First, the sugar in breast milk is mainly lactose, which comes from the glucose in the mother’s blood circulation and is converted into lactose in the mammary gland by the mammary cells, which is beneficial to the infant’s absorption. Secondly, the protein in breast milk comes from the amino acids in the mother’s blood, which are synthesized in the mother’s mammary gland by the mammary cells and transformed into small molecules of protein for the fetus to absorb. Thirdly, the small molecules of fat in breast milk come from fatty acids, cholesterol and phospholipids in the mother’s blood, which are transferred to the mother’s breast for transformation and synthesis by the mammary cells, and finally transformed into fat for the fetus to absorb. Fourth, the vitamins and inorganic salts in breast milk, as well as antibodies and other small molecules, come from the mother’s blood and are filtered through the mammary cells, as well as absorbed and stored in the mammary cells, in general, all the substances in breast milk come from the mother’s blood.