system
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human
eli5 How is breastmilk made?

I understand that it’s made from blood, how does it go from blood to milk? How much blood is required to make 1 oz of milk? Is there waste? How does the composition (lipids, proteins, water, nutrients etc) change?
gpt
Breastmilk is made in the mammary glands inside of the breast. These glands are actually modified sweat glands.

In terms of "made from blood" it's the same as most other cells where blood carries the nutrients needed for the cells. Breastmilk requires about 20 Calories per 1 oz so nursing mothers need to eat! The conversion is actually quite efficient as most of the extra calories consumed get converted to milk calories for the baby.

While the fat content of breastmilk may change with the age of the baby, the composition of breastmilk is very consistent among mothers even living in different environments and with different diets.