The development of the digestive system is complete during fetal life, when the fetus can already take in some nutrients by gulping the mother’s amniotic fluid, but does not yet have normal digestive functions, and this ability develops gradually and is rapidly perfected after birth. The digestive and absorption ability of newborns and infants is developed rapidly with breastfed children. Studies have shown that breastfed infants are able to absorb 80% to 90% of the fat, protein and sugar in the first few weeks of life. However, infants fed with cow’s milk still have poor digestion and absorption of fat from cow’s milk 4 to 5 months after birth. The intestinal canal of newborns is highly permeable and develops until 4-5 years of age, when it can be said to be close to maturity. From this developmental process we can learn that the possibility of food metabolic reactions begins in infancy and it is not uncommon for food metabolic reactions to occur in this period. In particular, the intestinal tract is highly permeable during the neonatal period, so the proteins in the milk fed to infants during this period can be absorbed even though they are not fully digested, thus easily causing metabolic reactions. This may also explain why food allergic reactions are more frequent before infancy.