Breast milk is considered to be the most appropriate nutrition for newborn babies and young infants. New data show that breast milk participates in the metabolic and immunological programming for the newborn, partially by facilitating the colonization of the infant's gut with beneficial bacteria, which are naturally present in the milk. However, not all breast milks are the same. The composition of breast milk depends on various parameters. Maternal diet and lifestyle can influence not only the nutritional and immunological value of breast milk, but also the types of bacterial strains present and their relative abundance.

Scientists believe that breast milk contains hundreds of unique bioactive substances, which protect against infection and inflammation, program immune maturation and promote organ development. For example, lactoferrin, an important protein found in human (and mammalian) breast milk is currently being investigated as a novel antimicrobial agent in a variety of clinical settings, such as prevention of hospital infections in neonatal intensive care units and diarrhea in children. Human milk also contains a unique class of sugars, known as type I oligosaccharides. Despite the fact that the baby does not have the enzymes to break them down, these undigested sugars eventually feed the first beneficial bacteria of the infant's gut (mainly Bifidobacteria). Breastfeeding provides the young child both superior nutrition and protection. The composition of breast milk depends on many factors; its composition changes constantly throughout the lactation period and is inevitably different between mothers. The overall concentration of protein and immune factors declines naturally during lactation. This trend could indicate that the immune and digestive system of the infant is considered mature enough to continue its development with reduced maternal nutritional input.

On the other hand, maternal diet is a major factor that affects the concentration of many important nutritional molecules in breast milk. There is substantial research evidence showing that presence of essential fatty acids (omega 3) in breast milk depends on the maternal dietary intake. A study published in The Journal of Nutrition, in 2012, investigated the effects of fish oil supplementation or salmon consumption during and after pregnancy. The results show that it is the mother's dietary patterns, during gestation and lactation that determine the adequate supply of omega 3s to their offspring before and after birth. The contribution of omega 3 fatty acids (mainly DHA and EPA) in health and wellbeing is well established.