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What Makes Human Milk So Special?

Mother feeding her child human milk,

We know about the benefits of breastfeeding, and have for years. But what makes it so very special? What are the components of human milk, and how do they change over time?

What about protein?

Protein is the macronutrient used for the growth and repair of cells. So, it makes sense that the preterm infant would need to grow and repair many cells, right? No wonder, then, that his mother’s milk is higher in protein compared to the milk of a mother who delivered a full-term infant. It’s also higher in two microminerals: sodium and chloride.

Keep in mind, though, that human milk is lower in protein than any other mammalian species.

What about taurine?

Taurine is an especially important amino acid (the building blocks of proteins) because of its function in brain and retinal development. The eyes are one of the last organs to fully develop in the infant.

Interestingly, taurine was not added to formula until about the mid 1980s.

How about fats?

Ah yes. Fats. Fats are one of several different types of lipids. But let’s talk fat.

Simply stated, fats are very variable. The amount of fat in human milk varies from woman to woman, from breast to breast, from day to day, from week to week, from morning to evening. And there’s more!

Researchers in western Australia have found that the more “empty” the breast is, the more concentrated the fat. However, the amount of fat the mother consumes in her diet does not affect the amount of fat in her milk.

Fat varies by time of lactation. The fat in colostrum is more concentrated than the fat in mature milk. But percentage-wise, it is greater than the other macronutrients. (And remember, fats are more difficult to digest than the other macronutrients.) Aksit and colleagues found that 2-6 months of lactation increases the milk’s creamatocrit by 28%.


Zinc is a micronutrient that is important for many biological functions. In the human infant, it’s especially important for brain development.

Yalcin and colleagues found that the amount of zinc in the human milk seems to decrease with more months of lactation, and the effect is more pronounced when iron stores are low.  Yet, maternal intake of zinc does not seem to affect to the amount in milk.

Zinc is present in about the same amounts in preterm human milk as term milk.  


Leptin is an interesting hormone. It appears that leptin plays a part in satiety, but the mechanism is not well understood.  

Leptin also appears to have a different effect on weight gain for girls than on boys.


You know that calcium builds strong bones and teeth. Mothers worry about not having enough calcium if they breastfeed one baby — or two or more.

But here’s what you might not know. Calcium levels actually increase if the mother breastfeeds the toddler while she is pregnant!


Lysozyme is an enzyme. Like all enzymes, it helps to break down specific components, but is never consumed. Lysozyme increases as lactation continues.

Immunoglobulin A

Secretory IgA is 10 times higher on the first day postpartum than any other time during lactation. It does diminish over time, but it remains until weaning.


Lactose, the carbohydrate in human milk, helps to feed and develop the brain. Increased lactose around day 3 postpartum triggers the volume we see when the milk “comes in.” But lactose amounts decrease after the last half of the first year or so. But no worries! That’s about the time the baby is starting to eat table food.

This isn’t everything you need to know, but I’ve found that most people have trouble learning the biochemistry of human milk.

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