Milk nutrients come from the blood stream.
1400 years ago people thought that milk came from the white fat near the nipples. However today this turned out to be false. Milk is produced in mammary glands which in turn get the nutrients directly from the blood stream. The blood stream gets it nutrients from the digestive system.
[Quran 16.66] And in cattle there is a lesson for you, We give you drink, from their bellies digested food and blood, pure liquid milk for those who drink.
How could an illiterate man who lived 1400 years ago have known that milk comes from the blood stream?