The final step in digesting proteins and carbohydrates and absorbing their components occurs among the intestinal microvilli. Mucosal epithelial cells produce peptidases that cleave small peptides into absorbable amino acids. These epithelial cells also produce the enzymes maltase, lactase, and sucrase that cleave the common disaccharides into absorbable monosaccharides—glucose, galactose, and fructose. There is also some lipase activity for fat digestion.

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Many humans stop producing the enzyme lactase in childhood and thereafter have difficulty digesting lactose (the sugar in milk). Lactose is a disaccharide and cannot be absorbed without being cleaved into its constituents, glucose and galactose. Unabsorbed lactose is metabolized by bacteria in the large intestine, causing gas, diarrhea, and abdominal cramps.

The mechanisms by which cells of the intestinal epithelium absorb nutrients and inorganic ions are diverse and include diffusion, facilitated diffusion, active transport, and secondary active transport. Many inorganic ions such as sodium, calcium, and iron are actively transported by these cells. For example, active Na⁺ transporters exist on the basal and lateral sides of the epithelial cells. They maintain a low concentration of Na⁺ in those cells so that Na⁺ can diffuse in from the chyme in the intestinal lumen. About 30 grams of Na⁺ are transported this way every day, and Cl^– follows.

The transport of Na⁺ and other ions is important for water absorption because it creates an osmotic concentration gradient. At least 7–8 liters of water per day move through the spaces between the epithelial cells in response to this osmotic gradient. Because the water moves through spaces between the cells and not through the cells themselves, it can carry with it nutrients that are in solution—a transport mechanism called solvent drag or bulk transport.

Many different kinds of transport proteins function in epithelial cell membranes. Some, such as the transport protein for fructose, only facilitate diffusion, and that requires a concentration gradient. Once fructose enters the cell it is converted to glucose, so the concentration of fructose in the cell is always low and the concentration gradient is maintained. Transport proteins known as symporters (see Figure 6.13) exploit the concentration gradient of Na⁺ between the inside and outside of the cell that is maintained by the Na⁺–K⁺ pump common to all cells (see Figure 6.14). Symporters combine the transport of Na⁺ and another molecule, such as glucose, galactose, or an amino acid. As Na⁺ moves down its concentration gradient into the cell, the “hitchhiking” molecules are carried along with it.

The absorption of the products of fat digestion is relatively simple. Triglycerides are hydrolyzed to diglycerides, monoglycerides, and fatty acids, all of which are lipid-soluble and thus able to pass through the cell membranes of the microvilli. In the intestinal epithelial cells, these molecules are resynthesized into triglycerides, combined with cholesterol and phospholipids, and coated with protein to form water-soluble chylomicrons (Figure 50.14B). Rather than enter the blood directly, chylomicrons pass into blind-ended lymph vessels called lacteals that are inside each villus (see Figure 50.8C). They then flow through the lymphatic system, entering the bloodstream through the thoracic ducts at the base of the neck. After a meal rich in fats, chylomicrons can be so abundant in the blood that they give the blood plasma a milky appearance. Chylomicrons deliver their triglyceride and cholesterol cargo as they circulate through tissues.

The bile molecules that emulsify fats are not absorbed along with the monoglycerides, diglycerides, and the fatty acids, but are shuttled back and forth between the gut contents and the microvilli. In the ileum, bile is actively reabsorbed and returned to the liver via the bloodstream.