Your infant's digestive tract is not fully functional and is vulnerable to infection. Anything that enters your baby's mouth makes its way into her gastrointestinal tract, which is not yet ready to fight off bacteria and other pathogens. In the first six months, your baby's digestive system will undergo enormous change as it develops the ability to produce enzymes to digest food and antibodies to protect itself.
During gestation, your baby received nutrients and disposed of waste products through the placenta. At birth, this changed abruptly, but your newborn's digestive system is still very immature. As a result, he may lose up to 10 percent of his body weight in the first days of his life, as he adjusts to using his digestive system. Because a newborn's stomach is small, your infant needs frequent feedings. Breast milk is high in fat because that's the most efficient way for your baby to meet his caloric needs. Ounce for ounce, fat contains over twice as many calories as proteins or carbohydrates.
Although a newborn can digest carbohydrates and proteins as well as fat, her pancreas is not fully developed, so your baby produces much lower levels of digestive enzymes than an older child. Enzymes in breast milk and your baby's saliva help make up for this shortcoming. In addition, the esophageal valve, which controls the entry of food into your infant's stomach, is underdeveloped. This is why babies frequently spit up. These digestive shortcomings, along with the immature state of the infant kidney, can put your baby at risk for dehydration, electrolyte imbalance and insufficient absorption of nutrients.
The human digestive system has a layer of mucous that protects the gastrointestinal tract from microbes and other contaminants that may be present in food or liquids. In infants, this protective barrier is immature, which puts your baby at risk of infection. Antibodies in breast milk help protect your baby until his digestive mucosal lining matures and he increases his ability to produce his own antibodies, which happens around the age of six months.
Vitamins and minerals help your baby's digestive system mature, but don't give her iron supplements or iron-fortified foods before the age of six months because this extra iron will reduce her ability to absorb iron from foods. In the first six months, breast milk is important to your baby's digestive maturation because it contains intestinal growth factors that help her to develop colonies of beneficial bacterial in her intestines. These bacteria help prevent invasion by pathogens, and they also help your child's intestinal lining to mature.
Although your baby may exhibit interest in solid foods, his digestive system won't be ready for the transition until he's about six months old. His body will not produce sufficient levels of enzymes to digest starches until around the age of six months, and enzymes that digest carbohydrates don't reach sufficient levels until approximately age seven months. Lipase and bile salts, which aid in fat digestion, don't reach full levels until the age of six to nine months. Moreover, between the ages of four and six months, infants have an "open gut," which allows whole proteins to pass directly from the small intestine into the bloodstream. The function of this opening is to allow antibodies from breast milk to enter the bloodstream, but large molecules from solid foods also can pass through and may cause allergies, or carry pathogens with them.