If you’ve found yourself scrolling through the breastfeeding world online, you know that people have a lot of strong opinions about breast milk. But what exactly do we know about the biology of it?

Such work has revealed distinct biological pathways for what is going wrong. Some women have too few lactocytes. Others may have lactocytes that are inefficient and secrete little breast milk.

We are now thinking of human milk not just as nutrition, but as a liquid biopsy of the mammary gland. Milk contains proteins, ...