direct answer
Human milk is a species-specific food whose composition changes from colostrum to mature milk, with lactose and whey protein predominating over casein. Compared with cow milk it contains much less protein and more lactose, with a whey-to-casein ratio of about 60 to 40 instead of about 20 to 80. It also carries secretory IgA, lactoferrin, lysozyme and long-chain fatty acids that are scarce in cow milk.
What you must remember
- Colostrum: thicker and yellowish, richer in protein, secretory IgA and minerals, with less fat and lactose than mature milk.
- Mature milk protein: about 0.9 to 1 g per 100 mL, far below the roughly 3.3 g in cow milk, which keeps the renal solute load low.
- Whey-casein ratio: about 60 to 40 in human milk against about 20 to 80 in cow milk, producing soft curds in the stomach.
- Lactose and oligosaccharides: lactose is higher in human milk, and human milk oligosaccharides favour Bifidobacterium growth in the infant gut.
- Lipids: human milk supplies DHA and arachidonic acid, plus bile salt-stimulated lipase, which aids fat digestion.
- Immune factors: secretory IgA, lactoferrin, lysozyme and live leucocytes protect the infant's gut and airways.
- Deficient nutrients: vitamin K, vitamin D and iron are low, so newborn prophylaxis and later complementary sources are needed.
Comparing human and cow milk
A mother asks why her baby grows well on breast milk alone, even though its protein content looks low next to cow milk. The answer lies in how the components are matched to an immature infant, as summarised below.
| Component | Human milk (mature) | Cow milk |
|---|---|---|
| Protein | About 0.9 to 1 g/100 mL | About 3.3 g/100 mL |
| Lactose | About 7 g/100 mL | About 4.7 g/100 mL |
| Whey:casein | About 60:40 | About 20:80 |
| Immune factors | IgA, lactoferrin, lysozyme | Much lower |
| Iron | Low content, well absorbed | Low content, poorly absorbed |
The lower protein load suits the immature infant kidney, while the whey-rich mix digests easily. Lactose supplies much of the infant's energy, so breast milk needs no added carbohydrate during the first six months, and the immune factors compensate for an infant's still-developing defences. Cow milk's higher protein and sodium loads, together with its low iron, explain why it is unsuitable as a drink before one year.
Where the exam tries to catch you
Stems usually test a single component and the reason it matters for the infant.
- Trap: calling human milk inadequate because its protein is low. Correct: the low, whey-rich protein is matched to the infant's kidney and digestion, and it supports adequate growth.
- Trap: starting cow milk early as a cheap drink. Correct: its protein and sodium load and low iron can cause iron deficiency and occult gut blood loss before one year.
- Trap: assuming human milk supplies enough vitamin D and iron. Correct: vitamin D is low, so supplementation is advised, and iron stores fall after about six months, needing complementary sources.
- Trap: dismissing colostrum because its volume is small. Correct: colostrum carries a high concentration of IgA and cells, so small early volumes deliver a substantial immune dose.
Frequently asked questions
Why is human milk lower in protein than cow milk?
Human milk protein is lower in quantity but matched to infant needs, with a whey-rich profile that is easier to digest. The lower protein load limits the solute burden on the immature kidney while still supporting normal growth.
What is the whey-to-casein ratio in human milk?
Mature human milk has a whey-to-casein ratio of about 60 to 40, whereas cow milk is closer to 20 to 80. Whey-rich milk forms softer curds in the stomach, which is one reason breast milk is easier to digest.
Which vitamins and minerals are low in human milk?
Vitamin K, vitamin D and iron are low in human milk. Newborns therefore receive vitamin K prophylaxis at birth, vitamin D supplementation is advised, and iron is needed from complementary foods after about six months.
Why is colostrum important despite its small volume?
Colostrum is rich in secretory IgA and cells that protect the gut, and it helps clear meconium early. Its volume is small, but the immune dose per millilitre is high, so it should be given from birth.
Why should cow milk not be the main drink before one year?
Cow milk has a high protein and sodium load, is low in iron and can cause occult gut blood loss. Breast milk or formula should therefore remain the main drink before one year of age.