Delaying solids gives baby's digestive system time to mature.
If solids are started before a baby's system is ready to handle them, they are poorly digested and may cause unpleasant reactions (digestive upset, gas, constipation, etc.). Protein digestion is incomplete in infancy. Gastric acid and pepsin are secreted at birth and increase toward adult values over the following 3 to 4 months. The pancreatic enzyme amylase does not reach adequate levels for digestion of starches until around 6 months, and carbohydrate enzymes such as maltase, isomaltase, and sucrase do not reach adult levels until around 7 months. Young infants also have low levels of lipase and bile salts, so fat digestion does not reach adult levels until 6-9 months.