It is widely accepted that human breast milk is the optimum food for infants. Not only does it provide essential nutrients for the developing infant, it changes in composition both during the lactation period and the lactation session to supply ideal nutrition according to the respective stages of development and hourly demand. For example, the fat content of breast milk is lowest early in the morning and highest in the afternoon. The amount of fat in breast milk also increases substantially towards the end of a session. The vitamin and mineral content of breast milk varies during the postpartum period, throughout nursing, and during weaning.

The mineral content of breast milk is highest immediately postpartum and decreases over time. During the first few days postpartum, colostrum is high in vitamin E, protein, zinc, magnesium, and antibodies. From day 3 to day 6 postpartum, colostrum enters a transitional form with increasing amounts of fat, lactose, and calories, while protein and immune factors gradually decrease. Magnesium levels begin to decrease after 18 months of lactation. Unlike other minerals, magnesium secretion is unaffected by constitutional variables such as adolescent motherhood, maternal malnutrition, smoking, gestation length, environment or stage of lactation.

The vitamin content transitions according to the fat- and water-solubility of vitamins and may vary according to the mother’s diet and genetic makeup. However, if a mother is deficient, the volume of milk produced varies more than the vitamin composition. Generally, water-soluble vitamins (e.g., B, C, B₆) increase over time except for vitamins C and B₆. Fat-soluble vitamins (e.g., A, D, E, K) generally decrease over time.