The development of gastrointestinal (GI) function begins in the fetus.  Beginning around the 4th month of gestation, the fetus begins to swallow amniotic fluid.  The swallowed fluid is absorbed in the GI tract and recycled to the amniotic sac by fetal urination.  Stooling does not begin until after birth.  Sucking and swallowing is a reflex that is present at birth.  In fact, an infant can begin bottle or breast feeding almost immediately after birth. Solid foods are not tolerated by infants until at least 3 months of age.  The normal, regular waves of muscle contraction that push milk through the gastrointestinal tract are usually functioning well by 10 days of age in term infants.  The first stool (meconium) is typically passed within the first 24 hours after birth.  Digestion of carbohydrates, lipids and proteins are described below.

Digestion of carbohydrates

Carbohydrates include sugars, starches and fiber.  They are essentially long chains of saccharides (simple molecules like glucose).  Enzymes such as amylase and disaccharidases help breakdown these chains to single molecules that can be absorbed by the intestines. 

Amylase is a protein (or enzyme) that helps break down long chains of saccharides.  It is found in breastmilk and saliva.  It is also secreted into the intestines by the pancreas.

Disaccharidases are proteins that help break 2-unit groups of saccharides into separate molecules.  Lactose (the main carbohydrate in milk) requires a disaccharidase called lactase for digestion.  The disaccharidases are found in the walls of the intestines.

Monosaccharides (such as glucose, galactose and fructose) can be absorbed by the intestinal wall directly with the help of water and sodium.

An excess of saccharides can cause diarrhea.  This is how some constipation medicines like lactulose (a disaccharide) work.  Excess fruit juice (fructose) or "sugar-free" foods (sorbitol) can also cause diarrhea for this reason.

Digestion of lipids

Lipids in milk provide about 50% of the energy requirements for infants.  They contain more than twice the number of calories per gram when compared to carbohydrates and proteins. 

Lipase is a protein that helps breakdown lipids (fats) for digestion.  Lipase is found in breastmilk and is produced by the tongue, stomach and pancreas. 

Bile acids are chemicals produced by the liver that are excreted into the intestines.  They help break down fat droplets that can be digested and absorbed more easily.

Digestion of proteins

Pepsin is a protein in the stomach that is activated by stomach acid to break down proteins. 

The pancreas also makes enzymes (such as trypsin and chymotrypsin) that are secreted into the small intestines to help break up protein molecules. 

Enzymes in the small intestinal walls complete the breakdown of proteins into small units called amino acids which are absorbed into the blood.

Absorption of vitamins and minerals

Vitamins and minerals are absorbed directly through the intestinal walls into the blood.