Fats are completely digested in the small intestine, unlike carbohydrates and proteins. The inner surface of the intestine has circular folds that more than triple the surface area for absorption. Villi covered with epithelial cells increase the surface area by another factor of 10. Each villus has a surface that is adjacent to the inside of the small intestinal opening covered in microvilli that form on top of an epithelial cell known as a brush border . Each villus has a capillary network supplied by a small arteriole.
Absorbed substances pass through the brush border into the capillary, usually by passive transport.
Enzymes in the cells convert these disaccharides into monosaccharides that then leave the cell and enter the capillary. Lactose intolerance results from the genetic lack of the enzyme lactase produced by the intestinal cells. Peptide fragments and amino acids cross the epithelial cell membranes by active transport . Inside the cell they are broken into amino acids that then enter the capillary. Gluten enteropathy is the inability to absorb gluten, a protein found in wheat.